Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L31: Focus Session: Atomic Quantum Systems IFocus Live

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Sponsoring Units: DQI DAMOP Chair: Stojan Rebic, APS 
Wednesday, March 17, 2021 8:00AM  8:12AM Live 
L31.00001: Entanglement formation in continuousvariable random quantum networks Bingzhi Zhang, Quntao Zhuang Entanglement is not only important for understanding the fundamental properties of manybody systems, but also the crucial resource enabling quantum advantages in practical information processing tasks. While previous works on quantum networks focus on discretevariable systems, lightas the only travelling carrier of quantum information in a networkis bosonic and thus requires a continuousvariable description. We extend the study to continuousvariable quantum networks. By mapping the ensembleaveraged entanglement dynamics on an arbitrary network to a randomwalk process on a graph, we are able to exactly solve the entanglement dynamics and reveal unique phenomena. We identify squeezing as the source of entanglement generation, which triggers a diffusive spread of entanglement with a parabolic light cone. A surprising linear superposition law in the entanglement growth is predicted by the theory and numerically verified, despite the nonlinear nature of the entanglement dynamics. The equilibrium entanglement distribution (Page curves) is exactly solved and has various shapes dependent on the average squeezing density and strength. 
Wednesday, March 17, 2021 8:12AM  8:24AM Live 
L31.00002: Generation of Photonic Matrix Product States with a Rydbergblockaded atomic array ZhiYuan WEI, Daniel Malz, Alejandro GonzalezTudela, Juan Ignacio Cirac In this work, we show how one can deterministically generate photonic matrix product states with high bond and physical dimensions with an atomic array if one has access to a Rydbergblockade mechanism. We develop both a quantum gate and an optimal control approach to universally control the system and analyze the photon retrieval efficiency of atomic arrays. Comprehensive modeling of the system shows that our scheme is capable of generating a large number of entangled photons. We further develop a multiport photon emission approach that can efficiently distribute entangled photons into free space in several directions, which can become a useful tool in future quantum networks. 
Wednesday, March 17, 2021 8:24AM  8:36AM Live 
L31.00003: Wigner negativity in spinj systems Jack Davis, Meenu Kumari, Robert Mann, Shohini Ghose The nonclassicality of simple spin systems as measured by Wigner negativity is studied on a spherical phase space. Several SU(2)covariant states with common qubit representations are addressed: spin coherent, spin cat (GHZ/N00N), and Dicke (W). We derive a bound on the Wigner negativity of spin cat states that rapidly approaches the true value as spin increases beyond j≈5. We find that spin cat states are not significantly Wignernegative relative to their Dicke state counterparts of equal dimension. We also find, in contrast to several entanglement measures, that the most Wignernegative Dicke basis element is spindependent, and is not the equatorial state │j,0〉 (or │j,±1/2〉for halfinteger spins). These results underscore the influence that dynamical symmetry has on nonclassicality, and suggest a guiding perspective for finding novel quantum computational applications. 
Wednesday, March 17, 2021 8:36AM  9:12AM Live 
L31.00004: Towards quantum error correction with ions: qubit loss correction and code stitching Invited Speaker: Thomas Monz Quantum computing promises to solve problems ranging from finance, via chemistry, to climate change. Before these potential applications can be targeted, quantum computers need to be scaled up to sufficiently large systems. With the size of the system growing, the likelihood for errors to occur increases  unless you try to correct for errors on the run. Error correction requires the redundant encoding of quantum information across several physical qubits in a socalled logical qubit. To date, quantum error correction has focused on correcting errors within the computational subspace and a single logical qubit. In this presentation the first experiments with respect to the correction of qubitloss, an error that can usually not be corrected, as well as the first implementation of operations between two logical qubits, will be presented. 
Wednesday, March 17, 2021 9:12AM  9:24AM Live 
L31.00005: Optimal state transfer and entanglement generation in powerlaw interacting systems Minh Tran, Abhinav Deshpande, Andrew Guo, Andrew Lucas, Alexey V Gorshkov We present an optimal protocol for encoding an unknown qubit state into a multiqubit GreenbergerHorneZeilingerlike state and, consequently, transferring quantum information in large systems exhibiting powerlaw (1/r^{α}) interactions. For all powerlaw exponents α between d and 2d+1, where d is the dimension of the system, the protocol yields a polynomial speedup for α>2d and a superpolynomial speedup for α≤2d, compared to the state of the art. For all α>d, the protocol saturates the LiebRobinson bounds (up to subpolynomial corrections), thereby establishing the optimality of the protocol and the tightness of the bounds in this regime. The protocol has a wide range of applications, including in quantum sensing, quantum computing, and preparation of topologically ordered states. 
