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arXiv:2608.12972

Always-on, highly efficient microwave photon detector based on a superconducting artificial molecule

Vyom Kulkarni, Mohammed Ali Aamir, Simon Sundelin, Simone Gasparinetti

quant-phSubmitted Aug 13, 2026• Announced Aug 14, 2026• 0 citations
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Abstract

Efficient detection of single microwave photons is a key capability for emerging quantum technologies. Yet, it remains far less developed than its optical domain counterpart. Realizing detectors that simultaneously achieve high efficiency, low dark counts, and continuous operation has proved challenging. Existing detectors operate cyclically, forcing a trade-off between efficiency and duty cycle. Here, we demonstrate a continuously operated microwave single-photon detector based on a superconducting artificial molecule. In our scheme, an incoming photon is captured by a bright state of the molecule and then transferred to a long-lived dark state via a driven-dissipative process. Photon ``clicks'' are revealed as quantum jumps in the continuously monitored dark state. We observe a cyclic detection efficiency of $0.73$, and a continuous detection efficiency of $0.47$ over a $5\,\mathrm{MHz}$ instantaneous bandwidth, with a $1\,μ\mathrm{s}$ temporal resolution and a $15\,μ\mathrm{s}$ dead time. By overcoming the trade-off between efficiency and duty cycle, this approach establishes continuous microwave photon detection for quantum sensing, quantum thermodynamics, and fundamental physics.

Comments
14 pages, 10 figures

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The detection of single microwave photons is becoming a cornerstone of fundamental experiments in circuit quantum electrodynamics and quantum sensing. Experiments in cavity quantum electrodynamics, photon-mediated entanglement, dark-matter axion searches and quantum thermodynamics increasingly rely on single-photon sensitivity. Current technologies exploit superconducting devices, including Josephson parametric detectors, and transmon-based non-demolition counters for dark-matter haloscopes. Hybrid double quantum dot-Josephson systems reach interesting levels of efficiency, while current-biased Josephson junctions show potential for near-quantumlimited resolution. Despite impressive progress, challenges remain in reducing dark counts, improving reset speed, and scaling for multiplexed arrays. Continued advances in superconducting materials, qubit coherence, and hybrid semiconductor-superconductor designs promise to extend detection efficiency and bandwidth. These developments are poised to enable the next generation of quantum-enabled experiments, from foundational tests in quantum optics to searches for new physics.
Single-photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an impedance-matched artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. Each signal photon deterministically induces a Raman transition in the Λ system and excites the qubit. The subsequent dispersive readout of the qubit produces a discrete 'click'. We attain a high single-photon-detection efficiency of 0.66±0.06 with a low dark-count probability of 0.014±0.001 and a reset time of ∼400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.
The realization of a high-efficiency microwave single photon detector is a long-standing problem in the field of microwave quantum optics. Here, we propose a quantum nondemolition, high-efficiency photon detector that can readily be implemented in present state-of-the-art circuit quantum electrodynamics. This scheme works in a continuous fashion, gaining information about the photon arrival time as well as about its presence. The key insight that allows us to circumvent the usual limitations imposed by measurement backaction is the use of long-lived dark states in a small ensemble of inhomogeneous artificial atoms to increase the interaction time between the photon and the measurement device. Using realistic system parameters, we show that large detection fidelities are possible.
Single-photon detection played an important role in the development of quantum optics. Its implementation in the microwave domain is challenging because the photon energy is five orders of magnitude smaller. In recent years, significant progress has been made in developing single microwave photon detectors (SMPDs) based on superconducting quantum bits or bolometers. In this paper we present a practical SMPD based on the irreversible transfer of an incoming photon to the excited state of a transmon qubit by a four-wave mixing process. This device achieves a detection efficiency $\ensuremath{\eta}=0.43$ and an operational dark count rate $\ensuremath{\alpha}=85\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, mainly due to the out-of-equilibrium microwave photons in the input line. The corresponding power sensitivity is $\mathcal{S}={10}^{\ensuremath{-}22}\phantom{\rule{0.2em}{0ex}}\mathrm{W}/\sqrt{\mathrm{Hz}}$, one order of magnitude lower than the state of the art. The detector operates continuously over hour time-scales with a duty cycle ${\ensuremath{\eta}}_{D}=0.84$, and offers frequency tunability of at least 50 MHz around 7 GHz.
Cyclically operated Single Microwave Photon Counter with 10−2210^\mathrm{-22}10−22 W/Hz\mathrm{W/\sqrt{Hz}}W/Hz​ sensitivity
Single photon detection played an important role in the development of quantum optics. Its implementation in the microwave domain is challenging because the photon energy is 5 orders of magnitude smaller. In recent years, significant progress has been made in developing single microwave photon detectors (SMPDs) based on superconducting quantum bits or bolometers. In this paper we present a practical SMPD based on the irreversible transfer of an incoming photon to the excited state of a transmon qubit by a four-wave mixing process. This device achieves a detection efficiency η=0.43η= 0.43η=0.43 and an operational dark count rate α=85α= 85α=85 s−1\mathrm{s^{-1}}s−1, mainly due to the out-of-equilibrium microwave photons in the input line. The corresponding power sensitivity is S=10−22\mathcal{S} = 10^{-22}S=10−22 W/Hz\mathrm{W/\sqrt{Hz}}W/Hz​, one order of magnitude lower than the state of the art. The detector operates continuously over hour timescales with a duty cycle ηD=0.84η_\mathrm{D}=0.84ηD​=0.84, and offers frequency tunability of at least 50 MHz around 7 GHz.
Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photon extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an "impedance-matched" artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ~400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.