Microwave photons are crucial for various quantum systems, including superconducting circuits, quantum dots and spin qubits. Detecting single microwave photons is critical for realizing the potential of various quantum technologies, from quantum computing and secure quantum communication to electron spin resonance spectroscopy or even the detection of dark matter. While optical photons can readily be detected at room temperature, single microwave photon detectors (SMPDs) must operate at cryogenic temperatures, where thermal fluctuations are sufficiently low to permit single-shot detection of indicvidual photons.
Our approach relies on circuit quantum electrodynamics (cQED) techniques to implement an efficient microwave photon counter using a superconducting qubit and resonant cavities.
The operating principle is to absorb an incoming microwave photon irreversibly in the qubit, mapping the photon state onto the qubit state. The qubit state is then read out projectively using another cavity, allowing detection of the photon arrival. Key innovations include using a four-wave mixing process to imprint the presence or absence of a photon onto the state of a transmon qubit and engineering the dissipation of an auxiliary cavity to ensure the passage of a photon through the device is irreversible, enabling continuous operation of the SMPD.
Compared to previous microwave photon counters, our SMPD achieves higher efficiency, lower dark counts, and continuous broadband operation. We have implemented design and fabrication improvements, including using tantalum thin films, to push the sensitivity to record levels exceeding 10^-22 W/√Hz. The exceptional performance of the SMPD enables breakthrough applications in quantum sensing. We have used it to detect single electron spins in crystals by electron spin resonance techniques, an achievement not possible with previous detectors. The SMPD is a versatile platform that can be extended to detect spins in a wide range of solid-state systems.
Ongoing work focuses on further improving the SMPD sensitivity and coupling it with spin ensembles. Our vision is to exploit the SMPD for high-resolution quantum sensing to image fields and correlations at the nanoscale using the techniques of magnetic resonance.
Above: A single microwave photon detector mounted inside its housing
Principal Investigator: Emmanuel Flurin