Atomic Physics Seminars History

Macroscopic quantum phenomena, such as observed in superfluids and superconductors, have led to promising technological advancements and some of the most important tests of fundamental physics. At present, quantum detection of light is mostly relegated to the microscale, where avalanche photodiodes are very sensitive to distinguishing single-photon events from vacuum but cannot differentiate between larger photon-number events. Beyond this, the ability to perform measurements to resolve photon numbers is highly desirable for a variety of quantum information applications, including computation, sensing and cryptography. True photon-number resolving detectors do exist, but they are currently limited to the ability to resolve on the order of 10 photons, which is too small for several quantum-state generation methods based on heralded detection. In this talk I’ll explain how we extended photon measurement into the mesoscopic regime by implementing a detection scheme based on multiplexing highly quantum-efficient transition-edge sensors to accurately resolve photon numbers between 0 and 100. Then I’ll demonstrate the use of our system by explaining how we implemented a quantum random-number generator with no inherent bias. This method is based on sampling a coherent state in the photon-number basis and is robust against environmental noise, phase and amplitude fluctuations in the laser, loss and detector inefficiency as well as eavesdropping. Beyond true random-number generation, our detection scheme serves as a means to implement quantum measurement and engineering techniques valuable for photonic quantum information processing.

Quantum computing with neutral atoms has progressed rapidly in recent years, combining large system sizes, flexible and dynamic connectivity, and quickly improving gate fidelities. The pioneering work in this field has been implemented using alkali atoms, primarily rubidium and cesium. However, divalent, alkaline-earth-like atoms such as ytterbium offer significant technical advantages. In this talk, I will present our progress on quantum computing using 171-Yb atoms, including high-fidelity imaging, nuclear spin qubits with extremely long coherence times, and two-qubit gates on nuclear spins using Rydberg states [1,2]. I will also discuss several unexpected benefits of alkaline-earth-atoms: an extremely robust and power-efficient local gate addressing scheme [3], and a novel approach to quantum error correction called “erasure conversion”, which has the potential to implement the surface code with a threshold exceeding 4%, using the unique level structure of 171-Yb to convert spontaneous emission events into erasure errors [4].

 

[1] S. Saskin et al, Phys. Rev. Lett. 122, 143002 (2019).
[2] A. P. Burgers et al, PRX Quantum 3, 020326 (2022).
[3] S. Ma, A. P. Burgers, et al, Phys. Rev. X 12, 021028 (2022).
[4] Y. Wu, et al, Nat. Comms. 13, 4657 (2022).

Inertial Navigation Systems (INS) are alternatives to GPS that operate using linear accelerometers and gyroscopes to calculate the user’s position, orientation, and velocity using no external reference. Optical Sagnac gyroscopes are part of modern day INS and are limited by their ability to measure small rotations as they need a very large enclosed area. The Bragg Interferometer Gyroscope in a Time Orbiting Potential Trap (BIGTOP) is a rotation detector using a Bose-Einstein Condensate (BEC) to execute two Sagnac interferometers in a magnetic trap. BIGTOP is an improvement upon a previous iteration of a dual Sagnac interferometer which demonstrated rotation sensing. We have achieved atom interferometry with BIGTOP and reached a Sagnac area of 8 mm² using multiple orbits, an improvement by a factor of 16. Additionally, we have taken our first large dataset over the course of 24 hours, which can be used to analyze the stability of our system. In tandem with BIGTOP, we have also worked to characterize and operate a compact atom chip interferometer system built by Cold Quanta (CQsystem). We report BIGTOP results, progress with the CQ system, and future work.

Robust and accurate acceleration tracking remains a challenge in many fields. For geophysics and economic geology, precise gravity mapping requires onboard sensors combined with accurate positioning and navigation systems. Cold-atom-based quantum inertial sensors can provide such high-precision instruments. However, current scalar instruments require precise alignment with vector quantities such as gravity. This presents a significant challenge in mobile environments. In recent work, we realized the first “vectorial” quantum accelerometer by combining three orthogonal atom interferometer measurements with a classical accelerometer triad. We demonstrate acceleration vector tracking with a 50-fold improvement in stability compared to our navigation-grade classical accelerometers. In this talk, I will give an overview of our vectorial quantum sensor and discuss future work moving beyond scalar quantum sensing.

The search for spontaneous pattern formation in equilibrium phases with genuine quantum properties is a leading direction of current research. We investigate the effect of quantum fluctuations - zero-point motion and exchange interactions - on the phases of an ensemble of bosonic particles with local and nonlocal interactions to determine their ground state properties. In the high-density limit, we observe patterns with 12-fold rotational symmetry compatible with periodic approximants of quasicrystalline phases and their connection to related phases in soft-matter physics. In the second part, I present results for a system of 2D trapped bosons in a quasiperiodic potential at finite temperature. Alongside the superfluid, normal fluid, and insulating phases, we demonstrate the existence of a Bose glass phase, which is robust to thermal fluctuations for a set of parameters within current experiments with quasi-2D optical confinement.

 

References:

[1] B. Abreu, F. Cinti, and T. Macrì, Phys. Rev. B 105, 094505 (2022)

[2] M. Ciardi, T. Macrì, and F. Cinti, Phys. Rev. A 105, L011301 (2022)

[3]  A. Mendoza-Coto, R. Turcati, V. Zampronio, R. Díaz-Méndez, T. Macrì, F. Cinti, Phys. Rev. B 105, 134521 (2022)

[4] N. Defenu, T. Donner, T. Macrì, G. Pagano, S. Ruffo, A. Trombettoni, arXiv:2109.01063 (2021)

It was shown that elastic tunneling through a p-n junction in gapped bilayer graphene can lead to oscillatory transmission as a function of bandgap [1], where a combination of the semiclassical considerations and numerical calculations were used. In this talk, I will first present how we confirm the numerical results of that work analytically by using the method of steepest descents, where we treat the momentum as time in Schrödinger's equation in momentum space. In the presence of phonons, we then use a similar approach and generalize it to the phonon-assisted tunneling, I will discuss how the presence of phonons, and the associated inelastic processes, can contribute to the transport across the p-n junction in gapped bilayer graphene. Near zero temperature, I will show phonon can enhance the transmission when an electron emits or absorb a phonon and jump from one branch point to another, and the conductance will behave like a step function in terms of voltage where the conductance increases with the square root of voltage firstly and eventually becomes constant.

[1] R. Nandkishore and L. Levitov, Proceedings of the National Academy of Sciences 108, 14021 (2011)

Reconfigurable arrays of neutral atoms are an exciting new platform to study quantum many-body phenomena and quantum information protocols. Their excellent coherence combined with programmable Rydberg interactions have led to intriguing observations such as quantum phase transitions, the discovery of quantum many-body scars, and the recent realization of a topological spin liquid phase.

Here, I will introduce new methods for controlling and measuring atom arrays that open up new directions in quantum state control, quantum feedback and many-body physics. First, I will introduce a dual species atomic array in which the second atomic species can be used to measure and control the primary species. This will lead to the possibility of performing quantum nondemolition measurements and new ways of engineering large, entangled states on these arrays. Furthermore, prospects of studying open systems with engineered environments will be discussed.

An alternative, hybrid approach for engineering interactions and scaling these quantum systems is the coupling of atoms to nanophotonic structures in which photons mediate interactions between atoms. Such a system can function as the building block of a large-scale quantum network. In this context, I will present quantum network node architectures that are capable of long-distance entanglement distribution at telecom wavelengths.

High temperature superconductivity is of high scientific interest. The underlying physics is captured by the Hubbard model. Based on this model, Anderson's resonating valence bond (RVB)concepts indicate that the strong correlation and frustration are keys to high temperature superconductivity. The triangular lattice Hubbard model is a paradigmatic model of a strongly correlated geometrically frustrated quantum system which exhibits a rich phase diagram including the spin-liquid state predicted by the RVB theory. However, this system is numerically difficult due to the frustration and the large ground state degeneracy. Quantum gas microscopes are at the forefront of quantum simulation, providing a direct site-resolved detection of experimental realizations of the Hubbard model. We realized site-resolved imaging of fermionic Mott Insulators in a novel triangular optical lattice. We measured the spin-spin correlations in these Mott insulators and compared the measured data to Quantum Monte Carlo simulations.

A tune-out wavelength is one at which the dynamical polarizability of an atom is zero; that is, the Stark shifts from higher- and lower-lying states cancel exactly.  Measurements of tune-out wavelengths provide vital experimental access to dipole matrix elements, which are currently the limiting factor in improving theoretical calculations of atomic parity violation.  I will discuss previous measurements of the scalar and vector tune-outs between the 5P excited states of 87Rb, as well as plans to improve measurement precision and complete a first measurement of the 6P tune-outs.

