Meter-scale quantum interconnects between large quantum processors
AuthorsNeereja M. Sundaresan, Nicholas Cotaj, Yadav Prasad Kandel, Eric Lewandowski, Christian Robert Longton, Yves Martin, Jae-woong Nah, Kathryn Pooley, Martin Sandberg, Michael Scanlon, Chi Xiong, Emanuil Sashev Yanev, David Abraham, Abhinandan Antony, Vincent Arena, Nicholas Bornman, Khanh Dang, Oliver Dial, Christopher Guarino, Gabriel Haspil, Jared Hertzberg, Kentaro Heya, Swetha Kamlapurkar, Michael Karschner, George Keefe, Eric Liniger, Laura Luo, Thomas G. McConkey, David Olson, Jason Orcutt, Dmitry Ovchinnikov, Timothy Phung, Sankeerth Rajalingam, Michael Selvanayagam, William Shanks, Dongbing Shao, Abigail Shearrow, Adam Stabile, Jiri Stehlik, Joseph Suttle, Jeng-Bang Yau, Eric Zhang
AffiliationsIBM Research, T. J. Watson Research Center, Yorktown Heights, NY 10598, USA · Anderon · Amazon Web Services
Researchers connected two large quantum processors over nearly a meter and used the links to transfer states and create entanglement.
Key results
Qubits in each IBM Nighthawk R1 processor.
Centimeters of superconducting coax in each L-coupler.
Mean relay-based transfer fidelity across the four links.
Mean fidelity of controlled-Z gates between processors.
Best measured interprocessor controlled-Z link.
Unmitigated fidelity of the six-qubit state spanning both processors.
What the paper found
A practical route to modular quantum computing is to connect separate processors with coherent, meter-scale links. This work joins two 120-qubit IBM Nighthawk R1 processors using four L-couplers, each built around 80 cm of superconducting aluminum coax. The links use relay-based state transfer between tunable L-qubits, with RF-SQUID-based i2m couplers and indium-to-indium pressure-bonded cable connections assembled after the modules were mounted. Mean state-transfer fidelity reaches 99.00%, and remote controlled-Z gates between lattice qubits in different processors average 94.75%, with a best link at 96.42%. The team also demonstrates simultaneous Bell-pair generation across neighboring links and creates a six-qubit GHZ state spanning both processors, with 67.67% unmitigated fidelity. A key engineering finding is that magnetic fields during cable cooldown can degrade superconducting cable quality, so the links require shielding. Together, the results demonstrate multiple high-fidelity quantum channels between large processors, while identifying cable coherence and transfer overhead as targets for further improvement.
Original abstract
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.
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