A frequency-agile microwave-optical interface for superconducting qubits

Abstract

Superconducting quantum processors operate at microwave frequencies in millikelvin environments, making it challenging to interconnect distant nodes using conventional microwave wiring. Coherent microwave-to-optical (M2O) transduction enables superconducting quantum networks by interfacing itinerant microwave photons with low-loss optical fiber. However, many state-of-the-art transducers provide efficient conversion only over a narrow frequency span, complicating deployment with heterogeneous superconducting devices that are detuned by gigahertz-scale offsets. Here we demonstrate a frequency-agile microwave-optical interface that overcomes this bandwidth mismatch by cascading an electro-optic M2O transducer with a multimode microwave-to-microwave (M2M) frequency converter, with in situ tunability of the microwave resonances in both stages. Using this architecture, we realize continuous frequency coverage from 5.0 to 8.5 GHz within a single system. As an application relevant to superconducting-qubit networking, we use the cascaded M2M-M2O interface to optically read out a superconducting qubit whose readout resonator is detuned by 1.7 GHz from the native M2O microwave resonance, demonstrating a scalable route toward fiber-linked superconducting quantum nodes.

Figures (9)

• Figure 1: Concept of a cascaded M2M--M2O microwave--optical interface with deterministic frequency matching.(a) Frequency-agile qubit readout via cascaded conversion. A fixed-frequency qubit readout tone is first sent to a multimode M2M converter, where an applied M2M pump enables coherent translation from a selected signal mode to a nearby idler mode. The translated microwave tone is then routed to an M2O transducer and converted to an optical sideband under optical pumping. (b) Deterministic frequency-matching protocol. The qubit readout frequency (black, fixed) is initially detuned from the M2O microwave resonance (green). The M2M spectrum (orange) is flux-tuned to bring a signal mode into resonance with the readout tone. The M2O microwave resonance is then flux-tuned to the nearest M2M mode, designated as the idler. With the M2M pump applied, the readout signal is coherently converted from the signal mode to the idler mode, bridging the frequency gap and enabling subsequent microwave-to-optical transduction in the M2O stage.
• Figure 2: Transducer characterization.(a) Simplified schematic of the setup to characterize the transduction efficiency. SSBM, single sideband modulator; AOM, acousto-optic modulator; LO, local oscillator. (b) Magnetic-field tuning of the microwave resonance. Measured frequency shift $\Delta f_{\mathrm{m}}$ (relative to the zero-field resonance) as a function of applied magnetic field. (c) Calibrated bidirectional electro--optic conversion efficiency spectrum $|S|^{2}$ at three magnetic-field bias points (0, 2.4, and 4.0 mT). Inset: pumping scheme with a strong optical pump at $\omega_-$ enables microwave to blue-sideband light conversion.
• Figure 3: M2M--M2O cascaded conversion setup and performance.(a) Experimental schematic of the cascaded M2M--M2O measurement. A microwave signal is first frequency-translated by the M2M stage and then converted to the optical domain by the M2O transducer. (b) Flux tuning of the M2M signal and idler frequencies relative to the fixed M2O operating frequency at 5.637 GHz. The shaded region indicates the accessible M2O tuning range. (c) Pulse diagram of the measurement sequence. (d) Calibrated conversion-efficiency spectra. Panel (i) shows the baseline M2O-only conversion efficiency at 5.637 GHz, which sets the reference performance (indicated by the dashed line). Panels (ii--vi) show the cascaded M2M--M2O conversion efficiency at several translated microwave signal frequencies from 5.0 to 8.5 GHz.
• Figure 4: Optical readout of a superconducting qubit using a cascaded M2M--M2O link.(a) Measurement schematic: the qubit readout tone is routed through an M2M frequency converter (providing frequency translation and gain) before entering the M2O transducer for optical detection. (b) Deterministic frequency matching among the qubit readout, the M2M mode pair, and the M2O microwave resonance. Flux tuning aligns the M2M signal mode (orange) with the fixed qubit readout frequency (orange dashed line) while shifting the corresponding idler mode (blue). The M2O microwave resonance (green) is then tuned to coincide with the idler. (c) Pulse sequence for the qubit $T_1$ experiment with optical readout. (d-f) Rabi oscillations, $T_1$, and $T_2$ measurements with optical (orange) and conventional microwave (blue) readout.
• Figure S1: Optical micrograph of the M2O device. (a) The superconducting resonator is deposited above the AlN double-ring resonator. (b) Expanded view of the capacitor and dc electrodes of the superconducting resonator.