Kerr-lens mode-locking (KLM) is the work-horse mechanism for generation of ultrashort pulses, where a non-linear lens forms an effective ultrafast saturable absorber within the laser cavity. According to standard theory, the pulse in the cavity is a soliton, with a temporal profile and power determined by the non-linearity to exactly counteract diffraction and dispersion, resulting in pulses, whose power and shape are fixed across a wide range of pump powers. I will present an experimental demonstration and theoretical modeling that a KLM laser in a linear cavity deviates from the soliton model due to the non-local Kerr lens. By breaking the spatial symmetry in the cavity between the forward and backward halves of the round-trip the laser efficiency can surpass the soliton limit in a single pulse, while maintaining stable cavity propagation. We model  the symmetry breaking by numerical simulation and confirm it experimentally in a mode-locked Ti:Sapphire laser with a quantitative agreement to the simulation results. Our numerical tool opens a new window into the crux of mode-locking physics by direct examination of the spatio-temporal dynamics within the Kerr medium, which is difficult (or even impossible) to observe experimentally.

Optical tweezers provide a non-contact method to manipulate microscopic objects and have many potential applications in precision measurements. Recently, we developed an optically levitated Cavendish torsion balance for quantum-limited torque and force sensing [Phys. Rev. Lett., 121, 033603  (2018)]. We have optically levitated nanoparticles in a vacuum and driven them to rotate up to 300 billion rpm (5 GHz). Using a levitated nanoparticle in a vacuum, we demonstrated ultrasensitive torque detection with a sensitivity several orders higher than the former record [Nature Nanotechnology 15, 89 (2020)]. This system will be promising to study quantum friction, Casimir torque, and gravity at short distances.  We also propose and demonstrate a scheme to achieve strong coupling between multiple micromechanical oscillators with virtual photons, i.e., quantum vacuum fluctuations. Quantum field theory predicts that there are random fluctuations everywhere in a vacuum due to the zero-point energy. The quantum electromagnetic fluctuations can induce a measurable force between neutral objects, which is known as the Casimir effect. We have achieved non-reciprocal energy transfer between two mechanical resonators coupled by quantum vacuum fluctuations [arXiv:2102.12857].

Imaging and addressing individual atoms in optical lattices with single-site resolution constitute a new approach to the study of quantum many-body problems. It provides microscopic information of quantum many-body states, such as correlation functions, and one can engineer arbitrary density patterns for the study of non-equilibrium quantum dynamics. Here, we report the first realization of the quantum gas microscope of Lithium-7 atoms in a square two-dimensional optical lattice and observation of the unity filling Mott insulator with few thousand atoms. We implement the Raman sideband cooling in the lattice and about 4,000 photons per atom are detected by high numerical aperture (NA=0.65) objective lens. The point spread function (PSF) of the imaging system is measured to be 630 nm (full width half maximum), small enough to resolve the lattice spacing (752 nm). In the talk, we will also introduce our journey (both successful and failed stories) when implementing the state-of-the-art imaging system.

A recent experiment, conducted in the group of Cass Sackett at the University of Virginia, implemented a dual-Sagnac atom interferometer (AI) for rotation sensing using a BoseEinstein condensate (BEC) confined in a TOP-trap potential. The BEC was split twice by laser light to create two pairs of counter-orbiting clouds in a harmonic potential trap where each cloud pair acted as a separate Sagnac interferometer. After one orbit the two overlapping cloud pairs were split a final time and the population of atoms in the zero momentum state were measured. We have studied the impact of the presence of anharmonic potential terms and atom-atom interactions on the performance of this rotation sensor. Our studies have been carried out using a variational model that approximates the rotating-frame Gross-Pitaevskii equation. We have used this model to study the impacts of using larger-number condensates and multiple-orbit protocols on sensor performance.

We present a technique for measuring the amplitude of weak microwave/rf fields of arbitrary frequency. The method uses Rydberg atoms in a vapor cell as a detection medium, and electromagnetically induced transparency (EIT) spectroscopy as an optical readout. Unlike other schemes that rely on resonant coupling between Rydberg states [1], our electrometer is based on non-resonant dressing of the Rydberg atoms in combined AC and DC fields. We demonstrate the technique in a room temperature Rb cell, where mixing of the AC and DC fields through the second-order Stark shift produces sidebands in the EIT signal, flanking the primary resonance feature associated with the optical 5p – 32s transition. The spectral location of the sidebands reveals the frequency of the AC field. The ratio of the sideband intensity to that of the central EIT feature gives the AC field amplitude through calculation. Field amplitudes less than 200 mV/cm have been measured at frequencies from 20 to 100 MHz.  

[1] J. A. Sedlacek et al., Nat. Phys. 8, 819 (2012). 

The spin-1 Haldane chain is the paradigmatic example of symmetry protected topological (SPT) phases, which are characterized by non-local order and edge states. In my talk, I will report on the experimental realization of such a phase using ultracold fermions in optical lattices (arXiv:2103.10421). 

Using the full spin and density resolution of our Fermi-gas microscope, we detect a finite non-local string correlator in the bulk of a Heisenberg two-leg ladder system and image localized spin-1/2 states at its edges. We find the phase to be robust to perturbations that preserve the spin symmetry. 

Going beyond the spin model, we then study the effects of charge fluctuations on the SPT phase in the more general Hubbard ladder. Finally, I will compare the non-local order seen here to spin-charge separation in doped spin-1/2 chains. 

 

Throughout many fields of physics, particle exchange plays an important role in the understanding of long-range interactions. From the exchange of massive bosons yielding the Yukawa potential to the phonon exchange underpinning Cooper pairing in superconductors, such mediated interactions can have profound consequences on the ground state of a many-body system. When a Bose-Einstein condensate (BEC) is immersed in a degenerate Fermi gas, exchange of a particle-hole pair of fermions gives rise to an attractive mediated interaction between bosons. These mediated interactions are analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism in condensed matter and are expected to give rise to novel magnetic phases and supersolids. In this talk, I will describe how we experimentally realize these mediated interactions in a quantum degenerate mixture of Li and Cs. We show that for suitable conditions, these mediated interactions can become dominant and convert a stable BEC into a train of \Bose-Fermi solitons". In the time remaining, prospects for other methods of engineering long-range interactions in quantum gases will also be discussed.
 

Scalability is the central challenge in universal quantum computing, which has long established revolutionary premises, such as exponential speedup of difficult to near-impossible computations. A promising platform towards scalable quantum computing is the quantum optical frequency comb, which leverages optical frequency multiplexing and produces thousands of unconditional EPR entanglement in a single oscillator. In this talk, I will present our recent work to miniaturize the quantum optical frequency comb to a photonic chip for the first time. Our work brings the power of microfabrication to quantum optical applications, and could enable low cost mass-production, which promises additional scalability. I will also briefly discuss the roadmap and the challenges towards scalable quantum computing with integrated photonic frequency combs. 

 

Ultracold atoms in optical lattices provide an excellent platform to study many-body quantum systems. Especially, a quantum gas microscope, which enables us to observe and control atoms at the single-site level, is a powerful tool for such studies. Our target is a quantum simulation of frustrated systems, which are expected to exhibit various phenomena and non-trivial quantum states such as quantum spin liquids. The simplest example of frustrated systems can be realized with a triangular lattice. In this talk, I will present single-site-resolved fluorescence imaging of ultracold rubidium-87 atoms in a triangular optical lattice [1].
Experimental parameters for the fluorescence imaging have been automatically optimized by using a machine learning technique. I will also introduce experimental results [1,2] of automatic optimization based on the Bayesian optimization.

[1] R. Yamamoto et al., “Single-site-resolved imaging of ultracold atoms in a triangular optical lattice,” New Journal of Physics 22, 123028 (2020).
[2] I. Nakamura et al., “Non-standard trajectories found by machine learning for evaporative cooling of 87Rb atoms,” Optics Express 27, 20435 (2019).

