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Advanced Quantum Technologies

Nonreciprocal Entanglement of Frequency‐Distinct Qubits

Sajjad Taravati

Published Jul 13, 2025• 3 citations
Publisher

Abstract

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.

DOI
10.1002/qute.202500171

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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.
Abstract We introduce space-time-modulated cryogenic-compatible Josephson metasurfaces as a versatile platform for polychromatic qubit coupling and frequency-selective interactions. These metasurfaces, based on arrays of gated superconductor-semiconductor Josephson field-effect transistors (JoFETs), enable dynamic four-dimensional (space-time) control of microwave fields at millikelvin temperatures. Through spatiotemporal modulation of the critical current density, the metasurface generates controllable Floquet harmonics, facilitating frequency conversion, parametric amplification, and nonreciprocal propagation. Full-wave electromagnetic simulations demonstrate selective excitation and multi-frequency state transitions in superconducting qubit arrays, where a photon emitted at frequency is efficiently redirected into multiple target frequencies to mediate long-range interactions between adjacent and non-adjacent qubits. In the circuit quantum electrodynamics framework, this yields an effective polychromatic Hamiltonian supporting frequency-multiplexed exchange couplings while providing spectral isolation that suppresses crosstalk. The engineered non-Markovian environment arising from the space-time modulation offers pathways to enhance qubit coherence times and entanglement fidelity through information backflow and noise suppression. Fabrication pathways using Al-proximitized InAs quantum wells and cryogenic measurement setups are discussed. This work establishes space-time-modulated Josephson metasurfaces as promising hardware enablers for frequency-selective control, improved connectivity, and robust operation in next-generation superconducting quantum systems.
The pursuit of scalable and robust quantum computing necessitates innovative approaches to overcome the inherent challenges of qubit connectivity, decoherence, and susceptibility to noise and crosstalk. Conventional monochromatic qubit coupling architectures, constrained by nearest-neighbor interactions and limited algorithmic flexibility, exacerbate these issues, hindering the realization of practical large-scale quantum processors. In this work, we introduce a paradigm leveraging a space-time-modulated cryogenic-compatible Josephson metasurface to enable polychromatic qubit coupling. This metasurface facilitates frequency-selective interactions, transforming nearest-neighbor connectivity into all-to-all qubit interactions, while significantly enhancing coherence, noise robustness, and entanglement fidelity. Our proposed approach capitalizes on the unique capabilities of space-time-modulated Josephson metasurfaces, including dynamic four-dimensional wave manipulation, nonreciprocal state transmission, and state-frequency conversion, to mediate multi-frequency qubit interactions. By isolating qubit couplings into distinct spectral channels, the cryogenic-compatible metasurface mitigates crosstalk and environmental decoherence, extending coherence times and preserving quantum state fidelity. Full-wave simulations and quantum performance analyses demonstrate a significant enhancement in the operational efficiency of a superconducting qubit array, showcasing improved connectivity, robustness, and entanglement stability. This study establishes the potential of space-time-modulated cryogenic-compatible Josephson metasurfaces as a transformative platform for next-generation quantum computing, addressing critical bottlenecks and paving the way for scalable, high-performance quantum processors.
This paper presents an on-chip nonreciprocal space-time-periodic Josephson frequency multiplier that up-converts a single low-frequency input tone into a comb of high-order harmonics while enforcing directional signal propagation along the bus. We show that the harmonic content and bandwidth of the generated comb can be controlled through the applied DC and RF flux bias, and that individual output channels can be selectively addressed by tuning the bus operating point. We characterize the resulting nonreciprocal isolation between channels and discuss the associated design trade-offs, including the influence of modulation depth on harmonic-generation efficiency and back-reflection. The proposed device delivers multiple mutually isolated frequency channels from a single low-frequency source and it is well suited to applications requiring multiplexed delivery of tones to spatially distributed, frequency-addressable loads; we discuss multiplexed control of superconducting qubits as one such potential application area. Master-equation simulations of a four-qubit array driven by the harmonic comb, together with a one-dimensional line model of the directional conversion, quantify the resulting crosstalk and directional asymmetry; all results are simulations, and no experimental demonstration is claimed. The proposed architecture offers a compact, integrated route toward on-chip frequency multiplexing and directional isolation for cryogenic microwave systems.
Scaling superconducting quantum processors to large qubit counts faces challenges in control-signal delivery, thermal management, and hardware complexity, particularly in achieving microwave signal multiplexing and long-distance quantum information routing at millikelvin temperatures. We propose a space-time modulated Josephson-junction metasurface architecture to generate and multiplex microwave control signals directly at millikelvin temperatures. Theoretical and numerical results demonstrate the generation of multiple frequency tones with controlled parameters, enabling efficient and scalable qubit control while minimizing thermal loads and wiring overhead. We derive the nonlinear wave equation governing this system, simulate beam steering and frequency conversion, and discuss the feasibility of experimental implementation. These results lay the groundwork for a next-generation cryogenic signal-delivery paradigm that may enable scaling of superconducting quantum processors to thousands of qubits without overwhelming limited dilution-refrigerator cooling power.
We present a framework for implementing two-qubit entangling operations between distant superconducting qubits using a space-time modulated Josephson junction (JJ) metasurface. By modulating the surface in both space and time, we engineer sidebands with controllable wavevectors that selectively couple target qubits. The metasurface acts as a reconfigurable coupling medium, where the interaction strength is determined by engineered transmission coefficientsTμ(kμ⋅r)T_{\mu}\left(\mathrm{k}_{\mu} \cdot \mathrm{r}\right)Tμ​(kμ​⋅r)rather than by exponentially decaying near-field coupling, thus reducing the dependence on physical proximity. We investigated the implementation of two-qubit interactions via iSWAP gates driven resonantly through the metasurface and controlled phase gates via geometric phase accumulation. Simulations show entangling fidelity exceeding 98% maintained over centimeter-scale separations.