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Delivering on the revolutionary promise of a universal quantum computer will require processors with millions of quantum bits (qubits)1–3. 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.

We demonstrate an easy-to-manufacture 25-mm-long ultra-stable optical reference cavity for transportable photonic microwave generation systems. Employing a rigid holding geometry that is first-order insensitive to the squeezing force and a cavity geometry that improves the thermal noise limit at room temperature, we observe a laser phase noise that is nearly thermal noise limited for three frequency decades (1 Hz to 1 kHz offset) and supports 10 GHz generation with phase noise near −100  dBc/Hz at 1 Hz offset and <−173  dBc/Hz for all offsets >600  Hz. The fractional frequency stability reaches 2×10^−15 at 0.1 s of averaging.

Lithium cobalt oxide (LiCoO2) was synthesized using two different sol−gel processes. The first method used stoichiometric molar ratios of cobalt 2-methoxyethoxide and lithium nitrate in alcoholic solutions; the second method used aqueous equimolar ratios of precursors of lithium nitrate and cobalt acetate with poly(ethylene glycol) (PEG) 200 as the chelating agent. Lithium cobalt oxide films were deposited on Si wafers and silica−soda−lime glass using a dipping technique. Based on differential thermal analysis/thermogravimetric analysis results, densification of the films was achieved by thermal treatment at both 500 and 800 °C in the case of silicon wafer substrates and at 500 °C for the silica−soda−lime glass. X-ray diffraction, spectroellipsometry, and atomic force microscopy were used to characterize the films. The correlation between the preparation procedure and the type of support on the structure and morphology of LiCoO2 compound films is discussed. Uniform ultrathin films were obtained on the glass (heat treated at 500 °C) and Si wafer (heat treated at 800 °C). The alcoholic synthetic procedure was also used to coat particles of LiMn2O4 with films of LiCoO2. The sol−gel method resulted in partial coverage of LiMn2O4 by LiCoO2 as determined by elemental mapping with scanning electron microscopy.

Surface reactions are responsible for the cycling performance of Li-ion rechargeable batteries based on LiMn2O4. We report structural and electrochemical studies of the LiMn2O4 cathode at room temperature in LiPF6 electrolyte using in situ electrochemical atomic force microscopy (EC-AFM) and lateral force microscopy (LFM). Surface dynamics were monitored in situ in organic electrolyte under potentiostatic conditions. After the first charge/discharge cycle, surface features were clearly visible. During the second charging process at 4.3 V vs Li/Li⁺, dissolution of surface particles was observed. The surface topography was quantitatively analyzed by height distribution functions. The dissolution rate (calculated from the change of particle volume with time) was found to follow the square root of time. During the second discharge process at 3.8 V vs Li/Li⁺, new particles were formed on the surface. However, there were no further changes in the surface topography with time during polarization at 3.8 V vs Li/Li⁺. The surface dynamics monitored for various charge/discharge conditions showed that a complex dissolution/precipitation reaction of manganese and lithium compounds is involved in the charge/discharge process of spinel LiMn2O4-based cathode material.

Surface chemistry of the capacity fading of the LixMn2O4 cathode was investigated using atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS). Measurements show a decrease in the cathode capacity from 124 mA h g^-1 before storage to 102 mA h g^-1 after storage in an electrolyte of 1 M LiPF6/EC + DMC + DEC at 70 °C for 5 days. Surface morphological changes of the LixMn2O4 cathode were monitored using contact and tapping AFM and lateral force microscopy in air. Nanoscale changes of the charged cathode before and after storage at 70 °C were observed. Before storage, homogeneous grains of approximately 100−200 nm are seen. After storage, fine and nearly round shaped structures of 10−30 nm in size are observed covering the larger grains on the surface of the cathode. This change in morphology suggests film deposition on the cathode's surface, which increases the resistance for Li⁺ ion transport in and out of the cathode. Results from EDAX show that compounds containing phosphorus and fluorine are also deposited on the surface of the cathode. Surface analysis of the cathode with XPS suggests the presence of MnF2. The conversion of the oxidation state of manganese on the surface of the cathode from MnO2 to MnO during storage at the elevated temperature was observed with XPS.

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We report the generation of optical pulse trains with 8.5 fs timing jitter (10 Hz to 10 MHz) from a mode-locked semiconductor laser, with a slab coupled optical waveguide amplifier used as the gain element. This is, to our knowledge, the lowest residual timing jitter reported to date from an actively mode-locked laser.