Quantum information science promises to hold substantial advantages over classical information by allowing for secure communication, measurement precision below standard limits, and an exponential increase in certain computational problems. Although there have been several recent advances, such as the claims at quantum supremacy with discrete quantum computation (QC), many challenges still remain. One large obstacle is the prevention of decoherence in large entangled systems, which leads to a scalability problem in qubit-based QC. The scalability problem can be solved with cluster states using continuous-variable (CV) quantum-optics, but this comes with its own difficulties. In order to achieve a quantum advantage and allow for error correction with CV systems, it is necessary to include quantum states with non-Gaussian distribution functions.  In this talk, I will discuss several experimentally accessible ways one can generate useful non-Gaussian states with photon-number-resolved detection. Some of these states are desirable for CVQC while others show potential for Heisenberg-limited metrology. I will then introduce our method of efficient quantum state characterization utilizing the photon-number-resolving measurement capabilities in our lab.

Systems of ultracold particles with strong interactions and correlations lie at the heart of many areas of the physical sciences, from atomic, molecular, optical, and condensed-matter physics to quantum chemistry. In condensed matter, strong interactions determine the formation of topological phases giving materials unexpected physical properties that could revolutionize technology through robustness to noise and disorder. In this talk I will report on our work towards the realization of a fractional Chern insulator state using our experimental apparatus producing degenerate Fermi gases of strontium. Our simulation of the topological insulating state will follow an optical flux approach, which engineers the lattice in reciprocal space through polychromatic beams driving a manifold of stimulated Raman transitions, and will benefit from ultracold strontium's low temperatures and reduced heating by spontaneous emission.

On the other hand, systems of ultracold particles without interactions reveal matter-wave properties with enhanced interferometric sensitivity. I will discuss our ongoing efforts to trap ultracold strontium atoms on the evanescent fields of nanophotonic waveguides and nanotapered optical fibers. The existence of magic blue and red detuned wavelengths lead to a trapping volume that can be continuously and robustly loaded with ultracold strontium via a transparency beam. Fundamental studies of Casimir and Casimir-Polder physics as well as several applications, such as field sensors and matter-wave interferometers, will be possible with these platforms.

The development of ultrashort, broadband light pulses in the vacuum ultraviolet enables resonant excitation and probing of the dynamics of isolated molecules. Since the total angular momentum of an isolated system is conserved, broadband excitation necessarily leads to a coherent wavepacket of angular momentum states. Coherences between states of different angular momentum physically manifest as a time varying alignment or orientation of the molecular axis, as well as a much faster variation in the alignment or orientation of the electronic probability distribution, which is synchronized with electronic dynamics occurring in the molecular frame. I will present a direct and selective measurement of this time varying electronic anisotropy, and the potential application of ultrafast scattering probes to this end. I will also briefly discuss the application of purely rotational coherences in the electronic ground state to extract molecular frame information, particularly in the context of photoionization. 

 

 

The sensitivity of classical Raman spectroscopy methods, such as coherent anti-stokes Raman spectroscopy (CARS) or stimulated Raman spectroscopy (SRS), is ultimately limited by shot-noise from the stimulating fields. I will present a squeezing-enhanced version of Raman spectroscopy that overcomes the shot-noise limit of sensitivity with enhancement of the Raman signal and inherent background suppression, while remaining fully compatible with standard Raman spectroscopy methods. By incorporating the Raman sample between two phase-sensitive parametric amplifiers that squeeze the light along orthogonal quadrature axes, the typical intensity measurement of the Raman response is converted into a quantum-limited, super-sensitive estimation of phase. The resonant Raman response in the sample induces a phase shift to signal-idler frequency-pairs within the fingerprint spectrum of the molecule, resulting in amplification of the resonant Raman signal by the squeezing factor of the parametric amplifiers, whereas the non-resonant background is annihilated by destructive interference. Seeding the interferometer with classical coherent light stimulates the Raman signal further without increasing the background, effectively forming squeezing-enhanced versions of CARS and SRS, where the quantum enhancement is achieved on top of the classical stimulation.

 

References

[1] Yoad Michael, Leon Bello, Michael Rosenbluh and Avi Pe’er, “Squeezing-enhanced Raman Spectroscopy”,  npj  – Quantum Information 5, 81 (2019) .

[2] Y. Shaked, Y. Michael, R. Vered, M. Rosenbluh and A. Pe’er, “Lifting the Bandwidth Limit of Optical Homodyne Measurement,” Nature Communications 9, 609 (2018).

TBA

Non-equilibrium systems are ubiquitous in nature. They are actively studied in a wide range of fields, from biological cell membranes to city traffic planning. For the past several years our lab has been studying the dynamics of non-equilibrium ultracold degenerate Fermi gasses. We probe the dynamics via fast radio-frequency pulses enabled by trapping the atoms close to a microfabricated chip. The kinds of dynamics we have probed include the diffusion of spin in a strongly interacting Fermi gas, the rise of correlations in the gas after a quench of the interaction strength, and the phase transition between two different dynamical phases. Dynamical phases and the transitions between them are one possible framework to extend the powerful ideas of equilibrium statistical mechanics to diverse non-equilibrium systems. In my talk I will discuss our work on dynamics, focussing on our recent observation of dynamical phase transitions in the collective Heisenberg spin model.

We present results from a programmable quantum computer comprised of a chain of individually trapped 171Yb+ ions. It features individual laser beam addressing and individual readout, and can be configured to run any sequence of single- and two-qubit gates [1]. We combine this setup with different classical optimization routines to implement a so-called hybrid system. Quantum-classical hybrid protocols offer a path towards the application of near-term quantum computers for different optimization tasks. They are attractive since part of the effort is outsourced to a classical machine resulting in shallower and narrower quantum circuits, which can be executed with lower error rates.
We have realized several experimental demonstrations relating to this approach, such as the training of shallow circuits for Generative Modeling using a Bayesian optimization routine [2], tackling the Max-Cut problem using the Quantum Approximate Optimization Algorithm (QAOA) [3], and the preparation of thermal quantum states [4].
Recent results, limitations of the above methods, and ideas for boosting these concepts for scaling up the quantum-classical hybrid architecture will be discussed.
[1] S. Debnath et al., Nature 563:63 (2016); [2] D. Zhu et al., Science Advances 5, 10 (2019); [3] O. Shehab et al., arXiv:1906.00476 (2019); [4] D. Zhu et al., arXiv:1906.02699 (2019)

Many-body quantum systems have come under intense focus in recent years to enable a number of quantum simulation and sensing tasks.  Neutral atoms, ions and solid state qubits have all emerged as key platforms for inquiry.  One key set of questions concerns the ability of these delicately tailored quantum systems to relax to equilibrium when they are isolated from the environment, and whether such dynamics might have universal features.  Research in our laboratory on magnetic quantum fluids comprised of spin-1 Bose-Einstein condensed atoms (BECs) has a remarkable potential to address this problem.  In this talk I will show data from our lab demonstrating the rich interplay between many actors--magnetic interactions between spins, the influence of external magnetic fields, and the spatial quantum dynamics of many interacting modes that all compete to determine the non-equilibrium behavior.

 

Dr. Raman Biosketch:  Dr Chandra Raman is Associate Professor in the School of Physics at Georgia Tech where he performs experimental research on ultracold atomic gases and builds miniature atomic systems for quantum sensing applications. His work aims to understand the basic physics of complex quantum systems to harness them for applications. His group at Georgia Tech has uncovered new properties of quantized vortices, spin textures and quantum phase transitions in ultracold Bose gases, work for which he was awarded Fellowship in the American Physical Society in 2013. From 2013-15 he took a leave of absence to work in industry to better understand real world atomic sensors, work which he has translated into his laboratory today. 

Ultracold plasmas provide us an opportunity to study exotic plasma regimes on a table-top laboratory scale. In particular, we can explore parameter spaces where strong coupling and electron magnetization effects play an important role like in some fusion and astrophysical systems. We have developed a new technique to measure electron-ion collision rates in ultracold plasmas using off-resonant RF heating of the electrons. By using the known variation in photoionization energy with photoionization laser wavelength and applying controlled sequences of electric fields, the amount of heating imparted can be calibrated and precisely measured. This allows the comparison of electron-ion collision rates as a function of plasma parameters such as electron temperature/degree of strong coupling and magnetization. A description of this technique and the experimental results obtained with it will be presented.

Atomic, molecular, and optical systems often exhibit long-range interactions, which decay with distance r as a power law 1/r^alpha. In this talk, we will derive bounds on how quickly quantum information can propagate in such systems. We will then discuss applications of these bounds to numerous phenomena including classical and quantum simulation of quantum systems, prethermal phases in Floquet systems, entanglement area laws, sampling complexity, and scrambling.

