IBM Unveils Meter-Scale Superconducting L-Couplers: Connecting Dual 120-Qubit Processors with 99% State Transfer Fidelity

IBM Quantum has demonstrated a crucial engineering milestone on its path toward modular fault-tolerant quantum computing, operating four meter-scale cryogenic quantum interconnects—designated L-couplers—between two full 120-qubit IBM Quantum Nighthawk R1 superconducting processors. Detailed in an arXiv preprint and an accompanying IBM Research technical overview, the deployment achieved an average inter-chip quantum state transfer fidelity of 99.00% (peaking at 99.29%) and supported inter-processor distant CZ entangling gates averaging 94.75% fidelity.

While previous academic and industrial demonstrations have linked few-qubit superconducting chips on laboratory benches, IBM’s implementation represents the first multi-channel, meter-scale interconnect architecture engineered specifically for the mechanical and thermal constraints of large, production-grade quantum processors mounted inside shared dilution refrigerators. The modular architecture serves as the physical backbone for IBM’s upcoming fault-tolerant processor, IBM Quantum Starling, planned for 2029.

Engineering for Assembly: Perpendicular Indium Joins and Cooldown Shielding

A primary bottleneck in multi-chip superconducting scaling has been mechanical: traditional wire-bonding and horizontal planar connectors consume valuable chip perimeter, preventing dense two-dimensional packing. IBM solved this by developing a perpendicular cable-to-chip mating geometry. Aluminum coaxial lines are mounted using spring-loaded fixtures that apply an indium-to-indium pressure bond directly to the chip surface at room temperature after both processor payloads are secured inside the cryostat.