Session L31: Focus Session: Atomic Quantum Systems I

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Entanglement is not only important for understanding the fundamental properties of many-body systems, but also the crucial resource enabling quantum advantages in practical information processing tasks. While previous works on quantum networks focus on discrete-variable systems, light--as the only travelling carrier of quantum information in a network--is bosonic and thus requires a continuous-variable description. We extend the study to continuous-variable quantum networks. By mapping the ensemble-averaged entanglement dynamics on an arbitrary network to a random-walk 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.   

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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 Rydberg-blockade 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 multi-port 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.   

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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 Wigner-negative relative to their Dicke state counterparts of equal dimension. We also find, in contrast to several entanglement measures, that the most Wigner-negative Dicke basis element is spin-dependent, and is not the equatorial state │j,0〉 (or │j,±1/2〉for half-integer spins). These results underscore the influence that dynamical symmetry has on nonclassicality, and suggest a guiding perspective for finding novel quantum computational applications.   

Live

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 so-called 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 qubit-loss, an error that can usually not be corrected, as well as the first implementation of operations between two logical qubits, will be presented.   

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We present an optimal protocol for encoding an unknown qubit state into a multiqubit Greenberger-Horne-Zeilinger-like state and, consequently, transferring quantum information in large systems exhibiting power-law (1/r^α) interactions. For all power-law 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 Lieb-Robinson 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.   

Live

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 double-dressing technique in continuous dynamical decoupling. We outline the minimal and optimal conditions for protection.

As an experimental case study, we focus on motional dephasing of a spin wave in an atomic ensemble. We employ light storage and retrieval for quantifying the coherence time, which without protection is limited by the ballistic atomic motion at random velocities along the spin wave. When applying the protection scheme, the effect of the drive field can be understood as a velocity-dependent light shift, maintaining the correlations between position and phase of the spin wave. We demonstrate complete suppression of the inhomogeneous dephasing, prolonging the memory time.

Our scheme is applicable to various gas, solid, and engineered systems suffering from dephasing due to variations of conditions in time, space, or other domains.   

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We construct quantum error-correcting codes that embed a finite-dimensional code space in the infinite-dimensional 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 non-Abelian groups and develop orthogonality relations for coset spaces, laying the groundwork for quantum information processing with exotic configuration spaces.   

Live

Quantum information platforms made great progress in the control of many-body 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 long-range entangling gates, while providing a natural candidate for quantum error-correction. 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 Gottesman-Kitaev-Preskill code paving the way to universal faul-tolerant quantum computation in ultracold atom systems.   

Live

Measurement-based quantum computation (MBQC) serves as route to universal quantum computation using just single-qubit measurements on an initial entangled resource state, typically a cluster state. MBQC is especially useful when two-qubit gates are slow and single-qubit 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 error-prone 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.   

Live

Multidimensional cluster states of photons are a powerful resource towards measurement-based quantum computing and robust quantum communication^[1,2]. Existing generation proposals rely on coupled spin-photon interfaces^[3] or complex optical feedback mechanisms^[4,5]. Instead, we propose to generate a multi-dimensional cluster state using a single, efficient spin-photon 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 silicon-29 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 spin-photon 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 large-scale quantum computing and quantum communication.

1 R. Raussendorf et al. Phys. Rev. Lett. 86, 5188 (2001)
2 M. Zwerger et al. Appl. Phys. B 122 (2016)
3 S. E. Economou et al. Phys. Rev. Lett. 105, 093601 (2010)
4 H. Pichler et al. PNAS 114, 11362 (2017)
5 Y. Pilnyak et al. Phys. Rev. A 95, 022304 (2017)   

Live

We show that stimulated Raman adiabatic passage (STIRAP) and its superadiabatic version (saSTIRAP) have a natural geometric two-star 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 spin-1 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.
Reference: Phys. Rev. Res. 2, 043079 (2020).   

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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 large-scale systems. We consider agents who have little control, implementing only general operations suited to macroscopic experimentalists: preparing small-scale 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, solid-state systems, atoms, and trapped ions. Consistently with known results, violations of our Bell inequality cannot disprove local hidden-variables theories. By rejecting the disproof goal, we show, one can certify nonclassical correlations under reasonable experimental assumptions.
arXiv:1911.09122   

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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 long-standing 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 non-equilibrium 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.