Aaron Somoroff, Patrick Truitt, Adam C. Weis, Jacob Bernhardt, Daniel T. Yohannes, Jason Walter, Konstantin Kalashnikov, Mario Renzullo, Raymond Mencia, Maxim Vavilov, Vladimir Manucharyan, I. V. Vernik, Oleg A. Mukhanov
The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for quantum information processing. In addition to requiring high-quality physical qubits, a quantum processor needs to be assembled in a manner that minimizes crosstalk and decoherence. In this paper, we report on fluxonium qubits packaged in a flip-chip architecture, where a classical control and readout chip is bump bonded to the quantum chip, forming a multichip module. The modular approach allows for improved connectivity between the qubits and control and readout elements, and separate fabrication processes. We characterize the coherence properties of the individual fluxonium qubits, demonstrate high-fidelity single-qubit gates with 6-ns microwave pulses (without DRAG), and identify the main decoherence mechanisms to improve on the reported results.
The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for quantum information processing. In addition to requiring high-quality physical qubits, a quantum processor needs to be assembled in a manner that minimizes crosstalk and decoherence. In this paper, we report on fluxonium qubits packaged in a flip-chip architecture, where a classical control and readout chip is bump bonded to the quantum chip, forming a multichip module. The modular approach allows for improved connectivity between the qubits and control and readout elements, and separate fabrication processes. We characterize the coherence properties of the individual fluxonium qubits, demonstrate high-fidelity single-qubit gates with 6-ns microwave pulses (without DRAG), and identify the main decoherence mechanisms to improve on the reported results.
The technological development of hardware heading toward universal fault-tolerant quantum computation requires a large-scale processing unit with high performance. While fluxonium qubits are promising with high coherence and large anharmonicity, their scalability has not been systematically explored. In this work, we propose a superconducting quantum information processor based on compact high-coherence fluxoniums with suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In this architecture, the qubits are readout dispersively using individual resonators connected to a common bus and manipulated via combined on-chip RF and DC control lines, both of which can be designed to have low crosstalk. A multi-path coupling approach enables exchange interactions between the high-coherence computational states and at the same time suppresses the spurious static ZZ rate, leading to fast and high-fidelity entangling gates. We numerically investigate the cross resonance controlled-NOT and the differential AC-Stark controlled-Z operations, revealing low gate error for qubit-qubit detuning bandwidth of up to 1 GHz. Our study on frequency crowding indicates high fabrication yield for quantum processors consisting of over thousands of qubits. In addition, we estimate low resource overhead to suppress logical error rate using the XZZX surface code. These results promise a scalable quantum architecture with high performance for the pursuit of universal quantum computation.
Transforming stand-alone qubits into a functional, general-purpose quantum processing unit requires an architecture where many-body quantum entanglement can be generated and controlled in a coherent, modular, and measurable fashion.Electronic circuits promise a well-developed pathway for large-scale integration once a mature library of quantum-compatible elements have been developed.In the domain of superconducting circuits, fluxonium has recently emerged as a promising qubit due to its high-coherence and large anharmonicity, yet its scalability has not been systematically explored.In this work, we present a blueprint for a high-performance fluxonium-based quantum processor that addresses the challenges of frequency crowding, and both quantum and classical crosstalk.The main ingredients of this architecture include high-anharmonicity circuits, multipath couplers to entangle qubits where spurious longitudinal coupling can be nulled, circuit designs that are compatible with multiplexed microwave circuitry, and strongly coupled readout channels that do not require complex, frequency-sculpted elements to maintain coherence.In addition, we explore robust and resource-efficient protocols for quantum logical operations, then perform numerical simulations to validate the expected performance of this proposed processor with respect to gate fidelity, fabrication yield, and logical error suppression.Lastly, we discuss practical considerations to implement the architecture and achieve the anticipated performance.
Abstract We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips—one quantum chip and one control chip—that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90 μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.
The path toward practical superconducting quantum processors requires the integration of a large number of high-performance qubits. Modular architectures could offer a way to address the scaling limitations of monolithic designs by partitioning a large quantum processor into physically separated modules, or chiplets, linked through long-range interconnects. In this context, although fluxonium qubits have emerged as a compelling platform for quantum computing due to their long coherence times and high-fidelity gates, existing coupling schemes remain restricted to qubits in close proximity on a single chip. This limitation inherently precludes the long-range interconnects essential for modular integration. In this work, we propose a long-range tunable coupler designed to interconnect fluxonium qubits separated by more than one centimeter, thereby supporting the realization of modular fluxonium quantum processors. Under realistic assumptions, the proposed coupler has the potential to achieve inter-module two-qubit gate performance, specifically sub-100-ns gates with intrinsic errors below 10−4, comparable to that of intra-module (intra-chiplet) gates, while enabling modular integration with low quantum crosstalk, a key requirement for scalable systems. We further discuss the integration of this coupler into modular fluxonium lattices and explore its feasibility for achieving the higher connectivity and longer coupling range required for complex quantum error correction codes. This work could contribute to the development of large-scale fluxonium quantum processors by bridging their demonstrated potential with modular scalability.
We have demonstrated superconducting interconnect technologies that enable a variety of flip-chip 3D integrated structures and packages compatible with high-coherence superconducting qubits. Superconducting indium micro-bumps and underbump metal (UBM) were used to join superconducting qubit chips to superconducting readout and control modules while maintaining high qubit coherence (T1, T2,echo> 20 μs) in the presence of capacitive and inductive coupling between the chips. Scanning electron microscope, X-ray, infrared and confocal microscopy were used to investigate the micro-structure, alignment accuracy, and parallelism of flip-chip qubits. The superconducting readout and control modules can accommodate both niobium and aluminum-based circuit and amplifier fabrication processes, including shadow-evaporated aluminum or Nb/Al-AlOx/Nb trilayer Josephson junctions (JJs). We present results for up to 16 active superconducting chips having trilayer junctions bonded to a passive superconducting module. The I-V characteristics and switching behavior were measured for flip-chip-connected JJ arrays with 40-20,000 JJs. Our approach maintained chip-level junction critical current and qubit coherence, demonstrating it to be a viable approach to building larger quantum computing systems. This paper also discusses packaging approaches to developing a quantum-to-classical interface in a cryogenic environment with multiple temperature stages.
Abstract Three-dimensional integration technologies such as flip-chip bonding are a key prerequisite to realize large-scale superconducting quantum processors. Modular architectures, in which circuit elements are spread over multiple chips, can further improve scalability and performance by enabling the integration of elements with different substrates or fabrication processes, by increasing the fabrication yield of completed devices, and by physically separating the qubits onto distinct modules to avoid correlated errors mediated by a common substrate. We present a design for a multi-chip module comprising one carrier chip and four qubit modules. Measuring two of the qubits, we analyze the readout performance, finding a mean three-level state-assignment error of 9×10−3 9 × 10 − 3 in 200 ns. We calibrate single-qubit gates and measure a mean simultaneous randomized benchmarking error of 6×10−4 6 × 10 − 4 , consistent with the coherence times of the qubits. Using a wiring-efficient static inter-module coupler featuring galvanic inter-chip transitions, we demonstrate a controlled-Z two-qubit gate in 100 ns with an error of 7×10−3 7 × 10 − 3 extracted from interleaved randomized benchmarking. Three-dimensional integration, as presented here, will continue to contribute to improving the performance of gates and readout as well as increasing the qubit count in future superconducting quantum processors.