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IEEE Transactions on Quantum Engineering

Integrated Superconducting High-$Q$ Seamless Cavity for Quantum Memory and Qubit Readout Applications

Mohamed F. Hagag, Kareem H. El‐Gendy, Bashar Zyoud, Thomas R. Jones, Karim G. Seddik, Dimitrios Peroulis

Published Jan 1, 2026• 0 citations
Publisher

Abstract

This work presents a novel micromachined, seamless sapphire–silicon hybrid evanescent-mode cavity architecture incorporating selectively engineered coupling channels and flux-tunable external coupling via a superconducting quantum interference device (SQUID). The proposed design enables two simulated operating states within a single device: an ultra-high- $Q$ memory configuration and a high-speed readout configuration with intrinsic Purcell filtering, which is expected to enhance readout fidelity while preserving the coherence of the stored quantum information. Unlike prior 3-D and multimode cavity approaches, the design employs a distributed embedded-capacitance network that confines electric fields into ultra-low-loss dielectric regions while distributing magnetic fields to an air-filled enclosure, thus substantially reducing simulated surface and dielectric losses beyond reported architectures. Moreover, the structure employs contactless superconducting metallization between two wafers, eliminating seam losses typically observed in micromachined resonators. The designed selective field-coupling strategy enables pure magnetic coupling through the lower silicon substrate, enabling fast readout, while pure electric coupling through the upper sapphire interface ensures strong qubit–cavity interaction, without mixed-coupling artifacts. High-fidelity mixed electromagnetic and circuit-level modeling predicts an intrinsic- $Q$ exceeding $10^{8}$ at 5554.5 MHz. The flux-tunable asymmetric DC-SQUID enables a dynamic external- $Q$ reconfiguration from $10^{9}$ (memory mode) to $10^{4}$ (readout mode). When coupled to a 6981.7 MHz transmon qubit, the architecture provides a nearly six-order-of-magnitude simulated Purcell-limited qubit lifetime improvement and achieves 66 $\%$ simulated memory efficiency at 1 ms under realistic coherence assumptions. These simulated results show that the proposed seamless, capacitively engineered architecture can work as a promising, high-coherence, and rapidly reconfigurable platform for scalable superconducting quantum systems.

