Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 67, Number 7
Monday–Friday, May 30–June 3 2022; Orlando, Florida
Session Q06: Open Quantum SystemsRecordings Available

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Chair: Ana AsenjoGarcia, U. of Columbia Room: Salon 1/2 
Thursday, June 2, 2022 8:00AM  8:12AM 
Q06.00001: Direct Observation of Quantum Backaction in a Bose Condensate Emine Altuntas, Ian Spielman The principal tenet of quantum technologies has two sides: quantum measurement and quantum control. We interpret the BoseEinstein condensate (BEC) imaging mechanism as a quantum measurement process in which the environment ``measured'' the scattered photons but this information was not passed on to the observer. BECs offer multiple minimally destructive imaging methods, which are weak measurement techniques that yield a controlled reservoir and consequently allow timeresolved study of the system evolution paving the way for realtime control of quantum gases. We employ phasecontrast imaging (PCI) technique — a nondestructive detection method — to acquire repeated measurements of the same BEC. In this measurement paradigm, by performing a pair of backtoback PCI measurements of the same BEC, we directly observe the quantum backaction of the first measurement as correlated excitations in the second measurement. To this end, I will describe our versatile highresolution ultracold atom microscope: a combined hardware/software system that recovers neardiffraction limited performance and minimizes the information loss. Our highfidelity digital correction technique reduces the contribution of photon shot noise to densitydensity correlation measurements which would otherwise contaminate the quantum projection noise signal in weak measurements [1]. I will discuss the experimental characterization of the measurement process via the PCI technique and the measurement induced overall heating of the condensate. [1] E. Altuntas, and I. B. Spielman, Physical Review Research, 3, 043087 (2021). 
Thursday, June 2, 2022 8:12AM  8:24AM 
Q06.00002: Combinatorial optimization with multimode cavity QED Brendan Marsh, Ronen Kroeze, David Atri Schuller, Sarang Gopalakrishnan, Jonathan Keeling, Benjamin L Lev We discuss how a particular drivendissipative quantum system, which is both pumped by an external source and is open to environmental interactions, might serve as a platform for information processing. This drivendissipative system is realized by coupling a network of ultracold atoms to a multimode optical cavity and its dynamics are shown to perform optimization of combinatorial problems. A compact description of multimode cavity QED using an appropriate mode basis, based on superpositions of bare cavity modes, makes tractable an exploration of the dynamics of the system across the superradiant phase transition. In the course of this transition and beyond, the system organizes into a state representing a solution to the optimization problem. The roles of entanglement and drivendissipative quantum dynamics in the optimization process will be explored, and experimental progress will be discussed that demonstrates the required ingredients for nearterm realization. 
Thursday, June 2, 2022 8:24AM  8:36AM 
Q06.00003: QSCOUT: Quantum Scientific Computing Open User Testbed Melissa C Revelle, Ashlyn D Burch, Daniel Lobser, Christopher G Yale, Susan M Clark The Quantum Scientific Computing Open User Testbed (QSCOUT) is a small quantum computer based on trappedion qubits designed to address the potential of nearterm quantum hardware for scientific computing applications. This system is based around a Sandia microfabricated surface ion trap, highfidelity Raman gates, and a custom assembly language JAQAL. Unlike many commercial alternatives, QSCOUT provides lowlevel access to the quantum hardware and enables users to adapt and modify the quantum gates and underlying pulse sequences. The Sandia scientists interact directly with the QSCOUT users and collaborators, allowing them to realize the full potential of the machine. This talk will discuss the specifications during the first round of users, plans for future developments and upcoming collaboration opportunities. 
Thursday, June 2, 2022 8:36AM  8:48AM 
Q06.00004: Optical precursors in waveguide quantum electrodynamics Silvia Fernanda Cardenas Lopez, Pablo Solano, Luis A Orozco, Ana AsenjoGarcia When a broadband signal propagates through a dispersive medium some frequency components move faster than the main pulse. This leads to the appearance of precursors, rapidly oscillating waves that emerge from the medium earlier than the main signal and seem to propagate superluminally. Here, we investigate the microscopic origin of precursors in a minimal setup: an array of qubits coupled to a waveguide. For large qubit numbers, our result for the linear transmission function converges to that of a continuous medium. The continuous description breaks down for small samples. Nevertheless, oscillations in the transmitted field persist down to only two qubits, which is the minimal number of elements required for the emergence of precursors. Precursors are best observed under conditions of electromagneticallyinduced transparency, as the main signal is significantly delayed. Under these conditions, just a single qutrit is enough to generate a precursor. Our results pave the way towards dispersion engineering of light with just a few qubits, and can be realized with superconducting qubits coupled to transmission lines or atoms coupled to fibers. 
