Space-Time-Modulated Cryogenic Josephson Metasurfaces for Polychromatic Qubit Coupling and Frequency-Selective Interactions in Superconducting Quantum Systems
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.
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.
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.
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.
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)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.
Josephson junctions, as pivotal components of modern technologies such as superconducting quantum computing, owe their prominence to their unique nonlinear properties at low temperatures. Despite their extensive use in static configurations, the study of dynamic Josephson junctions, particularly under space-time modulation, remains largely unexplored. This study investigates the interaction and transmission of electromagnetic waves through arrays of space-time-modulated Josephson junctions. A comprehensive mathematical framework is presented to model the propagation of electric and magnetic fields within and beyond these structures. We demonstrate how such dynamic arrays enable groundbreaking four-dimensional light manipulation, achieving angular-frequency beam multiplexing through a seamless integration of frequency conversion and beam-splitting functionalities. These advancements open new horizons for electromagnetic field engineering, with far-reaching implications for superconducting quantum technologies, next-generation wireless communications, biomedical sensing, and radar systems.
This thesis presents circuit quantum electrodynamic (cQED) experiments done with artificial atoms coupled to a high quality factor superconducting microwave resonator. This work discusses the theoretical framework, design, fabrication, implementation and characterization of a multi-qubit system using superconducting persistent current qubits, often referred as flux qubits. \n \nWe developed a new type of cryogenic aid for fast turnaround microwave experiments. The minimum turnaround time to perform microwave experiments in dilution refrigerators is typically a few days, as required by cooling down and warming up the entire refrigerator. The cryogenic microwave payload and wiring needed for each qubit and resonator add a significant heat load to \nthe system which typically only has a few µW s of cooling power at millikelvin temperatures. We present a new experimental approach we developed in which a suitable sample holder is attached to a cold-insertable probe and brought in contact with transmission lines permanently mounted inside the cryostat. The total turnaround time is reduced to 8 hours if the target temperature is 80 mK. This new measurement method increased our ability to successfully identify good devices with fast design-characterization iterations, while allowing months-long measurements at 30 mK. \n \nOur device architecture uses several qubits, each of which is controlled via on-chip fast flux bias lines. Readout of the qubit’s quantum state is performed by homodyne detection of the resonator field at large driving power. We observe long coherence times at the optimal bias point of the flux qubits and present decoherence measurements around this optimal point, where the \nflux qubits couple mostly to magnetic flux noise. Dynamical decoupling techniques employing fast microwave pulses are used to protect the qubit’s coherence and allow calculating the magnetic flux noise power spectral density of the qubit’s environment. Photon-mediated interaction between distant qubits in the resonator allows the implementation of a two-qubit controlled-NOT gate \nusing the selective darkening technique. \n \nThis multi-qubit superconducting design is an interesting platform to study the decoherence properties of flux qubits, the properties of strong interaction of electromagnetic radiation with macroscopic quantum systems. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.