A modular quantum computer links many smaller processors instead of relying on one giant chip, and IBM says it has passed a key milestone on that path. IBM researchers connected two IBM Quantum Nighthawk processors with four meter-scale links, called l-couplers, and moved quantum information between them with an average fidelity of 99%.
IBM described the work in a blog post on 27 October by Ryan Mandelbaum and Neereja Sundaresan, based on the team’s recently published paper. According to IBM, it is the first time superconducting quantum processors have been connected through multiple meter-scale quantum channels at that level of fidelity.
Why IBM is building a modular quantum computer
Useful quantum computers will need far more qubits than exist today. IBM says the way forward will not rely only on building bigger chips. Instead, many processors will work together, with information moving between them.
In IBM’s design, a modular quantum computer consists of individual processors joined by meter-scale interconnects inside modular cryogenics: dilution refrigerators that keep the hardware near absolute zero and can share a cold environment as they scale. IBM calls these long-range links l-couplers, as opposed to the c-couplers that connect qubits on a single chip.
Earlier work by IBM and others showed high-quality operations between connected modules, but IBM says those approaches did not fit easily with adding many links between large superconducting processors. The new design is built to be compatible with Nighthawk and to scale toward IBM Quantum Starling, the company’s planned fault-tolerant computer.
How the l-couplers work
Each l-coupler includes everything needed to pass quantum information between modules: an aluminum coaxial cable, simplified “l-qubits” at each end, tunable couplers that connect those qubits to the cable, and the associated controllers. The couplers are attached after the processors are mounted inside the dilution refrigerator.
In this experiment, two IBM Quantum Nighthawk R1 processors were linked by four l-couplers, each with an 80-centimeter cable. The chips were full Nighthawk processors, although only a subset of qubits was wired out for development.
Modular quantum computer results in numbers
- Quality factor: the l-coupler modes reached an internal quality factor of 940,000. Higher is better, and IBM notes that values on the order of one million suit superconducting quantum information transfer.
- State transfer: fidelity of up to 99.3%, with an average of 99%. In other words, the quantum state crossed the cable correctly 99% of the time on average.
- Two-qubit gates: distant CZ gates across the modules reached a fidelity of 94.75%, averaged over the four links.
Using those distant gates, the team created entangled states shared between the two modules, including running two l-couplers at the same time to create independent entangled states.
What comes next
IBM says its hardware focus will be error correction codes, which will require meter-scale couplers between neighboring chips. The company plans to improve the yield and reproducibility of l-couplers, lower their error rates and add more links that can be plugged and unplugged between larger processors. IBM also says there is room to improve every metric reported in the paper.
As I see it, the 94.75% gate fidelity is the number to watch, since it trails the 99% state transfer figure and IBM itself names error rates as a target. For related reading, see our stories on quantum skyrmions on a Penn chip and KIT’s common blueprint for quantum software.
FAQ
What is a modular quantum computer?
It is a quantum computer built from many processors joined by interconnects, rather than one very large chip. IBM’s design places linked processors inside shared cryogenic systems.
What are IBM’s l-couplers?
L-couplers are IBM’s long-range links between superconducting quantum processors. Each uses an aluminum coaxial cable, simplified l-qubits, tunable couplers and controllers.
How reliable were the l-coupler connections?
Quantum state transfer reached up to 99.3% fidelity, averaging 99%, and distant CZ gates reached 94.75% fidelity averaged over four links.
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