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Abstract:

The realization of a global quantum network holds the potential to enable groundbreaking applications such as secure quantum communication and blind quantum computing. However, building such a network remains a formidable challenge, primarily due to photon loss in optical fibers. In this work, we propose a quantum repeater architecture for distributing entanglement over intercontinental distances by leveraging low-Earth-orbit satellites equipped with spontaneous parametric down-conversion photon-pair sources and ground stations utilizing single-atom memories in optical cavities and single-photon detectors to implement the cavity-assisted photon scattering gates for high-fidelity entanglement mapping. The efficient entanglement swapping is achieved by performing high-fidelity Rydberg gates and readouts. We evaluate the entanglement distribution rates and fidelities by analyzing several key imperfections, including time-dependent two-photon transmission and time-dependent pair fidelity, for various satellite heights and ground station distances. We also investigate the impact of pair source fidelity, spin decoherence rate, and sky brightness on the repeater performance. Furthermore, we introduce a spatial-frequency multiplexing strategy within this architecture to enhance the design’s performance. Finally, we discuss in detail the practical implementation of this architecture. Our results show that this architecture enables entanglement distribution over intercontinental distances. For example, it can distribute over 10 000 pairs per flyby over 10 000 km with a fidelity above 90%, surpassing the capabilities of terrestrial quantum repeaters.

Abstract:

Controlling the coupling between different degrees of freedom in many-body systems is a powerful technique for engineering novel phases of matter. We create a bilayer system of two-dimensional (2D) ultracold Bose gases and demonstrate the controlled generation of bulk coherence through tunable interlayer Josephson coupling. We probe the resulting correlation properties of both phase modes of the bilayer system: the symmetric phase mode is studied via a noise-correlation method, while the antisymmetric phase fluctuations are directly captured by matter-wave interferometry. The measured correlation functions for both of these modes exhibit a crossover from short-range to quasi-long-range order above a coupling-dependent critical point, thus providing direct evidence of bilayer superfluidity mediated by interlayer coupling. We map out the phase diagram and interpret it with renormalization-group theory and Monte Carlo simulations. Additionally, we elucidate the underlying mechanism through the observation of suppressed vortex excitations in the antisymmetric mode.

Abstract:

We derive a general formula for the excitation threshold of parametric resonances of an oscillator with linear damping from consideration of the asymptotic properties of the Mathieu equation. This provides a good approximation for resonances of order m ≥ 2, and it is especially useful for high-order resonances in systems with light damping for which other approaches are cumbersome. Parametric resonance is ubiquitous in mechanical and electrical systems and its threshold is an important consideration, e.g., for systems that would be damaged by a high amplitude of resonantly excited motion. We present the expressions in a form useful for understanding systems with high quality factors such as trapped atomic ions, micro-mechanical devices and other oscillators, especially those with low dissipation in vacuum. High-order parametric resonances are extremely narrow making direct numerical simulation computationally intensive as well as less insightful.