# Atomic Physics Seminars History

## "Angular Momentum Coherences in the Ultrafast Dynamics of Isolated Molecules"

**Professor Varun Makhija , University of Mary Washington**

[Host: Bob Jones]

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.

**Special Atomic Seminar**

Thursday, January 30, 2020

3:30 PM

Physics Building, Room 204

## "Squeezing-Enhanced Raman Spectroscopy with Ultra-Broadband Parametric Homodyne Measurement"

**Avi Pe'er , Bar-Ilan University**

[Host: Olivier Pfister]

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).

## "Dynamical phases and transitions in an ultracold Fermi gas"

**Scott Smale , University of Toronto**

[Host: Cass Sackett]

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.

**Atomic**

Monday, November 11, 2019

4:00 PM

Physics Building, Room 204

## "Quantum-classical hybrid algorithms with trapped ions"

**Norbert Linke , Joint Quantum Institute/University of Maryland**

[Host: Peter Schauss]

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"

**Raman Chandra , Georgia Tech**

[Host: Peter Schauss]

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.

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

**John Guthrie , Colorado State University**

[Host: Cass Sackett]

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.

## "Dynamics of quantum systems with long-range interactions"

**Alexey Gorshkov , Joint Quantum Institute/University of Maryland**

[Host: Peter Schuass]

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.

**Atomic**

Monday, October 21, 2019

4:00 PM

Physics Building, Room 204

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

**Dmitry Green , AppliedTQC.com, ResearchPULSE LLC**

[Host: Peter Schauss]

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.

## "Cold atom gravimetry with atom interferometry: advances in technology at ANU and progress towards applications."

**John Close , Australian National University**

[Host: Cass Sackett]

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.

## "Simulations and Designs of Atom Chip Apparatus for BEC Interferometry"

**Zhe Luo , University of Virginia - Physics**

[Host: Cass Sackett]

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.

**Atomic**

Monday, April 29, 2019

3:00 PM

Mechanical & Aerospace Engineering Building, Room 346

## "A variational approach for phase transition in interacting electron-phonon system"

**Yuan Liang , University of Virginia - Physics**

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.

## "Beyond the Large-Scale Cluster-State Entanglement in the Quantum Optical Frequency Comb – the feasible way from 60 accessible qumodes to thousands"

**Chun-Hung Chang , University of Virginia**

[Host: Olivier Pfister]

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.

## "Decoherence in quantum systems: Measurement and control"

**Chengxing He , University of Virginia - Physics**

[Host: Bob Jones]

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.

## "Advances in Polarized Nuclear Imaging"

**David Keder , University of Virginia - Physics**

[Host: Gordon Cates]

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.

## "Measuring Core Polarizability of Rubidium-87 using RF Spectroscopy of Rydberg States"

**Seth Berl , University of Virginia - Physics**

[Host: Cass Sackett]

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.

## "Spin Transport in Fermi Gases Across the Superfluid Transition"

**Ariel Sommer , Lehigh University**

[Host: Peter Schauss]

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.

## "A large-area Sagnac interferometer using atoms in a time-orbiting potential"

**Eddie Moan , University of Virginia - Physics**

[Host: Cass Sackett]

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.

## "Rare-earth atoms in solids as a platform for quantum networks "

**Elizabeth Goldschmidt , US Army Research Laboratory**

[Host: Peter Schauss]

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.

## "Ultrafast intense laser interaction with molecules and nanostructures"

**Ali Azarm , University of Arizona**

[Host: Bob Jones]

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

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