Nonreciprocal Entanglement of Frequency‐Distinct Qubits

Abstract Entanglement is the linchpin of quantum mechanics and a pivotal enabler of quantum technologies, wherein the states of particles are intrinsically correlated, such that the state of one instantaneously influences the other, regardless of the distance between them. Reciprocal entanglement and coupling between qubits often lead to unwanted bidirectional interactions and reflections, which degrade quantum states and reduce quantum coherence. This paper introduces a cryogenic‐compatible metasurface that leverages space‐time modulation to enable nonreciprocal entanglement between frequency‐distinct superconducting qubits. This functionality is achieved through a reflective quantum state‐converting metasurface, specifically designed for millikelvin‐temperature quantum technologies. The metasurface utilizes cascaded space‐time‐modulated Josephson field‐effect transistors (JoFETs), offering a transformative platform for advanced quantum state manipulation and entanglement. This spatiotemporal superconductor‐semiconductor metasurface transcends the limitations of traditional linear space‐time metasurfaces by incorporating gate‐controlled Josephson junctions, offering highly efficient spurious‐free state‐frequency conversion. This study demonstrates that spatiotemporal superconducting metasurfaces, particularly those leveraging JoFETs, enable highly efficient quantum state conversion even for superconducting qubits with a high frequency distinction ratio.