Entanglement across separate silicon dies in a modular superconducting qubit device
Alysson Gold, Jean-Philip Paquette, Anna Stockklauser, Matthew J. Reagor, M. Sohaib Alam, Andrew J. Bestwick, Nicolas Didier, Ani Nersisyan, Feyza B. Oruc, Armin Razavi, Ben Scharmann, Eyob Alebachew Sete, Biswajit Sur, Davide Venturelli, Cody James Winkleblack, Filip Wudarski, Mike Harburn, Chad T. Rigetti
Abstract Assembling future large-scale quantum computers out of smaller, specialized modules promises to simplify a number of formidable science and engineering challenges. One of the primary challenges in developing a modular architecture is in engineering high fidelity, low-latency quantum interconnects between modules. Here we demonstrate a modular solid state architecture with deterministic inter-module coupling between four physically separate, interchangeable superconducting qubit integrated circuits, achieving two-qubit gate fidelities as high as 99.1 ± 0.5% and 98.3 ± 0.3% for iSWAP and CZ entangling gates, respectively. The quality of the inter-module entanglement is further confirmed by a demonstration of Bell-inequality violation for disjoint pairs of entangled qubits across the four separate silicon dies. Having proven out the fundamental building blocks, this work provides the technological foundations for a modular quantum processor: technology which will accelerate near-term experimental efforts and open up new paths to the fault-tolerant era for solid state qubit architectures.
Abstract Assembling future large-scale quantum computers out of smaller, specialized modules promises to simplify a number of formidable science and engineering challenges. One of the primary challenges in developing a modular architecture is in engineering high fidelity, low-latency quantum interconnects between modules. Here we demonstrate a modular solid state architecture with deterministic inter-module coupling between four physically separate, interchangeable superconducting qubit integrated circuits, achieving two-qubit gate fidelities as high as 99.1 ± 0.5% and 98.3 ± 0.3% for iSWAP and CZ entangling gates, respectively. The quality of the inter-module entanglement is further confirmed by a demonstration of Bell-inequality violation for disjoint pairs of entangled qubits across the four separate silicon dies. Having proven out the fundamental building blocks, this work provides the technological foundations for a modular quantum processor: technology which will accelerate near-term experimental efforts and open up new paths to the fault-tolerant era for solid state qubit architectures.
In addition to striving to assemble more and more qubits in a single monolithic quantum device, taking a modular design strategy may mitigate numerous engineering challenges for achieving large-scalable quantum processors with superconducting qubits. Nevertheless, a major challenge in the modular quantum device is how to realize high-fidelity entanglement operations on qubits housed in different modules while preserving the desired isolation between modules. In this work, we propose a conceptual design of a modular quantum device, where nearby modules are spatially separated by centimeters. In principle, each module can contain tens of superconducting qubits and can be separately fabricated, characterized, packaged, and replaced. By introducing a bridge module between nearby qubit modules and taking the coupling scheme utilizing a tunable bus, tunable coupling of qubits that are housed in nearby qubit modules could be realized. Given physically reasonable assumptions, we expect that sub-100-ns two-qubit gates for qubits housed in nearby modules, which are spatially separated by more than two centimeters could be obtained. In this way, the inter-module gate operations are promising to be implemented with gate performance comparable with that of intra-module gate operations. Moreover, with the help of through-silicon vias technologies, this long-range coupling scheme may also allow one to implement inter-module couplers in a multi-chip stacked processor. Thus, the tunable longer-range coupling scheme and the proposed modular architecture may provide a promising foundation for solving challenges toward large-scale quantum information processing with superconducting qubits.
Superconducting qubits provide a promising approach to large-scale fault-tolerant quantum computing. However, qubit connectivity on a planar surface is typically restricted to only a few neighboring qubits. Achieving longer-range and more flexible connectivity, which is particularly appealing in light of recent developments in error-correcting codes, however, usually involves complex multilayer packaging and external cabling, which is resource intensive and can impose fidelity limitations. Here, we propose and realize a high-speed on-chip quantum processor that supports reconfigurable all-to-all coupling with a large on-off ratio. We implement the design in a four-node quantum processor, built with a modular design comprising a wiring substrate coupled to two separate qubit-bearing substrates, each including two single-qubit nodes. We use this device to demonstrate reconfigurable controlled- Z gates across all qubit pairs, with a benchmarked average fidelity of 96.00 % ± 0.08 % and best fidelity of 97.14 % ± 0.07 % , limited mainly by dephasing in the qubits. We also generate multiqubit entanglement, distributed across the separate modules, demonstrating GHZ-3 and GHZ-4 states with fidelities of 88.15 % ± 0.24 % and 75.18 % ± 0.11 % , respectively. This approach promises efficient scaling to larger-scale quantum circuits and offers a pathway for implementing quantum algorithms and error-correction schemes that benefit from enhanced qubit connectivity. Published by the American Physical Society 2024
Abstract Connecting multiple smaller qubit modules by generating high-fidelity entanglement is a promising path for scaling quantum computing hardware. The performance of such a modular quantum computer depends on the quality and rate of entanglement generation. However, identifying optimal architectures and entanglement generation protocols remains an open question. How can modular quantum architectures be designed to achieve fault tolerance while requiring only feasible entanglement rates and hardware? Focusing on solid-state quantum hardware, we investigate the threshold and logical failure rate of a fully distributed surface code. We consider both emission-based and scattering-based entanglement schemes between the modules to link the performance to the physical hardware and identify the regime for fault tolerance. We compare architectures with one or two data qubits per module. For some entanglement schemes, thresholds nearing the thresholds of non-distributed implementations (~ 0.4%) appear feasible with future parameters minimizing the performance gap between modular and monolithic quantum processors.
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.
Modular quantum architecture requires long-distance quantum interconnects to stitch together individual processor modules. Such interconnects, long-distance couplers or L-couplers, must support coherent quantum operation over meter-scale distances and scale to multiple connections between any two modules. Here we demonstrate four L-couplers operating between two 120-qubit IBM Nighthawk R1 superconducting qubit processors. Each L-coupler joins two Nighthawk lattice qubits through two frequency-tunable L-qubits, two RF-SQUID based "i2m" couplers, and 80 cm of superconducting aluminum coax. Each cable installs perpendicular to the processor surface, to enable denser connections, and is joined to the chip by an indium-to-indium room temperature pressure bond made after both modules were mounted. L-coupler modes near 5 GHz, with internal quality factors of 0.86 to 1.09e6, carry relay-based state transfer at a mean network benchmarking fidelity of 99.00%, with a best link of 99.29 +/- 0.07%. We then realize distant CZ gates between lattice qubits in separate modules at a mean fidelity of 94.75%, with a best link of 96.42 +/- 0.28%, and use them to generate entangled states across the two processors, including a demonstration of simultaneous Bell pair generation using two adjacent L-couplers. These four high-fidelity L-couplers establish a practical path to scalable quantum interconnects for modular quantum computing.
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.