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# Control and readout of a superconducting qubit using a photonic link

## Abstract

Delivering on the revolutionary promise of a universal quantum computer will require processors with millions of quantum bits (qubits)1,2,3. In superconducting quantum processors4, each qubit is individually addressed with microwave signal lines that connect room-temperature electronics to the cryogenic environment of the quantum circuit. The complexity and heat load associated with the multiple coaxial lines per qubit limits the maximum possible size of a processor to a few thousand qubits5. Here we introduce a photonic link using an optical fibre to guide modulated laser light from room temperature to a cryogenic photodetector6, capable of delivering shot-noise-limited microwave signals directly at millikelvin temperatures. By demonstrating high-fidelity control and readout of a superconducting qubit, we show that this photonic link can meet the stringent requirements of superconducting quantum information processing7. Leveraging the low thermal conductivity and large intrinsic bandwidth of optical fibre enables the efficient and massively multiplexed delivery of coherent microwave control pulses, providing a path towards a million-qubit universal quantum computer.

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## Data availability

The experimental data and numerical simulations presented here are available from the corresponding authors upon request.

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## Acknowledgements

We thank J. Davila-Rodriguez, J. Campbell and E. Ivanov for early contributions to this work. We thank S. W. Nam and K. Lehnert for comments on the manuscript. This work was supported by the NIST Quantum Information Program.

## Author information

Authors

### Contributions

F.L., F.Q., J.A., S.A.D. and J.D.T. conceived and designed the experiment. F.L., F.Q. and J.D.T. built the experimental set-up. F.L. performed the experiment and F.L., F.Q. and J.D.T. analysed the data. K.C. fabricated the transmon qubit. All authors contributed to the manuscript.

### Corresponding authors

Correspondence to F. Lecocq, F. Quinlan or J. D. Teufel.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Joseph Bardin, Blake Johnson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Relaxation time in the presence of optical light.

Histogram of 280 measurements of the relaxation time of the qubit with 2 μW of optical power applied to the photodiode during the qubit evolution. Comparison with data in the absence of optical power confirm that the qubit relaxation time is not affected by stray optical photons, with an average relaxation time T1 ≈ 43 μs. Data were acquired using the set-up in Fig. 2a.

### Extended Data Fig. 2 Photodiode current–voltage characteristic.

Measured d.c. current through the photodiode as a function of voltage bias, in the absence of optical power. The dark current is below the 10-pA resolution of the current meter.

### Extended Data Fig. 3 Vector control with the photonic link.

Ramsey oscillation driven by the photonic link, as a function of the phase θ of the second π/2 pulse (same set-up as Fig. 2d). Data (dots) follow a clean sinusoidal dependence (line). $${R}_{x}^{{\rm{\pi }}/2}$$ denotes a π/2 qubit rotation around the x axis and $${R}_{\theta }^{{\rm{\pi }}/2}$$ denotes a π/2 qubit rotation around an axis with a variable angle θ within the xy plane. qb, qubit; cav., cavity.

### Extended Data Fig. 4 Dilution refrigerator wiring.

Details of the circuitry employed in the cryostat for qubit measurement and control experiments. The qubit cavity device is placed inside a double layer cryoperm shield. FPJA, field-programmable Josephson amplifier (see text); HEMT, high-electron-mobility transistor amplifier, LPF, low-pass filter.

### Extended Data Fig. 5 Simplified room-temperature set-up.

The FPJA pump, cavity local oscillator (LO) and demodulation LO share a 1-GHz reference clock and are locked to all other instruments via a 10-MHz reference clock. A master trigger, not shown, is shared via a distribution amplifier. There are slight differences in the set-up between the qubit control and measurement experiments. Amplification and attenuation levels are slightly different. The FPJA pump is pulsed on only during the qubit measurement. AWG, arbitrary waveform generator; BPF, band-pass filter; DR, dilution refrigerator; LPF, low-pass filter; VNA, vector network analyser.

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Reprints and Permissions

Lecocq, F., Quinlan, F., Cicak, K. et al. Control and readout of a superconducting qubit using a photonic link. Nature 591, 575–579 (2021). https://doi.org/10.1038/s41586-021-03268-x

• Accepted:

• Published:

• Issue Date:

• ### A photonic link for quantum circuits

• Koji Usami
•  & Yasunobu Nakamura

Nature Electronics (2021)

• ### A cryogenic electro-optic interconnect for superconducting devices

• Amir Youssefi
• , Itay Shomroni
• , Yash J. Joshi
• , Nathan R. Bernier
• , Anton Lukashchuk
• , Philipp Uhrich
• , Liu Qiu
•  & Tobias J. Kippenberg

Nature Electronics (2021)