We address a central problem in the creation and manipulation of quantum states: how to build topological quantum spin liquids with physically accessible interactions. Theorists have been studying models of quantum spin liquids that rely on "multi-spin" interactions since the 1970s, and, more recently, have realized that these models can be used for quantum computing. However, nature does not provide such interactions in real materials. We construct a lattice gauge model where the required, fully quantum, multi-spin interactions can in fact be emulated exactly in any system with only two-body Ising interactions plus a uniform transverse field. The latter systems do exist. Therefore, our solution is an alternative path to building a workable topological quantum computer within existing hardware.  Our bottom-up construction is generalizable to other  gauge-like  theories,  including  those  with  fractonic  topological  order  such  as the  X-cube model. Taken as a whole, our approach is a blueprint to emulate topologically ordered quantum spin liquids in programmable quantum machines.

I will discuss the cold atom gravimetry program at ANU. I will talk about our program in field deployed devices in broad terms and programs we are pursuing in squeezing and advanced detection methods.. I will also discuss our program to model and design fit for purpose cold atom gravimeters and accelerometers  for a variety of applications in Earth science, mineral exploration, hydrology and inertial navigation.

 

A Sagnac interferometer using Bose-Einstein condensates for rotation sensing is implemented in a harmonic trapping magnetic potential. The trapped cold atom cloud is manipulated by standing wave laser beams to produce two reciprocal interferometers. They provide common-mode rejection of accelerations, trap fluctuations and other noise sources while the Sagnac phase is differential between two interferometers. An image processing program is being developed to quickly extract positions and sizes of atom packets from their trajectories. Besides, a new atom chip is designed and constructed based on double layer spiral copper wires with different chirality. A supporting chamber for testing the atom chip is also designed and used to adjust trapping frequencies and allow laser beams coming through. The ultimate goal is to realize a compact and portable microchip-based atom gyroscope for rotation sensing and inertial navigation.

We employ a variational approach by optimizing the free energy of an anharmonic Hamiltonian with respect to strain tensor, interatomic coordinates and force constants in an interacting electron-phonon system. The goal is to predict possible phase transitions in crystal structures at finite temperatures. The variational method is based on Bogoliubov inequality to get an approximation to the Helmholtz free energy in a lattice with anharmonic potential energy terms. A harmonic trial Hamiltonian is used for the minimization. The optimization will give the set of equations corresponding to atomic displacements, lattice strain, IFCs and other order parameters, leading to phonon frequencies at each k-point for every temperature. The reliability of the approach is then checked in 1D/3D cases, comparing to available computational/experimental results and by applying DFT method to compute free energies of various phases at different temperatures.

In this talk, I will first give a short introduction about classical computers and quantum computers. I will also introduce cluster states, which are served as the calculating bases of one-way quantum computing. On top of that, I will talk about how we build the cluster states in our lab. Previously, our lab measured 60 qumodes which are simultaneously accessible, but we believe we should have far more than 60. I will explain why we thought we should have thousands accessible qumodes, what hinders us to get more than 60 qumodes, and a feasible way to overcome the difficulties. 

Quantum systems are fragile. Inhomogeneities in a sample’s environment can destroy its macroscopic coherence properties, while the coupling of components of a system to unmeasured/uncontrolled environmental degrees of freedom leads to microscopic decoherence. In this talk, I will discuss examples of my work related to macroscopic and microscopic coherence in cold atom ensembles, as well as possible approaches to preserving coherence. In addition, I will discuss a study of topological effects in atomic systems.

Polarized Nuclear Imaging (PNI) is a novel modality in which images of certain radioactive tracers are formed using conventional Magnetic Resonance Imaging (MRI) techniques by detecting asymmetries in gamma ray emission rates with respect to the nuclear magnetic moments of the tracer.  This modality combines the spatial resolution and contrast provided by MRI with the detection sensitivity of nuclear imaging, allowing for the production of an image using many orders of magnitude fewer nuclei than would be necessary with conventional MRI.  However, many challenges remain in bringing PNI from the laboratory to practice in a clinical setting.  For example, the first PNI image produced took 60 hours to acquire.  In my talk I will describe some novel techniques in currently in development intended to bridge the gap between laboratory and clinic.

The core electrons make a significant contribution to the total electric polarizability a of many-electron atoms like Rb. If the core contribution can be determined accurately, the remaining valence contribution to a provides constraints on the wave function and matrix elements of the valence electron. This can be useful for interpreting experiments such as parity violation or radiation shifts in atomic clocks. We report here on a measurement of the core polarizability based on radio-frequency spectroscopy of Rydberg states with large angular momentum. Preliminary results are 9.07 ± 0.01 a.u. for the dipole polarizability ad and 18.3 ± 0.5 a.u. for the quadrupole polarizability aq. These preliminary results are consistent with previous measurements, and uncertainties are reduced by approximately a factor of 4. The dipole polarizability is consistent with high-precision theoretical calculations, but a large discrepancy between theory and experiment persists for the quadrupole value.

Transport properties provide an important tool to characterize many-body systems. In particular, measurements of spin transport in strongly interacting Fermi gases can help to resolve the debate regarding the existence of a pseudogap--a pairing gap above the superfluid critical temperature--in the unitary Fermi gas. Studies of universal bounds on transport coefficients further motivate interest in spin transport in the unitary Fermi gas, which is expected to exhibit timescales approaching the "Planckian" limit set by the temperature, Boltzmann constant, and Planck's constant. I will describe proposed experiments to measure the spin transport coefficients in Fermi gases at low temperatures that can address these questions. Our experimental approach utilizes a homogeneous Fermi gas separated into three regions: a sample and two reservoirs. Non-equilibrium initial conditions in the reservoirs will drive a spin current through the sample, enabling measurements of the spin diffusivity. Our experimental approach can be extended to measurements of heat transport and non-equilibrium states. 

We describe the implementation of a dual Sagnac interferometer using a Bose-Einstein condensate confined in a harmonic time-orbiting potential magnetic trap, which is sensitive to rotations on the order of Earth’s rate.  Atoms are manipulated using Bragg laser beams to produce two reciprocal interferometers, providing common-mode rejection of accelerations, trap fluctuations, and other noise sources. The Sagnac rotation phase is differential between the two interferometers. The orbit of the atoms is nearly circular, with an effective Sagnac area of about 0.5 mm^2.  This technique has potential applications in terrestrial and space-based inertial navigation systems, which currently use more unstable and less rotation-sensitive optical gyroscopes.

I will give an overview of the emerging field of rare-earth atoms in solids as the basis for a variety of quantum information applications. These systems have a number of advantageous properties including long inherent coherence times, lack of motional dephasing or substantial spectral diffusion, and high density, that make them promising systems for important quantum information tasks, such as long-lived, efficient photonic quantum memory. A major challenge associated with most atom-like quantum emitters in solids, rare-earth atoms included, is the inhomogeneous broadening of the optical transition energy caused by site-to-site variation in the local environment. I will discuss initial experimental results on the effect of this broadening on electromagnetically induced transparency in a europium doped sample. Finally I will present our plans and ongoing work to mitigate the effects of inhomogeneity by investigating a new class of materials.

Strong laser field interaction with materials is rich in physics and chemistry, and gives rise to variety of spectacular phenomena ranging from multiphoton/tunneling ionization to high harmonics generation. In this talk, I will present three of these intriguing phenomena that I have investigated.

First, I will explain neutral dissociation of hydrogen molecule in strong laser field through multiphoton super-excitation. I will demonstrate the experimental results of fragmentation of hydrogen molecules in a strong laser field including observation of Balmer lines from hydrogen atoms and measuring the upper limit of the lifetime of the super-excited states by an ultrafast pump and probe experiment [1].

The second part of the talk is dedicated to optical gain and population inversion in ions at 428 nm wavelength through high-resolution spectroscopy. I will clarify how sufficient dissimilarity of rotational distributions in the upper and lower emission levels could lead to gain without net electronic or vibronic population inversion [2].

Finally, at the third part of the talk, I will show the results of use of femtosecond laser pulses to melt indium semi-spherical nanostructure (r~175 nm) and shape them by high spatial frequency laser induced periodic surface structures into linear microstructures of 2 μm long in the direction of laser polarization. The understanding of the modification process, melting and moving in the nano-grating structured field, pave the way to design nanostructures of arbitrary shapes at the sub-wavelength scale [3].

[1] A. Azarm, D. Song, K. Liu et al. J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 085601

[2] A. Azarm, P. Corkum, P. Polynkin, Phys. Rev. A Rapid Comm. 96 (2017) 051401(R)

[3] A. Azarm, F. Akhoundi, R. A. Norwood et al. Appl. Phys. Lett. 113 (2018) 033103

We investigate the transverse mode structure of the down-converted beams generated by a type-II optical parametric oscillator (OPO) driven by a structured pump. Our analysis focus on the selection rules imposed by the spatial overlap between the transverse modes of the three fields involved in the non-linear interaction. These rules imply a hierarchy of oscillation thresholds that determine the possible transverse modes generated by the OPO, as remarkably confirmed with experimental results.

