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Circuit-QED-enhanced quantum sensing

Generating, manipulating, amplifying, and detecting microwave photons are among the central experimental methods in state-of-the-art quantum science and technologies. In particular, the strong and controllable interaction between microwave photons and various solid-state artificial atoms is used to protect and read out quantum coherent states and information encoded in engineerable quantum electrical circuits. This versatile hardware architecture—known as circuit quantum electrodynamics (QED) —currently underlies many subdisciplines of quantum information processing, including quantum computing, networking, simulation, and quantum-limited precision sensing.

Advantages of employing microwave quantum measurement for sensing weak physical signals can be found in (i) enhanced sensitivity due to protected electromagnetic environments and low added noise, (ii) high measurement bandwidth for fast signal acquisition, averaging, and parametric sweep, and (iii) close-to-minimal back-action to the physical system and quantities under observation. Moreover, microwave quantum sensing has been empowered by the latest development of superconducting quantum technology, including design and optimization principles of circuit QED modules, the preparation and detection methods of microwave photon states, near-quantum-limited Josephson parametric amplifiers, as well as high-speed quantum control electronics and software tools.

Qµlab is closely collaborating with research groups in condensed matter physics and quantum material science to unleash the potential of microwave quantum sensing. The first launched project along this line is to integrate microwave readout circuits with scanning nanoSQUID-on-tip microscopy—an experimental technique with high sensitivity and spatial resolution for detecting local magnetic fields, which has recently featured in several important discoveries in low-dimensional quantum materials. Other prospective projects include all-microwave control and readout of solid-state spin ensembles for quantum-enhanced magnetometry, and applying non-classical states of microwave photons to achieve sensitivity beyond the standard quantum limit (SQL).

High frequency microwave processing

The ability to operate in high-frequency microwave regimes offers significant advantages in quantum measurement and computation.

In superconducting quantum computing platforms, qubit state measurement is typically performed by scattering a microwave tone off the qubit, with the readout frequency conventionally set close to the qubit's transition frequency. While this method is effective at low powers, increasing the readout power—necessary for faster measurements to mitigate decoherence—can activate spurious multi-excitation resonances. These resonances cause the qubit to leak into non-computational states, violating the quantum non-demolition (QND) nature of the measurement and introducing errors that error correction protocols cannot mitigate. Recent research [Kurilovich, Connolly et al., 2025] has shown that strongly detuning the readout frequency from the qubit frequency can exponentially suppress multi-excitation resonances, enabling high-fidelity QND measurements. By significantly increasing the readout frequency, it is possible to achieve high measurement fidelities while minimizing leakage into non-computational states.

These findings have spurred interest in exploring higher-frequency regimes in circuit quantum electrodynamics (cQED). High-frequency qubits—such as those fabricated using the niobium tri-layer process—also offer additional potential advantages, including higher-temperature operation and reduced unwanted multi-photon transitions. However, conventional electronic setups, both inside and outside dilution refrigerators, are typically constrained to frequencies around 25 GHz, limiting access to these higher operational ranges. For example, SMA connectors are restricted to 18 GHz (or 26.5 GHz in extended versions), and high-performance quantum-limited amplifiers above 20 GHz are currently unavailable.

To overcome these limitations, Qμlab is developing novel frequency conversion techniques to access previously unexplored frequency ranges. Specifically, we are leveraging three-wave mixing to up- and down-convert signals between different frequency domains. This process, in principle, can be performed without introducing additional noise, preserving quantum coherence and enabling high-fidelity operations.

Beyond expanding the accessible frequency range, achieving large frequency offsets between device operation and control/readout lines provides additional benefits, such as enhanced filtering capabilities and reduced noise, ultimately improving overall system performance.

Qμlab, in collaboration with researchers at Qulab at Yale University, is working to implement novel superconducting circuits for frequency conversion and demonstrate a high-frequency superconducting qubit readout chain operating above 20 GHz. This work will pave the way for next-generation high-frequency quantum devices, unlocking new possibilities for quantum information processing.