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1. Dynamics for the Haldane phase in the Bilinear-Biquadratic Model

   Arya Dhar, Daniel Jaschke (February 2021, ArXivorg)

   null (Ed.)

   The BBM is a promising candidate to study spin-one systems and to design quantum simulators based on its underlying Hamiltonian. The variety of different phases contains amongst other valuable and exotic phases the Haldane phase. We study the Kibble-Zurek physics of linear quenches into the Haldane phase. We outline ideal quench protocols to minimize defects in the final state while exploiting different linear quench protocols via the uniaxial or interaction term. Furthermore, we look at the fate of the string order when quenching from a topologically non-trivial phase to a trivial phase. Our studies show this depends significantly on the path chosen for quenching; for example, we discover quenches from \Neel{} to Haldane phase which reach a string order greater than their ground state counterparts for the initial or final state at intermediate quench times. 

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2. Parallel single-shot measurement and coherent control of solid-state spins below the diffraction limit

   https://doi.org/10.1126/science.abc7821  

   Chen, Songtao; Raha, Mouktik; Phenicie, Christopher M.; Ourari, Salim; Thompson, Jeff D. (October 2020, Science)

   Solid-state spin defects are a promising platform for quantum science and technology. The realization of larger-scale quantum systems with solid-state defects will require high-fidelity control over multiple defects with nanoscale separations, with strong spin-spin interactions for multi-qubit logic operations and the creation of entangled states. We demonstrate an optical frequency-domain multiplexing technique, allowing high-fidelity initialization and single-shot spin measurement of six rare-earth (Er³⁺) ions, within the subwavelength volume of a single, silicon photonic crystal cavity. We also demonstrate subwavelength control over coherent spin rotations by using an optical AC Stark shift. Our approach may be scaled to large numbers of ions with arbitrarily small separation and is a step toward realizing strongly interacting atomic defect ensembles with applications to quantum information processing and fundamental studies of many-body dynamics. 

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3. Controlling quantum many-body dynamics in driven Rydberg atom arrays

   https://doi.org/10.1126/science.abg2530  

   Bluvstein, D.; Omran, A.; Levine, H.; Keesling, A.; Semeghini, G.; Ebadi, S.; Wang, T. T.; Michailidis, A. A.; Maskara, N.; Ho, W. W.; et al (February 2021, Science)

   The control of nonequilibrium quantum dynamics in many-body systems is challenging because interactions typically lead to thermalization and a chaotic spreading throughout Hilbert space. We investigate nonequilibrium dynamics after rapid quenches in a many-body system composed of 3 to 200 strongly interacting qubits in one and two spatial dimensions. Using a programmable quantum simulator based on Rydberg atom arrays, we show that coherent revivals associated with so-called quantum many-body scars can be stabilized by periodic driving, which generates a robust subharmonic response akin to discrete time-crystalline order. We map Hilbert space dynamics, geometry dependence, phase diagrams, and system-size dependence of this emergent phenomenon, demonstrating new ways to steer complex dynamics in many-body systems and enabling potential applications in quantum information science. 

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4. Modeling Low- and High-Frequency Noise in Transmon Qubits with Resource-Efficient Measurement

   https://doi.org/10.1103/PRXQuantum.5.010320  

   Tripathi, Vinay; Chen, Huo; Levenson-Falk, Eli; Lidar, Daniel A. (February 2024, PRX Quantum)

   Transmon qubits experience open-system effects that manifest as noise at a broad range of frequencies. We present a model of these effects using the Redfield master equation with a hybrid bath consisting of low- and high-frequency components. We use two-level fluctuators to simulate 1/f-like noise behavior, which is a dominant source of decoherence for superconducting qubits. By measuring quantum state fidelity under free evolution with and without dynamical decoupling (DD), we can fit the low- and high-frequency noise parameters in our model. We train and test our model using experiments on quantum devices available through IBM quantum experience. Our model accurately predicts the fidelity decay of random initial states, including the effect of DD pulse sequences. We compare our model with two simpler models and confirm the importance of including both high frequency and 1/f noise in order to accurately predict transmon behavior. 

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5. Benchmarking highly entangled states on a 60-atom analogue quantum simulator

   https://doi.org/10.1038/s41586-024-07173-x  

   Shaw, Adam L.; Chen, Zhuo; Choi, Joonhee; Mark, Daniel K.; Scholl, Pascal; Finkelstein, Ran; Elben, Andreas; Choi, Soonwon; Endres, Manuel (April 2024, Nature)

   Abstract Quantum systems have entered a competitive regime in which classical computers must make approximations to represent highly entangled quantum states^1,2. However, in this beyond-classically-exact regime, fidelity comparisons between quantum and classical systems have so far been limited to digital quantum devices^2–5, and it remains unsolved how to estimate the actual entanglement content of experiments⁶. Here, we perform fidelity benchmarking and mixed-state entanglement estimation with a 60-atom analogue Rydberg quantum simulator, reaching a high-entanglement entropy regime in which exact classical simulation becomes impractical. Our benchmarking protocol involves extrapolation from comparisons against an approximate classical algorithm, introduced here, with varying entanglement limits. We then develop and demonstrate an estimator of the experimental mixed-state entanglement⁶, finding our experiment is competitive with state-of-the-art digital quantum devices performing random circuit evolution^2–5. Finally, we compare the experimental fidelity against that achieved by various approximate classical algorithms, and find that only the algorithm we introduce is able to keep pace with the experiment on the classical hardware we use. Our results enable a new model for evaluating the ability of both analogue and digital quantum devices to generate entanglement in the beyond-classically-exact regime, and highlight the evolving divide between quantum and classical systems. 

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