At Tanya Zelevinsky’s lab at Columbia, our current effort focuses on characterizing the strontium molecule with the goal to develop an ultra-precise molecular clock---similar to better-known atomic optical clocks---with unique sensitivity to the fundamental constants of nature such as the gravitational constant G and the electron-to-proton mass ratio. Through precision measurements, one may investigate fundamental problems that are otherwise studied in high-energy (accelerator) research and astrophysical observations.

The implementation of a molecular clock relies on detailed knowledge of the Sr₂ molecule. Studies of photodissociation, combined with spectroscopic data, have helped develop a state-of-the-art quantum chemistry model. The predictive value of the model is tested against experimental photodissociation data with remarkable complexity. The model faithfully reproduces the photofragment distributions and helps illuminate a quantum-to-classical crossover in dissociation dynamics.

We have demonstrated the operation of a molecular clock by coherently transferring molecules from a shallow bound state to near the bottom of the molecular potential. Using a magic wavelength technique, we have improved transition quality by 3.5 orders of magnitude, projecting a clock accuracy better than 10^-14.

Spin-specific trapping and mechanical control of ultracold atoms is difficult with current techniques, but offers the possibility of exploring new physics systems, notably spin-dependent trapped atom interferometers, as well as quantum gates, 1D many-body spin gases, and novel cooling schemes. Microwave near-field potentials based on the AC Zeeman effect provide a mechanism for such spin-specific control of atoms: in principle, independent potentials can be targeted to different spin states simultaneously. We present recent experimental progress in implementing such control by using AC near-fields on an atom chip to drive hyperfine transitions and manipulate ultracold rubidium atoms.

CaH⁺ is an astrophysically relevant molecule with proposed applications in fundamental physics. We use CaH⁺ co-trapped with Doppler cooled Ca⁺ to perform spectroscopy using two-photon photodissociation with a frequency doubled, mode locked Ti:sapph laser. This method was used to measure the vibronic spectrum of the 1¹ Σ, v = 0  2¹ Σ, v' = 0, 1, 2, 3 transitions. Spectroscopy on the deuterated isotopologue, CaD⁺ confirmed a revised assignment of the CaH⁺ vibronic levels and a disagreement with MS-CASPT2 theoretical calculations by approximately 700 cm^-1. Updated high-level coupled-cluster calculations that include core-valence correlations reduce the disagreement between theory and experiment to 300 cm^-1.  The broad bandwidth of the pulsed Ti:sapph provided an advantage for the initial search for transitions, but did not allow spectral resolution of rotational transitions. Pulse shaping was applied to spectrally narrow the linewidth of the pulsed laser to obtain rotational constants for the 2¹ Σ, v' = 0, 1, 2, 3 and 1¹ Σ, v = 0 states. This measurement has value in the control of quantum states of the molecule for high precision measurements of rovibrational transitions using quantum logic. Molecule-cold atom collisions for possible buffer gas cooling can also be tested using this method.

Quantum state preparation and characterization are essential to emerging near-term quantum technological applications. In particular, single-photon Fock states are of interest as their exciting applications in linear optical quantum computing, quantum internet, quantum communication, quantum sensing, and quantum imaging.

In this work we prepare a single-photon Fock state by heralding two-mode spontaneous parametric down conversion in a PPKTP based optical parametric oscillator (OPO). We then reconstruct the Wigner quasi-probability distribution by Photon-Number-Resolving (PNR) based quantum state tomography. This method circumvents the need for numerical tomographic reconstruction of the state by inverse Radon transform with the balanced homodyne detection method. We perform PNR measurements using transition edge sensor which can resolve up to five photons at 1064 nm. Here we report our recent results of reconstructed negative Wigner function with a final detection efficiency of 56 %.

Towards the end of my talk, I will discuss a method known as Fock state filtering to generate non-classical states using single-photon states, linear optics and PNR resolving measurements.

We describe progress towards a rotation sensor using ultracold atoms.  A Bose-Einstein condensate is loaded into weak cylindrically symmetric harmonic trap.  The condensate is split and recombined using off-resonant Bragg laser pulses.  After the clouds are split, they oscillate in the trap.  By analyzing the trajectories of the clouds we can optimize the trap such that the frequency is equal along different axes.  Next, we split the condensate twice, creating four clouds that traverse a circular path around the trap center.  After completing an integer number of orbits the clouds can be recombined, forming two reciprocal interferometers whose phase difference is sensitive to rotations but rejects other common-mode noise sources.  We have observed closed circular trajectories with a diameter of 0.6 mm, corresponding to a Sagnac phase of 1500 seconds times the rotation rate, or about 0.1 rad for an Earth-rate rotation.

When a material is exposed to an intense laser field, the absence of inversion symmetry at the surface can result in the formation  of a non-linear surface polarization and surface second harmonic (SSH) emission. We find that the SSH yield from a metal can be dramatically influenced by the presence of an additional THz field. In the experiments, collinear 100fs 780nm laser and 2ps single-cycle THz beams are focused at grazing incidence along a gold surface. The SSH yield is measured as a function of the THz intensity, relative laser-THz delay, and laser/THz polarizations relative to the surface normal. The yield from an optically flat gold mirror increases by as much as a factor of three in the presence of a 20kV/cm THz eld. Interestingly, the SSH enhancement for the same THz field is as large as a factor of 27 if the gold mirror is replaced by a gold-coated diffraction grating, apparently due to either a local THz field enhancement or increased sensitivity of the non-linear polarization to the THz field near grating micro-structures. We are exploring the use of THz-enhanced SSH emission to characterize the THz-field enhancement and/or response of micro-structured metal surfaces with other geometries.

The use of cold atoms has led to a substantial increase in the accuracy achievable in many atomic physics measurements. This has most notably been demonstrated in the atomic clock relying on the interference of internal states of weakly interacting atoms in free fall. However, it has also led to an additional layer of experimental complexity which, combined with the physical size of state-of-the-art setups, impose significant limitations on wider practical applications. Progress will be reported on the development of a compact atomic clock based on cold atoms. 

Unprecedented precision has also been demonstrated in atom interferometers relying on the  detection of differential phase shifts between atomic wavefunctions of e.g. different motional states. Sensitivity to external interactions results in a shift of the atomic phase relative to a lab-frame reference, typically the spatial phase of an optical standing wave. This is a limitation to practical measurements as it requires long temporal stability and has motivated the investigation of an atom interferomenter inherently insensitive to the phase noise of the readout system. This relies on an atomic homodyne detection allowing the entire interferometric signal to be read out in a single shot.

Quantum computing harnesses purely non-classical features of quantum physics to perform computations that would be otherwise infeasible on a traditional (classical) computer. Highly squeezed states are a crucial resource for many quantum technologies, primarily fault tolerant quantum computing. As with any quantum resource, squeezing is very fragile and arduous to generate experimentally. Because of this, the 20.5 dB squeezing level germane to the fault tolerance threshold (for an error rate of 0.00001) of continuous variable (CV) quantum computing has yet to be obtained, despite recent progress. I propose an experimental method designed to breach this threshold by unitarily redistributing multitudinous-mode squeezing into a highly squeezed single qumode (the CV analog of a qubit, or quantum bit). This new paradigm utilizes multi-mode states as a squeezing resource, by effectively transferring small levels of squeezing per qumode over N modes into a single qumode with N-times the squeezing.

In this talk, I'll report on a systematic experimental and numerical study of the electron temperature in ultra-cold plasmas which evolve from samples of cold Rydberg atoms. Specifically, we have measured the asymptotic expansion velocities of ultra-cold plasmas (UNPs) which evolve from cold, dense, samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the initial plasma electron temperature, as a function of the original Rydberg atom density and binding energy. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that, for n > 40, the electron temperature in the Rydberg plasma is insensitive of the initial ionization mechanism which seeds the plasma. Instead, it is determined principally by the plasma environment when the UNP decouples from the Rydberg atoms at the end of the avalanche regime, and this occurs when the plasma electrons are too cold to ionize the remaining Rydberg population. On the other hand, plasmas from Rydberg samples with n < 40 evolve in a different manner. There is very little additional ionization after the plasma reaches threshold as the electrons in the plasma have insufficient energy to ionize the parent atoms. Consequently, the only significant interaction between the plasma and the parent atoms causes the Rydberg atoms to be de-excited, and the electron temperature equilibrates at a fraction of the initial Rydberg atom binding energy.

