arXiv subjects

Xu-Ke Gu

Publications and source records attributed to Xu-Ke Gu.

4 recordsLinked to original sources

arXiv2026

Magnon squeezing in the quantum regime

Squeezed states, crucial for quantum metrology and emerging quantum technologies, have been demonstrated in various platforms, but quantum squeezing of magnons in macroscopic spin systems remains elusive. Here we report the experimental observation of quantum-level magnon squeezing in a millimeter-scale yttrium iron garnet (YIG) sphere. By engineering a strong dispersive magnon-superconducting qubit coupling via a microwave cavity, we implement a significant self-Kerr nonlinearity to generate squeezed magnon states with their mean magnon number less than one. Harnessing a magnon-assisted Raman process, we perform Wigner tomography, revealing quadrature variances of $\sim\!0.8$ ($\sim\!1.0$~dB squeezing) relative to the vacuum. These results lay the groundwork for quantum nonlinear magnonics and promise potential applications in quantum metrology.

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arXiv2025

Exploring Dynamics of Open Quantum Systems in Naturally Inaccessible Regimes

Markovian open quantum systems are governed by the Lindblad master equation where the dissipation contains two parts, i.e., the anti-Hermitian operator and the quantum jumps, which share a common dissipation rate. We generalize the Lindblad master equation via postselection to a generalized Liouvillian formalism in which the effective damping rate of the anti-Hermitian operator can be different from the quantum jump rate. Our formalism provides a parameter space with regimes inaccessible in naturally-occurring systems. We explore these new regimes and find several interesting results including negative damping rates and generalized Liouvillian exceptional points. In a previously unexplored zero-damping Liouvillian regime where the damping rate is negligible, we investigate the effect only due to the quantum jumps and show an unusual polynomial decay of the excited state. This generalized Liouvillian formalism offers opportunities to explore novel phenomena and quantum technologies associated with the peculiar behavior of quantum jumps.

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arXiv2024

Macroscopic Bell state between a millimeter-sized spin system and a superconducting qubit

Entanglement is a fundamental property in quantum mechanics that systems share inseparable quantum correlation regardless of their mutual distances. Owing to the fundamental significance and versatile applications, the generation of quantum entanglement between {\it macroscopic} systems has been a focus of current research. Here we report on the deterministic generation and tomography of the macroscopically entangled Bell state in a hybrid quantum system containing a millimeter-sized spin system ($\sim 1\times10^{19}$ atoms) and a micrometer-sized superconducting qubit. The deterministic generation is realized by coupling the macroscopic spin system and the qubit via a microwave cavity. Also, we develop a joint tomography approach to confirming the deterministic generation of the Bell state, which gives a generation fidelity of $0.90\pm0.01$. Our work makes the macroscopic spin system the {\it largest} system (in the sense of atom number) capable of generating the maximally entangled quantum state.

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arXiv2023

Quantum control of a single magnon in a macroscopic spin system

Non-classical quantum states are the pivotal features of a quantum system that differs from its classical counterpart. However, the generation and coherent control of quantum states in a macroscopic spin system remain an outstanding challenge. Here we experimentally demonstrate the quantum control of a single magnon in a macroscopic spin system (i.e., 1~mm-diameter yttrium-iron-garnet sphere) coupled to a superconducting qubit via a microwave cavity. By tuning the qubit frequency {\it in situ} via the Autler-Townes effect, we manipulate this single magnon to generate its non-classical quantum states, including the single-magnon state and the superposition state of a single magnon and vacuum. Moreover, we confirm the deterministic generation of these non-classical states by Wigner tomography. Our experiment offers the first reported deterministic generation of the non-classical quantum states in a macroscopic spin system and paves a way to explore its promising applications in quantum engineering.

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