Researchers Achieve 99.915% Fidelity in Qubit Control Via Light

Scaling up superconducting quantum computers has been limited by increasing numbers of signal cables within dilution refrigerators due to space constraints and heat build-up. Full control of superconducting qubits is now achieved using optically transmitted signals instead of traditional coaxial cables. A new method controls quantum bits, known as qubits, with light in place of metal cabling inside dilution refrigerators. These specialised cooling systems are essential for maintaining ultra-low temperatures required for superconductivity but become increasingly congested with wiring as qubit numbers rise.

Transmitting microwave signals via laser beams and converting them to electrical current using photodiodes inside the refrigerator circumvents space limitations and reduces heat build-up. The University of Science and Technology of China team has fully controlled superconducting qubits using light rather than conventional wiring within dilution refrigerators, which maintain temperatures colder than those achievable with conventional refrigeration. Dilution refrigerators are key for superconductivity, yet face increasing congestion as quantum computers scale up because each qubit traditionally requires a dedicated signal cable.

Modulating microwave signals onto laser beams at room temperature then reconverting them into electrical current inside the refrigerator using photodiodes effectively bypasses spatial restrictions and diminishes heat build-up; this is similar to sending information via fibre optic cables instead of copper wires. Achieving single and two-qubit gate fidelities exceeding 99.6%, their method meets requirements for error correction techniques like surface code, adding redundancy to protect calculations from errors.

Optical Control Achieves Record Fidelity for Single and Two-Qubit Gates

Single-qubit gate fidelities now exceed 99.915% ±0.005%, a substantial improvement over previous limitations imposed by cabling congestion within dilution refrigerators. Achieving this threshold is key for implementing surface code quantum error correction, a technique demanding exceedingly precise qubit manipulation to protect fragile quantum information. Until recently, increasing heat loads and spatial constraints caused by numerous control cables presented significant hurdles in scaling superconducting quantum computers.

The institution has also achieved high-fidelity performance on two-qubit gates; these reached 99.676% ±0.041% fidelity, further supporting the viability of this optical approach for complex quantum computations. Detailed noise characterisation revealed that fluctuations on the transmitted signal closely matched expected behaviour, a combination of low frequency (1/f) noise, white noise, and Lorentzian peaks, enabling accurate modelling of error rates down to timescales relevant for qubit control.

Ramsey oscillation and Spin echo techniques confirmed sufficiently low levels of phase noise impacting gate accuracy. The Idle gate error rate was approximately 0.04% observed at a frequency of 10−3Hz.

High fidelity optical control supports progress towards fault-tolerant quantum computation

Scaling superconducting quantum computers demands new solutions to manage increasing complexity within dilution refrigerators; these specialised cooling systems struggle with both physical space and heat dissipation as qubit numbers grow. While optically-assisted transmission demonstrates high fidelity control, the team acknowledges that extending this method beyond a limited number of qubits presents significant hurdles not fully addressed by this work. Control errors were estimated via Cross-entropy benchmarking and Speckle Purity Benchmarking as being below 0.053% for two-qubit operations, exceeding decoherence limits but currently representing performance within a small test system.

By modulating microwave signals onto laser light at room temperature and converting them back into electrical current cryogenically via photodiodes, the group bypassed spatial limitations and reduced heat build-up inside the refrigerator; these specialised cooling systems are important for maintaining ultra-low temperatures required for superconductivity yet become increasingly congested with wiring as qubit numbers rise. This new approach could simplify scaling up delicate processors in the future and begin to unlock their full potential.

This research demonstrated control of superconducting qubits using photocurrent generated from modulated laser intensities, achieving single-qubit gate fidelities of 99.915% ±0.005% and two-qubit gate fidelities of 99.676% ±0.041%. The optically-assisted transmission line offers an alternative to traditional coaxial cables, potentially easing space and heat load constraints within dilution refrigerators used for quantum computing. Results indicate this method supports complex computations with error rates below 0.053% for two-qubit operations; the authors note further work is needed to extend this approach beyond a limited number of qubits.