Highly efficient single photon detectors are ubiquitous in quantum optics and atomic physics. However, many of the theories of single photon detection still arise from the Glauber theory of photodetection which was primarily developed for inefficient detectors (weak light-matter coupling). The first goal of the talk is to contrast the photon counting mechanism in photomultiplier tubes (PMTs), single photon avalanche diodes (SPADs), single electron transistor based photodetectors (SET-PDs) and superconducting nanowire single photon detectors (SNSPDs). This can help develop a general model for single photon detection beyond Glauber's theory. 

Secondly, we will present experimental results on time-correlated single photon counting experiments that demonstrate long range dipole-dipole interactions between quantum emitters mediated by metamaterials.   We will discuss a fundamental limit to the efficiency of energy transfer between quantum emitters and discuss routes to achieve this limit through induced coherence. Our approach to engineering dipole-dipole interactions can motivate experiments from atomic systems (eg: Rydberg blockade) to biochemistry (Forster/Dexter resonance energy transfer).

An upcoming measurement at Jefferson Laboratory (JLab) of the electric form factor of the neutron (GEn) will utilize a polarized 3He target at high luminosity. While polarized 3He targets at JLab have previously been made entirely of glass, we describe progress toward six liter, convection style target cells incorporating metal windows for the electron beam. We have found good performance by using Oxygen Free High Conductivity (OFHC) copper substrates electroplated with gold. We have further established that Uranium glass (Canary glass) has excellent spin-relaxation properties, and can serve as a transition glass from Pyrex to Aluminosilicate glass (GE180). We also present polarimetry results of our first production, “Stage 1”, all-glass target cell that is to be used in the measurement of the virtual photon spin asymmetry of the neutron (A1n) at JLab. Further, we present preliminary results of a custom built, low noise NMR system which will be used in the precision measurement of the atomic parameter k0, which is important for the accurate understanding of the 3He polarization and which characterizes the 3He-Rb system.

Continuous-variable (CV) approaches to quantum computing have certain advantages over standard qubit-based approaches. The most striking of these is the ability to make extremely large resource states for measurement-based quantum computing using small optical setups. Furthermore, local measurements on one of these states will transform it into a CV version of a topological quantum code, which has potential applications in condensed-matter theory, anonymous broadcasting, and quantum error correction. My talk will discuss the latest theoretical and experimental developments in this research area.

https://www.college-de-france.fr/site/jean-dalibard/guestlecturer-2015-2016.htm

The ability of electron cryo-microscopy (cryoEM) to rapidly and routinely determine the atomic structures for biological complexes has transformed the interface of chemistry and biology. Previously, determining structures at this resolution was only ossible by X-ray crystallography and NMR spectroscopy, which could be slow, difficult, and for many complexes impossible.

Over the last few years the EM database shows a dramatic increase in the number of cryoEM maps at a resolution higher than 4 Å:

Year   # 3D maps
2012        4
2013        7
2014       36
2015     114
2016     167 (as of Oct. 7th)

Dr. Chiu has been a major contributor to this "resolution revolution,” and his presentation is entitled "CryoEM of Molecular Machines at Atomic Resolution.”

Dr. Chiu received his BA in Physics (1969) and PhD in Biophysics (1975) from the University of California, Berkeley. He is the Alvin Romansky Professor of Biochemistry and the Distinguished Service Professor at Baylor College of Medicine in Houston, Texas. He is a pioneer in methodology development for electron cryo-microscopy. His work has made multiple transformational contributions in developing single particle electron cryo-microscopy as a tool for the structural determination of molecular machines towards atomic resolution.

For three decades, Dr. Chiu has directed an NIH funded 3DEM Resource Center. He has solved many cryoEM structures including viruses, chaperonins, membrane channels, cytoskeleton protein complexes, protein-DNA complexes and RNA complexes in collaboration with many scientists around the world. His 3DEM Resource Center continues to establish high standard testing and characterization protocols for cryoEM instrumentation and to develop new image processing and modeling algorithms for cryoEM structure determination. 

Dr. Chiu is the co-founder of the W.M. Keck Center for Computational Biology and the graduate program in Structural and Computational Biology and Molecular Biophysics in the early 1990s. These cross-disciplinary and cross-institutional programs involve hundreds of faculty from 7 academic institutions in the Greater Houston Area and have trained many eminent scientists fluent in quantitative biomedicine.

Dr. Chiu’s research, collaboration and training efforts have been recognized by his elected membership to the Academia Sinica, Taiwan (2008) and the United States National Academy of Sciences (2012).  Other honors include the Distinguished Science Award from the Microscopy Society of America (2014) and an Honorary Doctorate of Philosophy from the University of Helsinki, Finland (2014).

Dr. Chiu's visit is sponsored and hosted by the
     - UVa Program in Biophysics (Dr. Robert Nakamoto)
     - Department of Molecular Physiology and Biological Physics (Drs. Wladek Minor, Zygmunt Derewenda and Mark Yeager)
     -Department of Biochemistry and Molecular Genetics (Drs. Ed Egelman and Anindya Dutta).

This seminar will present an optical atomic clock based on a two-photon transition at 778 nm in rubidium that could be made small and robust enough to be used outside of a laboratory environment. We will cover the clocks principals of operation, fundamental limitations and data supported current status. We will show that this system is fundamentally capable of besting a hydrogen MASER in frequency stability and size.

Two cases of strong manifestation of electron correlation upon innervalence photoionization of rare gas atoms with syncrhtron radiation will be discussed.
1. Photoelectron recapture due to post-collision interactions between Auger electrons and photo electrons.  In our new high resolution Auger electron spectroscopy results, the Rydberg series structure of ionic final states appear in the structure of the Auger peak.  Rich structure due to angular momentum states were resolved and angular distribution information was obtained for the first time.
2. We demonstrated that fluorescence lifetime measurements upon innervalence excitation can be very sensitive to electron correlation and provide information regarding configuration interaction that cannot otherwise be obtained

Strong-field ionization (SFI) and the resulting electronic dynamics are important for a range of processes such as high harmonic generation, photodamage, and ionization-triggered charge migration. Modeling ionization dynamics in molecular systems from first-principles can be challenging due to the large spatial extent of the wavefunction which stresses validity basis sets, and the intense fields which require non-perturbative electronic structure methods. In this talk I present recent developments towards extending time-dependent density functional theory (TDDFT) for SFI using atom-centered basis sets and tuned range-separated hybrid DFT functionals. Unlike traditional TDDFT, this approach has the correct long-range Coulomb potential and reduced self-interaction errors.  The resulting laser intensity, angle, and molecular orbital-dependent ionization rates for N2 and iodoacetylene show good agreement with experimental values. This method opens the door to predictive simulations of ionization and ionization-triggered dynamics in large molecular systems without inputs from experiment. For more details see: Sissay et al, J. Chem. Phys. 145, 094105 (2016).

While small isolated quantum systems undergo well-defined wave packet dynamics, the observables in very large, locally isolated quantum systems generally relax to states of maximum entropy. To explain this, the eigenstate thermalization hypothesis (ETH) holds that the unitary dynamics of arbitrary superpositions yield equilibrium expectation values as a time-average [1, 2]. Thus, in this picture – despite the deterministic nature of the Schro¨dinger equation and the absence of outside perturbations – an arbitrarily prepared isolated quantum system relaxes to a thermal equilibrium that is somehow hardwired in its eigenstates. Indeed, unimolecular rate theory depends on energy randomization, and quantum systems as small as three transmon qubits exhibit ergodic dynamics. [3].

 

But, theory predicts the existence of certain interacting many-body systems that lack intrinsic decoherence and preserve topological order in highly excited states. These systems exhibit local observables that retain a memory of initial conditions for arbitrarily long times. Such behaviour has important practical and fundamental implications. For this reason, experimental realizations of isolated quantum systems that fail to thermalize have attracted a great deal of interest [4, 5].

 

Here we describe particular conditions under which an ultracold plasma evolves from a molecular Rydberg gas of nitric oxide, adiabatically sequesters energy in a reservoir of mass transport, and relaxes to form a spatially correlated strongly coupled plasma. Short-time electron spectroscopy provides evidence for complete ionization. The long lifetime of the system, particularly its stability with respect to recombination and neutral dissociation, suggest a robust process of self-organization to reach a state of arrested relaxation, far from thermal equilibrium.

