IBM researchers report achieving 99% average quantum state transfer fidelity when connecting superconducting quantum processors using multiple meter-scale quantum channels. This demonstration marks a critical advance for scaling quantum computers toward fault tolerance, exceeding previous results and moving beyond solely building larger single chips.
The team’s newly designed l-couplers are compatible with both the existing IBM Quantum Nighthawk processors and the future IBM Quantum Starling computer, indicating a clear roadmap for expansion. “This is a critical component of the quantum computing architecture that truly unlocks scalability,” the researchers state, enabling more powerful error correction and larger quantum algorithms.
L-Couplers Enable High-Fidelity Interconnection of Nighthawk Processors
This level of fidelity was achieved by IBM researchers connecting processors within a shared cryogenic environment, shifting the focus from solely increasing the size of single chips to a modular approach. The team used four aluminum l-couplers, each with an 80-centimeter cable, to link two IBM Quantum Nighthawk R1 processors inside a dilution refrigerator. While the experiment used only a subset of the qubits for development, the processor chips themselves represent complete Nighthawk processors capable of more complex calculations.
These l-couplers encompass all components needed for inter-module quantum operation, including the aluminum coaxial cable and simplified “l-qubits” that interface with the processors. Internal quality factors of 940,000 were reported for the l-coupler modes, a dimensionless number indicating the system’s ability to store energy. Higher values are desirable for superconducting quantum information transfer. The design allows for l-coupler installation after processor payloads are mounted inside the cryostat, streamlining the integration process.
This compatibility ensures that investments in l-coupler technology will continue to yield benefits as the company advances its quantum computing platforms. With connected processors, more powerful error correction codes and larger quantum algorithms can be run across a higher number of entangled qubits.
99% Quantum State Transfer Achieved with Meter-Scale L-Couplers
Quantum state transfer fidelity reached 99.3% during recent tests of meter-scale connections between superconducting processors, a result indicating progress toward fault-tolerant quantum computing. The achievement used aluminum l-couplers, long-range interconnects differing from on-chip connections, within a newly debuted modular cryogenic platform designed for scaling quantum computers. Four of these l-couplers facilitated quantum information transfer between processor payloads housed inside a dilution refrigerator, establishing links between the front and rear assemblies.
The team benchmarked performance by measuring quantum state transfer fidelities, reporting an average of 99% accuracy, meaning the quantum state was correctly transmitted over the cable in roughly nine out of ten attempts. Beyond simple transfer, researchers constructed distant CZ gates using these links, achieving a two-qubit gate fidelity of 94.75% averaged across the four connections.
This allowed for the generation of entangled states shared between the two modules, and the simultaneous use of two l-couplers to create independent entangled states was also demonstrated.
L-Coupler Design Supports Scalability to IBM Quantum Starling
This design diverges from previous approaches that lacked compatibility with large-scale superconducting qubit processors, establishing a clear roadmap for future development. The company’s modular cryogenic platform, unveiled earlier this year, provides the environment for these connected processors to operate within a dilution refrigerator. IBM anticipates improvements in l-coupler yield and reproducibility, alongside reductions in error rates, as they build connected Nighthawk processors and progress toward the IBM Quantum Starling computer.
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