IBM links two quantum processors with meter-scale cables, moving quantum states at 99% fidelity

IBM has connected two superconducting quantum processors with cables about a meter long and moved quantum information between them with high accuracy, IBM Research said on 27 October. The company calls it the first time superconducting quantum processors have been linked by multiple meter-scale quantum channels.

Why quantum computers need to be modular

A single chip can only hold so many qubits, the basic units of quantum computers. To build machines large enough to correct their own errors, IBM plans to connect many processors that sit together in the same ultra-cold refrigerator and pass information between them. The new links are designed for that.

IBM calls them l-couplers. Each consists of an aluminum coaxial cable with simplified qubits at each end, tunable couplers and controllers, and they are connected after the processors are mounted in the dilution refrigerator. In the test, four l-couplers with 80 cm cables linked two IBM Quantum Nighthawk R1 processors.

99% of states arrived intact

Quantum states moved from one chip to the other at a fidelity of up to 99.3%, and 99% on average. The cable modes reached a quality factor of 940,000, close to the roughly one million IBM says suits this kind of transfer. A two-qubit gate performed across the link, which is harder, reached 94.75% fidelity on average across the four links. The team also ran two l-couplers at once to create independent entangled states.

“For the first time, superconducting quantum processors have been connected using multiple meter-scale quantum channels.” — IBM Research

Room to improve

The authors write that there is “room to improve all of the metrics.” Next, IBM wants to improve the yield and reproducibility of the couplers, cut error rates, and scale to more couplers that can be plugged and unplugged. The links are designed to work with IBM's Nighthawk processors and to scale toward Starling, the fault-tolerant quantum computer on IBM's roadmap. Only part of each chip's qubits was wired up for this development test.

The work was described by IBM's Ryan Mandelbaum and Neereja Sundaresan. More physics stories are collected under quantum physics.