Superconducting quantum processors are wired with individual microwave coaxial lines per qubit, and the passive heat load and wiring complexity of these lines are widely expected to cap processor sizes at a few thousand qubits. Lecocq et al. (Nature 591, 575 (2021)) demonstrated an alternative: a photonic link in which modulated laser light travels down an optical fiber to a cryogenic photodiode at millikelvin temperature, which delivers shot-noise-limited microwave pulses that drive and read out a single superconducting qubit with high fidelity. The paper identified a coherent set of unresolved issues — consolidated in the outlook of the published Nature discussion — that must be settled before such links can be deployed at processor scale: the active optical dissipation per qubit and its duty-cycle budget, drive-line impedance trade-offs, experimental demonstration of multiplexing many microwave channels onto one optical carrier (including nonlinear mixing products and broadband photodiode white noise on simultaneously addressed qubits), dense optical/microwave integration, and suppression of the measured background thermal occupancies. No demonstration at processor scale and no settled characterization of multiplexed-control crosstalk is known in the literature to date, so the problem remains open.

Background

A superconducting quantum processor stores quantum bits (qubits) in lithographed microwave circuits cooled to millikelvin temperatures in a dilution refrigerator (the standard cryostat for such processors, whose coldest stage typically provides only tens of microwatts of cooling power); the most common qubit type, the transmon, is read out dispersively by coupling it to a microwave cavity whose resonance frequency shifts with the qubit state. Every qubit needs microwave drive ('control') lines delivering coherent pulses, and state readout requires detecting faint microwave fields, both conventionally done with room-temperature electronics connected through heavily attenuated coaxial cables. These cables impose a passive heat load (measured at about 14 nW per line at the coldest stage) and enormous wiring complexity, so the cryogenic input/output (I/O) interface is a recognized bottleneck for scaling beyond a few thousand qubits.

One proposed replacement is the photonic link: an optical fiber (with negligible passive heat load, about 3 pW) carries modulated laser light from room temperature to a high-speed photodiode at millikelvin temperature, and the photocurrent directly generates the microwave signals at the qubit. Lecocq, Quinlan, Cicak, Aumentado, Diddams and Teufel (Nature 591, 575-579 (2021)) demonstrated this for a single qubit: the link drove Rabi oscillations (coherent oscillations of the qubit state under a resonant microwave drive) and high-fidelity control and readout, and they used the qubit itself as a sensitive noise meter to show that the generated microwave noise is set by the optical shot noise (the fundamental fluctuation of photon arrival, giving a white current-noise spectrum up to the photodiode bandwidth). They also modeled scaling: with passive heat load negligible, the active heat load — optical power dissipated in the photodiode during gates — becomes the limit, so the maximum qubit count for a given cooling power scales inversely with the duty cycle (fraction of time each qubit is driven). Because the photodiode acts as a near-ideal current source, raising the drive-line impedance ZZ above the standard 50 ohms reduces the photocurrent, and hence optical power, needed for a given microwave power, further increasing the supported qubit count. A parallel research line replaces the dissipative photodiode with coherent electro-optic or optomechanical transduction, in which microwave and optical resonators are coupled parametrically so microwave signals convert to optical signals (and back) without dissipating optical power at the qubit chip; examples include the cryogenic electro-optic interconnect of Youssefi et al. (Nature Electronics 4, 326 (2021)), low-backaction electro-optic qubit readout by Delaney et al. (Nature 606, 489 (2022)), and coherent optical control of a transmon via an electro-optic transducer by Warner et al. (Nature Physics 21, 831 (2025)).

The 2021 demonstration also measured unwanted background thermal occupancy of the qubit's microwave environment — about 1% for the readout cavity and about 10% for the qubit at zero photocurrent — attributed to imperfect thermalization and filtering, which would need to be suppressed for fault-tolerant operation. The paper's closing discussion (published Nature version) states that several practical and physics-level issues must be addressed before photonic links can be deployed at processor scale (many thousands to millions of qubits operating continuously): active optical dissipation per qubit, impedance-engineering trade-offs, cryogenic multiplexing of many microwave channels onto shared optical carriers, dense low-loss integration, and background thermal occupancy. Reviews and hardware roadmaps through 2024-2026 (e.g., Joshi and Moazeni on cryogenic RF-photonics; the quantum-supercomputer roadmap of Mohseni et al.) treat cryogenic photonic I/O as a leading but still-open scaling route.

Problem Statement

Establish and experimentally demonstrate a photonic-link architecture that can carry the control and readout signals of a processor-scale superconducting quantum processor — that is, resolve, jointly and at the multi-qubit level, the issues that Lecocq et al. (2021) identified as blocking deployment of photonic links at processor scale: (1) reduce the active optical dissipation per qubit (through photodiode/link efficiency, higher drive-line impedance, multiplexing many microwave channels per optical carrier, or short optical pulses) so that continuous-duty-cycle operation supports processor-scale qubit counts within realistic dilution-refrigerator cooling budgets; (2) settle the engineering trade-offs of the drive-line impedance choice for the photodiode-based link — the trade-off among drive-line impedance (50 ohms versus raised values such as 10 kOhm), photodiode electrical bandwidth and required optical power, with effects such as standing-wave constraints on longer interconnects and consequences for gate fidelity; (3) demonstrate and characterize cryogenic multiplexing of many microwave channels onto shared optical carriers or via time-domain multiplexing, including the nonlinear mixing products of the link and the impact of broadband photodiode white (shot) noise on all simultaneously addressed qubits; (4) achieve dense, low-loss, low-stray-photon optical and microwave routing and connectorization compatible with large systems; and (5) suppress the background thermal occupancies of the qubit environment (the roughly 1% cavity and 10% qubit backgrounds measured in the source experiment) by appropriate cryogenic means, such as improved filtering, attenuator thermalization and shielding. The overarching objective is a photonic link whose measured heat load, noise, crosstalk and fidelity at multi-qubit scale are jointly compatible with continuous operation of a large superconducting processor; the five items are linked facets of that single objective, not independent targets. The operative completion criterion is a multi-qubit experimental demonstration (at least two qubits) that jointly resolves the five facets, each facet addressed by quantitative estimates, with the demonstration's own measured per-qubit dissipation, noise and duty cycle — not the source's published baselines re-inserted — establishing processor-scale viability through the source's own heat-load/duty-cycle scaling analysis rather than by operating a many-thousand-qubit processor; an architecture design or scaling analysis alone, without such multi-qubit measurements, does not satisfy the criterion. Quantitatively, the frame is set by the source's own scaling analysis and measured baselines (about 20 uW of mixing-chamber cooling power; about 3 pW passive load per fiber versus about 14 nW per coaxial line; qubit count inversely proportional to per-qubit duty cycle, with recent processors operating at duty cycles of order 1% or less; the roughly 1%/10% background occupancies).

References

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