Wednesday, March 17, 2021 9:24AM  9:36AM Live 
L31.00006: Continuous protection from inhomogeneous dephasing Ran Finkelstein, Ohr Lahad, Itsik Cohen, Omri Davidson, Shai Kiriati, Eilon PoemKalogerakis, Ofer Firstenberg We present a scheme for protecting a qubit from inhomogeneous dephasing. The scheme relies on continuously dressing the qubit with an auxiliary state, which exhibits an opposite and potentially enhanced sensitivity to the same source of inhomogeneity. By employing a pair of driving fields, we increase the protection range, circumvent qubit phase rotation, and obtain robustness to drive noise, similarly to the doubledressing technique in continuous dynamical decoupling. We outline the minimal and optimal conditions for protection. 
Wednesday, March 17, 2021 9:36AM  9:48AM Live 
L31.00007: Robust Encoding of a Qubit in a Molecule Victor Albert, Jacob P Covey, John P Preskill We construct quantum errorcorrecting codes that embed a finitedimensional code space in the infinitedimensional Hilbert space of rotational states of a rigid body. These codes, which protect against both drift in the body’s orientation and small changes in its angular momentum, may be well suited for robust storage and coherent processing of quantum information using rotational states of a polyatomic molecule. Extensions of such codes to rigid bodies with a symmetry axis are compatible with rotational states of diatomic molecules as well as nuclear states of molecules and atoms. We also describe codes associated with general nonAbelian groups and develop orthogonality relations for coset spaces, laying the groundwork for quantum information processing with exotic configuration spaces. 
Wednesday, March 17, 2021 9:48AM  10:00AM Live 
L31.00008: Universal quantum computation and quantum error correction with ultracold atomic mixtures Valentin Kasper, Daniel Gonzalez Cuadra, Apoorva Hegde, Andy Xia, Alexandre Dauphin, Felix Huber, Maciej Lewenstein, Fred Jendrzejewski, Philipp Hauke Quantum information platforms made great progress in the control of manybody entanglement and the implementation of quantum error correction, but it remains a challenge to realize both in the same setup. Here, we propose a mixture of two ultracold atomic species as a platform for universal quantum computation with longrange entangling gates, while providing a natural candidate for quantum errorcorrection. In this proposed setup, one atomic species realizes localized collective spins with tunable length, which form the fundamental unit of information. The second atomic species yields phononic excitations, which are used to entangle collective spins. Finally, we illustrate how to encode a qubit in the collective spin using a finite version of the GottesmanKitaevPreskill code paving the way to universal faultolerant quantum computation in ultracold atom systems. 
Wednesday, March 17, 2021 10:00AM  10:12AM Live 
L31.00009: Quantification of entanglement in small onedimensional cluster states Zhangjie Qin, WooRam Lee, Vito W Scarola Measurementbased quantum computation (MBQC) serves as route to universal quantum computation using just singlequbit measurements on an initial entangled resource state, typically a cluster state. MBQC is especially useful when twoqubit gates are slow and singlequbit measurements are fast and accurate. Error in gates used to create cluster states can, however, degrade entanglement. Furthermore, an efficient measure of entanglement on small cluster states would be useful for experiments as they scale up their system sizes. I will discuss a simple fidelity measure to diagnose entanglement in cluster state chains based on teleportation across the chain. Teleportation is a crucial ingredient in MBQC and thus offers a valuable probe of small cluster states. We test the fidelity measure on cluster state chains built from errorprone interactions we expect to be relevant in atomic and molecular systems, e.g., Ising and XY interactions. We establish fidelity thresholds sufficient for establishing enough entanglement to realize teleportation in these cluster states in the laboratory. 