 

[1] Rigol M, Dunjko V, Olshanii M: Thermalization and its mechanism for generic isolated quantum systems. Nature

2008,  452(7189):854–858.

[2] Eisert J, Friesdorf M, Gogolin C: Quantum many-body systems out of equilibrium. Nature Physics 2015, 11(2):124– 130.

[3] Neill C, et al: Ergodic dynamics and thermalization in an isolated quantum system. arXiv: 2016, 1601.00600v2. [4] Kondov SS, McGehee WR, Xu W, DeMarco B: Disorder-Induced Localization in a Strongly Correlated Atomic

Hubbard Gas. Phys Rev Lett 2015, 114(8):083002.

[5] Schreiber M, Hodgman SS, Bordia P, Lu¨schen HP, Fischer MH, Vosk R, Altman E, Schneider U, Bloch I: Observation of many-body localization of interacting fermions in a quasi-random optical lattice. Science 2015, 349:842–845.

The outcome of an experiment should not depend on the orientation of the apparatus in space. This important cornerstone of physics is deeply engrained into the Standard Model of Physics by requiring that all fields must be Lorentz invariant. However, it is well-known that the Standard Model is incomplete. Some theories conjecture that at the Planck scale Lorentz symmetry might be broken and measurable at experimentally accessible energy scales. Therefore, a search for violation of Lorentz symmetry directly probes physics beyond the Standard model. We present a novel experiment utilizing trapped calcium ions as a direct probe of Lorentz-violation in the electron-photon sector. We monitor the energy between atomic states with different orientations of the electronic wave-functions as they rotate together with the motion of the Earth. This is analogous to the famous Michelson-Morley experiment. To remove magnetic field noise, we perform the experiment with the ions prepared in the decoherence-free states. Our result improves on the most stringent bounds on Lorentz symmetry for electrons by 100 times. The experimental scheme is readily applicable to many ion species, hence opening up paths toward much improved test of Lorentz symmetry in the future.

http://www.phys.virginia.edu/Announcements/talk-list.asp?GID=speakers_committee&PERIOD=this%20term

Symmetry-protected topological phases have gapless surface (edge) states. These states are robust against any single-body perturbation that has the symmetry, as long as the gap in the bulk spectrum is not closed. Recently, it is found that these gapless surface states can be gapped when many-body interaction is included. In this talk, I will discuss what kind of many-body interaction opens a gap in the surface of topological superconductor without breaking its symmetry.

Time reversal symmetric topological superconductor carries gapless Majorana fermion on the surface. I will discuss the “Coupled Wire Model” which we built to mimic the massless Majoranas in the surface. I will talk about how the explicit many-body inter-wire interaction, that we introduced, preserves time reversal symmetry but opens a gap in the surface spectrum. I will also discuss the resulting gapped surface carrying non-trivial topological order evident from the anyon structure that we find. 

 

Entanglement of bright beams of light is a useful resource for applications in information protocols. By using continuous variables, much in the spirit of the original analysis by Einstein, Podolsky, and Rosen, it is possible to perform deterministic tasks and also to use detection tools available to classical communications systems.  We have concentrated our efforts on nonlinear optical processes using bulk crystals inside optical cavities.  The cavity bandwidth sets a maximum repetition rate for information processing and communications.  An interesting pathway consists in investigating similar effects in miniaturized cavities on photonic chips, with the additional benefit of compatibility with micro-electronics.  In this talk, I will present an overview of our research on the generation of continuous-variable entanglement in bulk nonlinear crystals, as well as nonclassical light generated in silicon chips. 

Rydberg Atoms have pronounced dipole moments and dipole – dipole interactions lead to the pairing up of Rydberg atoms to form transient diatomic Rydberg molecules under special conditions. Resonant microwave transitions from pairs of Rb atoms have been observed and a configuration interaction model can be used to understand this phenomena. 

Amorphous ferrimagnetic TbFeCo and TbSmFeCo thin films are found to exhibit exchange bias and bi-stable magneto-resistance states near compensation temperature. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive spectroscopy mapping reveal two nanoscale amorphous phases with different Tb concentrations distributed within the amorphous films. The observed exchange anisotropy originates from the exchange interaction between the two nanoscale amorphous phases. The micromagnetic model is adopted to study this heterogeneous magnetic material with two interpenetrating nanoscale phases. This study can serve as a platform for developing exchange bias materials with ferrimagnet.

Indirect measurements, of which non-demolition measurements is a specific subclass, describe a physical situation in which an information about a quantum system is acquired through a direct measurement on a sequence of probes subsequently interacting with the system. Recent interest in the field originates in the photon counting experiments of Haroche. Mathematically the problem is equivalent to the study of statistics of long products of completely positive maps. I describe this mathematical theory with a special focus on the non-demolition subclass -- this is the case when all the map commutes -- and its small perturbation.

 

I will report a measurement of a light wavelength at which the ac electric polarizability equals zero for Rb87 atoms in the F=2 ground hyperfine state. The experiment uses a condensate interferometer to find this “tune-out” wavelength for the scalar polarizability, which lies at 790.032388(32) nm. This result can be used to determine the ratio of matrix elements R = 1.99221(3), a 100-fold improvement over previous experimental values. I will discuss techniques for accurate determination and control of light polarization as well as progress on measurements of the vector polarizability between the D1 and D2 spectral lines. Measurements of tune-out wavelengths and the vector polarizability between multiple lines allows separation of individual contributions to the polarizability from higher-lying states and the core up to ratios of matrix elements. Accurate knowledge of these ratios should serve useful as a theoretical benchmark and in atomic parity violation experiments.

The strong long-range interaction between ultracold Rydberg atoms gives rise to a number of interesting phenomena that have been studied in recent years including resonant energy transfer collisions, many-body quantum simulations, quantum information processing, and ultracold plasmas. The dipole-dipole interaction between a pair of Rydberg atoms can result in a state-changing interaction if the energy defect for the process is small. The collisional energy transfer process can be tuned into resonance via the Stark effect. Such resonances are known as Förster resonances. In this talk, we will discuss a recent study of the time dependence of resonant energy transfer process and of a catalysis effect in the resonant energy transfer between ultracold 85Rb Rydberg atoms. We have investigated the energy transfer process of 34p + 34p → 34s + 35s, and observed Stark-tuned Förster resonances. When additional Rydberg atoms of 34d state are included in the interaction, an increase in the population of 34s states atoms was observed. Although the 34d state atoms do not directly participate in the resonant energy transfer process that produces 34s state atoms (shown above), they add an additional interaction channel 34p + 34d → 34d + 34p that is resonant for all electric fields and results in a change in the rate in which 34s atoms are produced. We will present our experimental results and compare them with model simulations. 

[1]          R. Schmeissner, J. Roslund, C. Fabre, and N. Treps, 113, 263906 (2014).

[2]          P. Jian, O. Pinel, C. Fabre, B. Lamine, and N. Treps, Opt Express 20, 27133 (2012).

[3]          J. Roslund, R. M. De Araujo, S. Jiang, and C. Fabre, Nature Photonics 8, 109 (2014).

[4]          S. Gerke, J. Sperling, W. Vogel, Y. Cai, J. Roslund, N. Treps, and C. Fabre, 114, 050501 (2015).

[5]          G. Ferrini, J. P. Gazeau, T. Coudreau, C. Fabre, and N. Treps, New J Phys 15, 093015 (2013).

 

 

 

For more information about Prof. Sevag Gharibian please see the following link:

http://computer-science.egr.vcu.edu/faculty/sevag-gharibian/

A central direction of research in condensed matter physics is the determination of properties of local Hamiltonian systems. Unfortunately, computing such properties is often intractable, in that the complex matrices involved are typically of exponential size. Over the last 15 years, a burgeoning area of research at the intersection of condensed matter physics and computational complexity theory, known as Quantum Hamiltonian Complexity, has made significant strides in characterizing the complexity of such computational tasks. In this talk, we begin with a gentle introduction to the field of Quantum Hamiltonian Complexity. We then show that a central and basic task in condensed matter physics, computing the expected value of a local observable against the ground state of a local Hamiltonian, is an intractable task in the worst case (assuming standard complexity theoretic conjectures), even if the observable acts on just a single qubit.

This talk is based on joint work with Xiaodi Wu (U Oregon) and Justin Yirka (VCU).