DOI
10.1109/tqe.2026.3728781

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Integrated Superconducting High-QQQ Seamless Cavity for Quantum Memory and Qubit Readout Applications
This work presents a novel micromachined, seamless sapphire–silicon hybrid evanescent-mode cavity architecture incorporating selectively engineered coupling channels and flux-tunable external coupling via a superconducting quantum interference device (SQUID). The proposed design enables two simulated operating states within a single device: an ultra-high- QQQ memory configuration and a high-speed readout configuration with intrinsic Purcell filtering, which is expected to enhance readout fidelity while preserving the coherence of the stored quantum information. Unlike prior 3-D and multimode cavity approaches, the design employs a distributed embedded-capacitance network that confines electric fields into ultra-low-loss dielectric regions while distributing magnetic fields to an air-filled enclosure, thus substantially reducing simulated surface and dielectric losses beyond reported architectures. Moreover, the structure employs contactless superconducting metallization between two wafers, eliminating seam losses typically observed in micromachined resonators. The designed selective field-coupling strategy enables pure magnetic coupling through the lower silicon substrate, enabling fast readout, while pure electric coupling through the upper sapphire interface ensures strong qubit–cavity interaction, without mixed-coupling artifacts. High-fidelity mixed electromagnetic and circuit-level modeling predicts an intrinsic- QQQ exceeding 10810^{8}108 at 5554.5 MHz. The flux-tunable asymmetric DC-SQUID enables a dynamic external- QQQ reconfiguration from 10910^{9}109 (memory mode) to 10410^{4}104 (readout mode). When coupled to a 6981.7 MHz transmon qubit, the architecture provides a nearly six-order-of-magnitude simulated Purcell-limited qubit lifetime improvement and achieves 66 %\%% simulated memory efficiency at 1 ms under realistic coherence assumptions. These simulated results show that the proposed seamless, capacitively engineered architecture can work as a promising, high-coherence, and rapidly reconfigurable platform for scalable superconducting quantum systems.
Superconducting 3D microwave cavities offer state-of-the-art coherence times and a well controlled environment for superconducting qubits. In order to realize at the same time fast readout and long-lived quantum information storage, one can couple the qubit both to a low-quality readout and a high-quality storage cavity. However, such systems are bulky compared to their less coherent 2D counterparts. A more compact and scalable approach is achieved by making use of the multimode structure of a 3D cavity. In our work, we investigate such a device where a transmon qubit is capacitively coupled to two modes of a single 3D cavity. The external coupling is engineered so that the memory mode has an about 100 times larger quality factor than the readout mode. Using an all-microwave second-order protocol, we realize a lifetime enhancement of the stored state over the qubit lifetime by a factor of 666 with a ZZZ-fidelity of 82%82\%82%. We also find that this enhancement is not limited by fundamental constraints.
This study introduces a superconducting quantum chip architecture designed to simultaneously preserve entanglement and readout fidelity, addressing one of the key trade-offs in the development of scalable quantum hardware. In conventional quantum circuits, strong qubit qubit coupling enhances entanglement but often leads to undesired crosstalk, dephasing, and reduced measurement fidelity. To mitigate these effects, we propose a hybrid multiqubit configuration consisting of nine transmon qubits organized into interior and exterior groups, interconnected via a flux tunable qubit and a network of distributed resonators. The interior qubits along with tunable qubit form an entanglement core, while the exterior qubits operate in the dispersive regime under large detuning to enable readout. The degree of entanglement can be dynamically tuned by adjusting the coupling between the central tunable qubit and the interior qubits. The total Hamiltonian includes all significant coupling contributions, encompassing effective exchange interactions among interior and exterior qubits, as well as their mediated couplings through interface resonators. By numerically solving the complete Hamiltonian alongside the Lindblad master equation, the system dynamics are characterized, allowing evaluation of both spectroscopic features and separation fidelity. Simulation results demonstrate that the proposed design maintains strong entanglement by creating the avoided-crossing region while sustaining measurement fidelity around 0.995 under realistic noise conditions. These findings confirm that entanglement strength and readout fidelity can be co-optimized within a single, reconfigurable architecture, establishing a viable route toward high-performance and scalable superconducting quantum processors.
Dispersive readout of superconducting qubits relies on a transverse capacitive coupling that hybridizes the qubit with the readout resonator, subjecting the qubit to Purcell decay and measurement-induced state transitions (MIST). Despite the widespread use of Purcell filters to suppress qubit decay and near-quantum-limited amplifiers, dispersive readout often lags behind single- and two-qubit gates in both speed and fidelity. Here, we experimentally demonstrate junction readout, a simple readout architecture that realizes a strong qubit-resonator cross-Kerr interaction without relying on a transverse coupling. This interaction is achieved by coupling a transmon qubit to its readout resonator through both a capacitance and a Josephson junction. By varying the qubit frequency, we show that this hybrid coupling provides intrinsic Purcell protection and enhanced resilience to MIST, enabling readout at high photon numbers. While junction readout is compatible with conventional linear measurement, in this work we exploit the nonlinear coupling to intentionally engineer a large Kerr nonlinearity in the resonator, enabling bifurcation-based readout. Using this approach, we achieve a 99.4 % assignment fidelity with a 68 ns integration time and a 98.4 % QND fidelity without an external Purcell filter or a near-quantum-limited amplifier. These results establish the junction readout architecture with bifurcation-based readout as a scalable and practical alternative to dispersive readout, enabling fast, high-fidelity qubit measurement with reduced hardware overhead.
Superconducting 3D microwave cavities offer state-of-the-art coherence times and a well-controlled environment for superconducting qubits. In order to realize at the same time fast readout and long-lived quantum information storage, one can couple the qubit to both a low-quality readout and a high-quality storage cavity. However, such systems are bulky compared to their less coherent 2D counterparts. A more compact and scalable approach is achieved by making use of the multimode structure of a 3D cavity. In our work, we investigate such a device where a transmon qubit is capacitively coupled to two modes of a single 3D cavity. External coupling is engineered so that the memory mode has an about 100 times larger quality factor than the readout mode. Using an all-microwave second-order protocol, we realize a lifetime enhancement of the stored state over the qubit lifetime by a factor of 6 with a fidelity of approximately 80% determined via quantum process tomography. We also find that this enhancement is not limited by fundamental constraints.
Superconducting quantum circuits typically use capacitive charge-based linear coupling schemes to control interactions between elements such as qubits. While simple and effective, this coupling scheme makes it difficult to satisfy competing circuit design requirements such as maintaining large qubit anharmonicity and coherence along with a high degree of qubit connectivity and packing density. Moreover, tunable interactions using linear coupling elements produce dynamical variations in mode hybridization, which can induce non-adiabatic transitions, resulting in leakage errors and limiting gate speeds. In this work we attempt to address these challenges by proposing a junction-based coupling architecture based on SQUID (superconducting quantum interference device) couplers with relatively small Josephson energies. SQUID couplers provide intrinsic cross-Kerr interactions that can be controlled by external fluxes and that do not rely on mode hybridization. The small Josephson energies of the coupler maintain the interaction at a perturbative scale, which limits undesired higher-order mixing between coupled elements while achieving a sufficiently strong cross-Kerr interaction originating from diagonal coupling elements. Based on these properties, we show that a SQUID coupler can be used to implement a fast, adiabatic, and high-fidelity controlled-Z gate without introducing extra modes, and the operation is robust against junction asymmetry for high-frequency qubits. Although unconventional crosstalk may arise due to junction asymmetries and parasitic hybridization with spectator qubits, we show that these effects are sufficiently small for realistic circuit parameters. As an example of the utility of such junction-based coupling schemes, we present a scalable tiling strategy for a miniaturized superconducting quantum processor based on merged-element transmon qubits.
We describe a tunable-cavity quantum electrodynamics (QED) architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both microwave readout of tunneling and dispersive measurements of the qubit. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling, and detuning. A tunable-cavity frequency provides a way to strongly vary both the qubit-cavity detuning and coupling strength, which can reduce Purcell losses, cavity-induced dephasing of the qubit, and residual bus coupling for a system with multiple qubits. With our qubit-cavity system, we show that dynamic control over the cavity frequency enables one to avoid Purcell losses during coherent qubit evolutions and optimize state readout during qubit measurements. The maximum qubit decay time ${T}_{1}=1.5\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}$s is found to be limited by surface dielectric losses from a design geometry similar to planar transmon qubits.