Atomic Physics Seminars History

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)

"Quantum spins in space: the rich phases of spinor Bose gases"

"Off-resonant RF Heating of Ultracold Plasmas to Measure Collision Rates "

"Dynamics of quantum systems with long-range interactions"

"Path to building quantum spin liquids and topological qubits within existing quantum hardware"