 

Reserved for Chemistry Seminar

The combination of the polarization and spatial photonic degrees of freedom open interesting possibilities in the quantum optics and quantum-information domains. The ability to produce and transform beams carrying orbital angular momentum – the optical vortices - has allowed the development of important techniques with potential applications to quantum information. In this seminar I will present some optical devices to quantum information and I will talk about the topological phase, associated with the double connectedness of the SO [3] representation in terms of maximally entangled states. Under local unitary operations on their polarization and transverse degrees of freedom, the vector vortices can only acquire discrete geometric phase values, 0 or π, associated with closed paths belonging to different homotopy classes on the SO(3) manifold. As the meaning results I will show that the topological phase can be evidenced through interferometric measurements and that a quantitative relationship between the concurrence and the fringes visibility can be derived as well.

[1] C. E. R. Souza, J. A. O. Huguenin and A. Z. Khoury Topological phase structure of vector vortex beams, JOSA-A 31, No. 5 1007 (2014).

[2] C. E. R. Souza, J. A. O. Huguenin, P. Milman and A. Z. Khoury Topological Phase for Spin-Orbit Transformations on a Laser Beam PRL 99, 160401 (2007).

[3] C. E. R. Souza and A. Z. Khoury A Michelson controlled-not gate with a single-lens astigmatic mode converter Optics Express 18, No. 9 9207 (2010).

Broadband time-energy entangled photon pairs (bi-photons) from a narrowband pump are in many ways the 'black sheep of the family' in quantum information. Although they are very easily produced in large quantities and demonstrate extreme nonclassical behavior, broadband bi-photons are rarely used in quantum information, mainly because of the bandwidth limitations in detecting them with standard photo-detectors and homodyne techniques. We demonstrate complete measurement of the bi-photons wave-function (amplitude and phase) with near-unit efficiency, using a quantum interference between the generation amplitudes of bi-photons in two separated nonlinear media. I will describe experiments that employ this method with two different ultra-bright and ultra-broadband sources of bi-photons: one based on spontaneous down conversion (SPDC) in a nonlinear crystal, and the other on spontaneous four-waves mixing (FWM) in a nonlinear fiber. With SPDC we measure the quantum purity and the spectral phase of the bi-photon wave-function from the observed fringe pattern and its visibility. With the FWM source we explore the quantum-to-classical transition as pump intensity is varied, and observe quantum collapses and revivals of the interference contrast that are the signature of bi-photon generation with imaginary gain – a unique quantum regime of FWM.

1. Yaakov Shaked, Roey Pomerantz, Rafi Z. Vered and Avi Pe'er, "Observing the nonclassical nature of ultra-broadband bi-photons at ultrafast speed", New J. Phys. 16, 053012 (2014).

2. Rafi Vered, Yaakov Shaked, Michael Rosenbluh and Avi Pe'er, "The Classical-to-Quantum Transition with Broadband Four-Waves Mixing - Observing Bi-photon Generation with Real or Imaginary Gain", Submitted (2014)

3. Faina Shikerman and Avi Pe’er, "Sum-frequency generation as a detector of high-power two-mode squeezing", Phys. Rev. A. 88, 043808 (2013)

Atom traps promise great improvements on a wide range of technologies: Atom interferometry, gravimetry, magnetometry, navigation. Despite orders of magnitude in improvements promised, atom technology has seen limited commercial adoption due to voluminous instrumentation size. We have designed a new type of trap which reduces the apparatus volume by a factor of ten with satisfactory performance. Our design choices and component research will be discussed. We aim to develop a self-contained Rb vapor cell capable of sustaining a magneto-optical trap at a decent background pressure over a long period of time.

The quantum computer, whose information is encoded in "qubits” obeying quantum mechanics laws, will be able to perform some calculations exponentially faster than the classical computer whose information is encoded in "bits”. There are two principal models of quantum computing: the circuit model and the measurement-based model. The measurement-based model is crucially based on the cluster state, a type of highly entangled quantum state that serves as the resource and material for the whole calculation. This talk will discuss an original experimental work for the largest cluster state ever created whose modes (optical versions of qubits) are all available simultaneously. The entanglement proceeds from interfering multiple EPR pairs generated from a nonlinear crystal in an optical parametric oscillator, into a very long dual-rail wire cluster state. These highly scalable cluster states serve as building blocks of the universal quantum computer, and also are important resources for studying quantum mechanics in large systems.

The BosonSampling Problem is to sample output photon-coincidence probabilities given vacuum and single-photon inputs to a passive interferometer with more channels than photons. This problem is classically hard to simulate as these probabilities are weighted by computationally hard permanents of sub-matrices of the interferometer transition matrix yet efficient to execute quantumly. Our innovation [1,2] introduces distinguishability between photons by controlling arrival times of otherwise identical photons in order to test the model, assess sampling errors and generalize BosonSampling beyond permanent-weighted to immanant-weighted probabilities.
[1] S.-H. Tan, Y. Y. Gao, H. de Guise and B. C. Sanders, “SU(3) Quantum Interferometry with single-photon input pulses”, Physical Review Letters110 (11): 113603 (5 pp.), 12 March 2013, arXiv.org:1208.5677.
[2] H. de Guise, S.-H. Tan, I. P. Poulin and B. C. Sanders, “”Immanants for three-channel linear optical networks”, arXiv.org:1402.2391.

In this talk I will elaborate on what the Brout-Englert-Higgs mechanism is and how it leads to masses of the elementary particles in the Nature. I will also talk about what remains to be done on the theoretical front beyond the standard model of particle physics after the discovery of the Higgs boson at LHC and what our research group is doing in that direction. This will be a semi-technical talk understandable to graduate and undergraduate students.

Sponsored by Department of Physics and UVA Graduate Council

It is now widely known that the Brout-Englert-Higgs mechanism (popular by the name Higgs mechanism) leads to the masses of the elementary particles in Nature. But the answer to 'why do we need Higgs mechanism to give these masses?' is not well known to many outside the field of particle physics. Hence, in this talk I will try to explain answer to this question in a semi-technical way, which is understandable to graduate students having basic conceptual idea about quantum mechanics.

Dynamic holography using a spatial light modulator is a very versatile tool for creating arbitrary optical potentials by spatially shaping a laser beam. These potentials can be used for cold atoms manipulation. In order to produce accurate optical potentials by shaping a laser beam we need high quality holograms. Nevertheless SLMs suffer from defects which limit the quality. To measure defects we have developed a method based on polarimetry to get the birefringence map of the SLM. Birefringence mapping is suitable to monitor the hologram without disturbing the on-going experiment. It's an in-situ measurement. After the measurement, defects are corrected by a feedback on the input hologram.

As a demonstration, we used our SLM to produce Laguerre-Gaussian beams in order to guide over a long distance a cold rubidium beam supplied by a 2D-MOT. We have strongly reduced the divergence of this source and thus increasing by a factor of 200 the atomic density after a propagation of 30 cm. Furthermore we showed that in such a guide the spontaneous emission is lower than in the usual red detuned gaussian guide case.

This general talk will give an overview of the very successful Standard Model (SM) of Physics, and the reasons that make us believe there is physics beyond it that we haven't discovered yet. This is the motivation for the Q_weak experiment in Jefferson Lab, that uses the violation of the parity symmetry to probe the weak interaction and test the SM predictions with high precision. The first results from the experiment constitute the first-ever determination of the proton's weak charge, the neutral-weak analog to the electric charge. The experimental measurement and the first results will be presented.

Graduate students do not need to attend.

Cluster states are an entangled resource state that enable quantum computing using adaptive measurements alone. This is surprising when one considers what this means: one can quantum compute simply by *looking* at a quantum systems in a particular way! The continuous-variable incarnations of these states are simple to make using lasers and can be scaled up with ease. In this talk, I will describe the theoretical underpinnings of measurement-based quantum computation using continuous-variable systems, and I will report on their experimental realization, including the recent demonstration of a 10,000-mode (!) cluster state. Issues related to error correction and fault tolerance -- many of which remain open problems -- will also be discussed.

We report the excitation of metastable Li Rydberg atoms in the presence of a strong 38.3 GHz microwave field. We detect approximately 5% of the initial population in very high Rydberg states with n > 215 after the microwave pulse for a wide range of initial binding energies. The surviving population of atoms displays a periodic comb structure in energy with a periodicity matching the structure of the 38.3 GHz microwave field. A small static field displaces the entire comb to lower energy, and the high lying states disappear when the static field exceeds 30 mV/cm. We also perform measurements of microwave ionization thresholds in Li and, in spite of the fact that the pulse is 8000 cycles long, detect approximately 5% of the initial population in extremely high-lying states when the microwave pulse is subsequent to the laser excitation of a Rydberg state of any binding energy. We suggest that these atoms are trapped in metastable atom-field states during the microwave pulse and relax to the high-lying states when the field is turned off.