Xue Han

Nonreciprocity is a key ingredient for realizing chiral quantum networks. We propose a scheme to achieve the nonreciprocal enhancement of bipartite and tripartite entanglement in a hybrid cavity optomechanical (COM) system comprising a χ ⁽²⁾ nonlinear resonator, an atomic ensemble, and a coherent feedback loop. The phase-matching condition of the χ ⁽²⁾ nonlinear crystal confines the OPA-induced directional squeezing to the counterclockwise (CCW) mode, resulting in nonreciprocal quantum entanglement. We find that the synergistic interplay between directional squeezing and coherent feedback not only enhances the nonreciprocal bipartite and tripartite entanglement, but also substantially improves its dissipation tolerance. Our results demonstrate an alternative mechanism for generating and manipulating nonreciprocal quantum correlations in stationary optomechanical platforms. These findings open a promising avenue for quantum information processing, offering a robust platform for the development of nonreciprocal quantum devices.

We investigate a quantum Stirling engine whose working medium is a three‐qubit Heisenberg XXZ spin chain incorporating the Dzyaloshinskii‐Moriya interaction. The integration of a regenerator, which recovers internal energy during the isochoric processes, significantly enhances the efficiency of the engine, allowing it to approach the Carnot efficiency under appropriate parameters. Our analysis reveals a pronounced stage dependence of quantum correlations: at the end of the isothermal expansion, quantum entanglement and coherence of the thermal equilibrium state are positively correlated with the cycle efficiency, whereas at the end of the isothermal compression, they are negatively correlated with the efficiency. Furthermore, we demonstrate how external magnetic fields and coupling coefficients regulate these quantum correlations and the engine performance, including the work output and efficiency. This work provides a systematic comparison of the stage‐dependent roles of entanglement and coherence in a three‐qubit Stirling cycle, offering insights for designing high‐performance regenerative quantum thermal devices.

Photon blockade (PB) is generally classified into conventional and unconventional types based on their distinct physical mechanisms. Recent studies have aimed to bridge the two regimes to realize a universal photon blockade (universal PB) that combines their respective advantages. Here, we propose a scheme for achieving universal PB based on second-order nonlinearity. The second-order nonlinear coupling between two cavity modes mediates the fundamental interaction, while an optical parametric amplifier embedded in one cavity introduces additional quantum interference pathways. We analytically derive the optimal conditions for universal PB, and the analytical results show excellent agreement with numerical simulations. By comparing universal PB with conventional PB, we highlight its unique performance advantages: the scheme operates effectively in both strong and weak coupling regimes, relaxes experimental constraints, and achieves an ultra-low equal-time second-order correlation function g ⁽²⁾ (0) down to 10 ⁻⁷ , while maintaining a moderate mean photon number N _a ∼ 10 ⁻² . This work provides a feasible and tunable platform for realizing high-purity single-photon sources and exploring quantum statistical properties in nonlinear cavity systems.

The sensitivity of optomechanical sensors is fundamentally constrained by the combined effects of shot noise and quantum back‐action. Here we show that the joint action of cross‐Kerr nonlinearity and an optical parametric amplifier (OPA) can effectively suppress these noises. By tailoring the cross‐Kerr strength, the force‐sensing sensitivity can surpass the standard quantum limit (SQL), while an optimal driving power broadens the detection bandwidth. We further find that the system becomes unstable when the detuning falls below . Importantly, the inclusion of an OPA enhances robustness against thermal fluctuations, allowing the sensitivity of force measurement to surpass the SQL even at temperatures up to 300 K. A detailed noise analysis reveals that cross‐Kerr nonlinearity predominantly suppresses back‐action noise, whereas the OPA provides a strong reduction of shot noise. These results establish a promising hybrid strategy for realizing optomechanical sensors that combine high sensitivity with thermal resilience, with broad applications ranging from medical diagnostics to gravitational‐wave astronomy.

Previous investigations of mechanical squeezing have primarily been concentrated on a single manipulation method. We propose an innovative scheme to address the research gap by leveraging the synergy of an optical Kerr media and mechanical parametric amplification (MPA) in an optomechanical system to efficiently generate strong mechanical squeezing. By reasonably choosing the system parameters, this scheme has the following outstanding advantages: the squeezing effect of the cavity field induced by the optical Kerr medium can be transferred to the mechanical oscillator, which has been squeezed by the mechanical parametric amplifiers. The squeezing of the mechanical oscillator is higher than that of either state alone, and through this joint effect the squeezing can be significantly increased, ultimately realizing a strong mechanical squeezing. The scheme also exhibits notable robustness against both thermal noise and decay of the mechanical oscillator. The presented scheme provides an efficient and experimentally feasible platform for achieving strong mechanical squeezing, paving the way for generating other quantum effects.