Wednesday, March 17, 2021 10:12AM  10:24AM Live 
L31.00010: Multidimensional Photonic Cluster States Using a Single SpinPhoton Interface Coupled to a Nuclear Register Cathryn Michaels, Jesús Arjona Martínez, Romain Debroux, Luca Huber, Alexander Stramma, Ryan Parker, Carola Purser, Dorian A Gangloff, Mete Atature Multidimensional cluster states of photons are a powerful resource towards measurementbased quantum computing and robust quantum communication^{[1,2]}. Existing generation proposals rely on coupled spinphoton interfaces^{[3]} or complex optical feedback mechanisms^{[4,5]}. Instead, we propose to generate a multidimensional cluster state using a single, efficient spinphoton interface coupled to nuclear spins. We use the hyperfine interaction to enable universal quantum gates between the interface spin and a local nuclear register and funnel the resulting entanglement to photons. We show that in silicon29 vacancy centres in diamond coupled to nanophotonic structures, 2x4 sized cluster states of fidelity F>0.5 and repetition rate of 50 kHz are achievable. As spinphoton interfaces equipped with a nuclear mode continue to develop, the fidelity and repetition rate at which multidimensional cluster states could be generated using our proposal will improve, providing a route to realising largescale quantum computing and quantum communication. 
Wednesday, March 17, 2021 10:24AM  10:36AM Live 
L31.00011: Majorana representation of adiabatic and superadiabatic processes in threelevel systems Shruti Dogra, Antti Vepsäläinen, Gheorghe Sorin Paraoanu We show that stimulated Raman adiabatic passage (STIRAP) and its superadiabatic version (saSTIRAP) have a natural geometric twostar representation on the Majorana sphere. In the case of STIRAP, we find that the evolution is confined to a vertical plane. A faster evolution can be achieved in the saSTIRAP protocol, which employs a counterdiabatic Hamiltonian to nullify the nonadiabatic excitations. We derive this Hamiltonian in the Majorana picture, and we observe how, under realistic experimental parameters, the counterdiabatic term corrects the trajectory of the Majorana stars toward the dark state. We also introduce a spin1 average vector and present its evolution during the two processes, demonstrating that it provides a measure of nonadiabaticity. We show that the Majorana representation can be used as a sensitive tool for the detection of process errors due to ac Stark shifts and nonadiabatic transitions. Finally, we provide an extension of these results to mixed states and processes with decoherence. 
Wednesday, March 17, 2021 10:36AM  10:48AM Live 
L31.00012: Nonlinear Bell inequality for macroscopic measurements Adam Bene Watts, Nicole Yunger Halpern, Aram Harrow The correspondence principle suggests that quantum systems grow classical when large. Classical systems cannot violate Bell inequalities. Yet agents given much control can violate Bell inequalities proven for largescale systems. We consider agents who have little control, implementing only general operations suited to macroscopic experimentalists: preparing smallscale entanglement and measuring macroscopic properties while suffering from noise. That experimentalists so restricted can violate a Bell inequality appears unlikely, in light of earlier literature. Yet we prove a Bell in equality that such an agent can violate, even if experimental errors have variances that scale as the system size. A violation implies nonclassicality, given limitations on particles' interactions. A product of singlets violates the inequality; experimental tests are feasible for photons, solidstate systems, atoms, and trapped ions. Consistently with known results, violations of our Bell inequality cannot disprove local hiddenvariables theories. By rejecting the disproof goal, we show, one can certify nonclassical correlations under reasonable experimental assumptions. 
Wednesday, March 17, 2021 10:48AM  11:00AM Live 
L31.00013: Observing Quantum Phases and Multiparticle Entanglement Dynamics in a Central Qudit Ising Model Joseph Szabo, Nandini Trivedi Quantum spin models are host to interesting phenomena including criticality and novel dynamical response. Detecting and understanding these phases as well as the underlying fluctuations and more importantly entanglement physics, is a longstanding issue. Through the use of an ancillary central qudit coupled to the paradigmatic transverse Ising model, we show how simple probes on the external ancilla provide details about the ground state and nonequilibrium phase of the system as well as how the phases are modified by this nonintegrable coupling. More interstingly, we observe that the development of fluctuations that captures mulitparticle entanglement in the pure Ising system also quantifies how entanglement and correlations serve to thermalize this composite system. Our findings are robust to the environment Hilbert space and physical couplings between spins and ancilla. 
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