IBM Links Two 120-Qubit Chips With 80 cm Cables at 99% Fidelity

Researchers at IBM have connected two 120-qubit superconducting quantum processors with four cables, each 80 centimetres long, and moved quantum states between the chips with a mean accuracy of 99.00 percent. The work, led by a team including Neereja Sundaresan, was posted on October 21 as a preprint on arXiv with the title "Meter-scale quantum interconnects between large quantum processors"; it has not yet been peer reviewed.

Why chips need cables

A quantum computer stores information in qubits. Unlike an ordinary bit, which is either 0 or 1, a qubit can be in a combination of both until it is measured. Packing ever more qubits onto a single chip is hard, so IBM is pursuing a modular approach: several processors, each a module, linked together. For that, the links must carry fragile quantum states over distances that are long by the standards of a chip, and a machine will need many identical links between any two modules.

The team calls its links long-distance couplers, or L-couplers. In this work four of them joined two IBM Nighthawk R1 processors, each with 120 qubits.

How an L-coupler is built

Each L-coupler joins two qubits of the processors' lattices through two frequency-tunable "L-qubits", two couplers based on a superconducting RF-SQUID circuit, and 80 centimetres of superconducting aluminium coaxial cable. The cable stands perpendicular to the processor surface, which allows denser connections, and it is attached to the chip by an indium-to-indium pressure bond made at room temperature after both modules were already mounted in the cryostat, the refrigerator that keeps the qubits cold. The modes of the links sit near 5 gigahertz, with internal quality factors of 0.86 to 1.09 million, a measure of how little energy they lose.

What the numbers show

• State transfer through the links, measured by network benchmarking, reached a mean fidelity of 99.00 percent. The best link reached 99.29 ± 0.07 percent.
• Two-qubit CZ gates between qubits located on different chips ran at a mean fidelity of 94.75 percent, with a best link of 96.42 ± 0.28 percent.
• The researchers used those gates to create entangled states across the two processors, including simultaneous Bell pairs through two adjacent L-couplers.

Fidelity here is the share of the intended quantum state that arrives intact; 100 percent would be a perfect transfer. Entanglement is a quantum link between particles in which the state of one cannot be described without the other.

Limits

As reported by ScienceAlert, the cables lost very little energy in use and stayed operational through three cycles of warming up and cooling down. The researchers also noticed magnetic interference that affected signal quality, and further efficiency improvements will be needed before the L-couplers are ready for practical use. The authors write that, to their knowledge, this is the first demonstration of a multi-channel quantum interconnect between large-qubit superconducting processors.

IBM has said it aims to deliver a fault-tolerant quantum computer by 2029. More technology coverage is in our Technology section.