Thursday, June 2, 2022 8:48AM  9:00AM 
Q06.00005: The rise and fall, and slow rise again, of operator entanglement under dephasing David Wellnitz, Guillermo Preisser, Vincenzo Alba, JEROME DUBAIL, Johannes Schachenmayer Operator space entanglement entropy, or simply "operator entanglement" (OE), is an indicator of the complexity of quantum operators and of their approximability by Matrix Product Operators (MPO). We study the OE of the density matrix of a 1D spin chain undergoing dissipative evolution. It is expected that, after an initial linear growth reminiscent of unitary quench dynamics, the OE should be suppressed by dissipative processes as the system evolves to a simple stationary state. Surprisingly, we find that this scenario breaks down for one of the most fundamental dissipative mechanisms: dephasing. Under dephasing, after the initial "rise and fall" the OE can rise again, increasing logarithmically at long times. Using a combination of MPO simulations for chains of infinite length and analytical arguments valid for strong dephasing, we argue that this logarithmic growth is inherent to a U(1) conservation law, universal, and trace it back to an anomalous classical diffusion process. 
Thursday, June 2, 2022 9:00AM  9:12AM 
Q06.00006: Adiabatic Control of DecoherenceFreeSubspaces in an Open Collective System Jarrod Reilly, Simon B Jäger, John Cooper, Murray J Holland We propose a method to adiabatically control an atomic ensemble using a decoherencefree subspace (DFS) within a dissipative cavity. We can engineer a specific eigenstate of the system's Lindblad jump operators by injecting a field into the cavity which deconstructively interferes with the emission amplitude of the ensemble. In contrast to previous adiabatic DFS proposals, our scheme creates a DFS in the presence of collective decoherence. We therefore have the ability to engineer states that have high multiparticle entanglements which may be exploited for quantum information science or metrology. We further demonstrate a more optimized driving scheme that utilizes the knowledge of possible diabatic evolution gained from the socalled adiabatic criteria. This allows us to evolve to a desired state with exceptionally high fidelity on a time scale that does not depend on the number of atoms in the ensemble. By engineering the DFS eigenstate adiabatically, our method allows for faster state preparation than previous schemes that rely on damping into a desired state solely using dissipation. 
Thursday, June 2, 2022 9:12AM  9:24AM 
Q06.00007: Threephoton entanglement transfer and control in multiemitter chiral waveguide quantum electrodynamics Imran M Mirza, Dingyu Guo, Logan Patrick, Umar Arshad We study the threephoton entanglement transfer and manipulation in chiral waveguides side coupled to a chain of twolevel quantum emitters (QEs). By applying the machinery of Fockstate master equations, we pay special attention to how entanglement is transiently distributed among the QEs. Therein, we compare how chirality aids to maintain higher values of entanglement among QEs for longer times as compared to the case of bidirectional waveguide quantum electrodynamics. We envisaged quantum networking and longdistance quantum communication as the two main areas of applications of this work in quantum information science. 
Thursday, June 2, 2022 9:24AM  9:36AM 
Q06.00008: Calculating how much time resonant photons spend as atomic excitations before being transmitted Kyle E Thompson, Kehui Li, Daniela Angulo Murcillo, VidaMichelle Nixon, Josiah J Sinclair, Howard M Wiseman, Aephraim M Steinberg When a single photon traverses a cloud of 2level atoms on resonance, how much time does it spend as an atomic excitation, as measured by weakly probing the atoms? It turns out that the answer, on average, is simply the spontaneous lifetime, multiplied by the probability of the photon being scattered into a sidemode. An obvious inference from this is that photons that are transmitted spend no time as excited atoms, and photons that are scattered spend, on average, one spontaneous lifetime. Our recent experimental work (PRX Quantum 3, 010314) shows that this inference is incorrect. However, a complete theoretical treatment of the opensystem dynamics for such a system has never, to our knowledge, been carried out. We examine this problem using the weakvalue formalism, and find that transmitted photons in general spend a nonzero amount of time as atomic excitations, and that this time can even be negative. We also determine the corresponding time for scattered photons, which turns out to be related to the "Wigner time" associated with elastic scattering. This work provides insight into the complex histories of photons travelling through absorptive media. 
Thursday, June 2, 2022 9:36AM  9:48AM 
Q06.00009: A nonHermitian optical atomic mirror YiCheng Wang, JhihShih You, HsiangHua Jen Explorations of symmetry and topology have led to important breakthroughs in quantum optics, but much richer behaviors arise from the nonHermitian nature of lightmatter interactions. A highreflectivity, nonHermitian optical mirror can be realized by a twodimensional subwavelength array of neutral atoms near the cooperative resonance associated with the collective dipole modes. Here we show that exceptional points develop from a nondefective degeneracy by lowering the crystal symmetry of a square atomic lattice, and dispersive bulk Fermi arcs that originate from exceptional points are truncated by the light cone. We also find, although the dipoledipole interaction is reciprocal, the geometrydependent nonHermitian skin effect emerges. Furthermore, skin modes localized at a boundary show a scalefree behavior that stems from the longrange interaction and whose mechanism goes beyond the framework of nonBloch band theory. Our work opens the door to the study of the interplay among nonHermiticity, topology, and longrange interaction. 
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