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Abstract:

Abstract Quantum entanglement allows for correlations between distant objects that go beyond any classical theory. These additional correlations can be exploited to gain practical advantage in certain non-local games. In recent years there has been interest in games defined on graph structures and involving mobile agents. One of these is the graph domination game, where quantum advantage has been discovered recently on some finite graphs by numerical optimization [1]. Here we study quantum advantage in the 1-step, 2-player version of this game, focusing on cycle graphs. We study it numerically, analytically and through the use of noisy, intermediate scale quantum (NISQ) processors. We find explicit strategies and show that they realise the numerical bounds that were found recently for the case of small graphs [1]. We then generalise our strategies to cycles of arbitrary size. Finally, we run our strategies for 5-, 6-, and 7-site cycles on NISQ hardware and find measurable advantage (compared to the optimal classical strategies) in all cases.

Abstract:

Quantum harmonic oscillators model phenomena from electromagnetic fields to molecular vibrations, with excitations represented by bosons such as photons or phonons. Linear interactions that create or annihilate single bosons generate coherent states of light or motion. Introducing higher-order nonlinear interactions produces richer quantum behaviour: second-order interactions enable squeezing, whereas higher-order interactions generate non-Gaussian states useful for continuous-variable quantum computation. However, such interactions are usually weak or require specialized hardware. Hybrid systems, where a linear interaction couples an oscillator to a spin, offer an alternative. Here we combine two spin-dependent linear bosonic interactions to implement up to fourth-order nonlinear bosonic interactions in a single trapped ion, focusing on generalized squeezing. We demonstrate and characterize squeezing, trisqueezing and quadsqueezing; reconstruct the Wigner functions of the resulting states; and achieve quadsqueezing over 100 times faster than conventional methods. The approach has no fundamental limit on the interaction order and applies to any platform supporting spin-dependent linear interactions.

Abstract:

We characterise an efficient optically-heated neutral atom source for ion trapping. We observe loading rates of up to 24(3) s−1 with heating powers below 85 mW, and demonstrate loading of a single ion in under 30 s with 41.4(4) mW of optical power in a room-temperature ion trap system with an ionisation probability of 1.50(5) × 10−5 . We calibrate a thermal model for the source’s internal temperature by imaging the fluorescence of a collimated flux of neutral calcium that effuses from the source at various optical heating powers. We show that the thermal performance of this source is mainly limited by radiative losses. We explore the effect of second-stage photo-ionisation laser power on the loading rate, and identify a path beyond the loading rates reported in this study. We predict that this source is also well-suited to a wide range of metals used in ion trapping.

Abstract:

We report the first experimental demonstration of the odd-cycle game. We entangle two atoms separated by ∼ 2 m and the players use them to win the odd-cycle game with a probability ∼ 26σ above that allowed by the best classical strategy. The experiment implements the optimal quantum strategy, is free of loopholes, and achieves 97.8(3) % of the theoretical limit to the quantum winning probability. We perform the associated Bell test and measure a nonlocal content of 0.54(2) – the largest value for physically separate devices, free of the detection loophole, ever observed.

Abstract:

Distributed quantum computing (DQC) combines the computing power of multiple networked quantum processing modules, ideally enabling the execution of large quantum circuits without compromising performance or qubit connectivity1, 2. Photonic networks are well suited as a versatile and reconfigurable interconnect layer for DQC; remote entanglement shared between matter qubits across the network enables all-to-all logical connectivity through quantum gate teleportation (QGT)3, 4. For a scalable DQC architecture, the QGT implementation must be deterministic and repeatable; until now, no demonstration has satisfied these requirements. Here we experimentally demonstrate the distribution of quantum computations between two photonically interconnected trapped-ion modules. The modules, separated by about two metres, each contain dedicated network and circuit qubits. By using heralded remote entanglement between the network qubits, we deterministically teleport a controlled-Z (CZ) gate between two circuit qubits in separate modules, achieving 86% fidelity. We then execute Grover’s search algorithm5—to our knowledge, the first implementation of a distributed quantum algorithm comprising several non-local two-qubit gates—and measure a 71% success rate. Furthermore, we implement distributed iSWAP and SWAP circuits, compiled with two and three instances of QGT, respectively, demonstrating the ability to distribute arbitrary two-qubit operations6. As photons can be interfaced with a variety of systems, the versatile DQC architecture demonstrated here provides a viable pathway towards large-scale quantum computing for a range of physical platforms.