A single optical fibre could replace the entire tangle of power-hungry amplifiers and cables needed to read out a quantum processor, solving a critical bottleneck in scaling up these machines. Today’s superconducting quantum computers are limited by the heat generated by the many semiconductor amplifiers required to read out each qubit. Inside a dilution fridge—where the processor operates near absolute zero—this heat becomes unmanageable beyond a few hundred qubits, making it impossible to build the large systems needed for error correction or useful computation. The team behind QC-SOAR has already shown that a microwave-to-optical transducer chip, developed by the startup QphoX, can read out a single qubit with the same fidelity as conventional electronics, but without adding heat inside the fridge. If this approach works on a full 9-qubit processor, it will clear a path toward 1000+ qubit machines that can run algorithms no classical computer can simulate. The immediate impact is on the quantum computing industry itself: it would allow companies to scale processors inside existing commercial dilution fridges, accelerating progress toward practical quantum advantage in fields such as drug discovery, materials design, and cryptography.
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**The Opportunity:** For quantum computing to reach its potential to solve problems beyond today's supercomputers, we need to build a large 1000+ qubit scale quantum processor with sufficient gate fidelity to allow prototype error correction schemes to be tested. In this regime, the quantum algorithm cannot be simulated on a classical computer, and we can determine the requirements for these systems to demonstrate useful computation. For superconducting quantum processors, the technical challenge is to do this on commercially available dilution fridges such as the new large format fridges (the Oxford Instruments QX and Bluefors Kide systems), which require the system thermal budgets to be controlled. This is currently impossible at the 1000 qubit scale because of the power required by the readout, which includes many semiconductor amplifiers. **The Approach:** An optical readout via a fibre would eliminate the entire amplifier and cabling in the readout circuit. Using a transducer to convert the microwave signal to the optical regime allows the signal to be carried on an optical fibre out of the fridge with significantly lower thermal conductivity than a coaxial cable and no active amplifier heat sources inside the cryostat. This proposal brings together three teams: QphoX, a startup company in the Netherlands with technology to convert microwaves to optical signals; Rigetti UK, a company that builds superconducting quantum computers; and the UK National Quantum Computer Centre (NQCC). **Innovation and Benefits:** QphoX has designed and fabricated a microwave-to-optical transducer chip that works in a dilution fridge at the ultra-low powers present in the readout resonator of a transmon qubit. Rigetti and QphoX have demonstrated a transmon qubit readout using this transducer with the same fidelity as a conventional microwave readout. This proposal aims to demonstrate optical readout with an array of QphoX transducers on a full 9-qubit Rigetti quantum processor at the NQCC. This will allow QphoX to create a new optical readout product by further developing the existing prototypes, Rigetti to evaluate this technology for future larger superconducting quantum processor systems, and the NQCC to demonstrate the results for the wider quantum community and report the results as a trusted source.
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