Chalmers Team Builds 47% Efficient Microwave Photon Detector

A continuous detection efficiency of 0.47 over a 5MHz bandwidth represents an advance in microwave single-photon detection. Previous detectors operated cyclically, creating a trade-off between detection efficiency and continuous operation duration. Vyom Kulkarni and colleagues at Chalmers University of Technology have engineered a continuously operated detector, utilising a superconducting artificial molecule to capture incoming photons. This new approach overcomes previous limitations, enabling continuous microwave photon detection for applications including quantum sensing and fundamental physics.

The device achieves an efficiency of 0.47, enabling continuous operation vital for applications like quantum sensing and exploring fundamental physics. Existing detectors functioned cyclically, compromising between photon detection efficiency and uninterrupted operation. Vyom Kulkarni and colleagues have unveiled a new microwave photon detector with continuous operation, a sharp advancement over previous devices.

Existing detectors functioned cyclically, registering photons efficiently only during specific ‘on’ periods, missing signals during reset or measurement phases. This new detector utilises a superconducting artificial molecule, a tiny engineered circuit mimicking natural molecule behaviour but controlling properties using superconductivity, to continuously capture incoming photons. The team employs a carefully controlled driven-dissipative process to manage energy flow within the artificial molecule and reveal photon “clicks” as subtle changes in its state. Achieving a continuous detection efficiency of 0.47 over a 5MHz bandwidth, this technology promises to unlock new possibilities in quantum sensing and fundamental physics.

Superconducting transmons enable efficient single-photon detection via energy state transitions

A continuously operated microwave single-photon detector utilising a superconducting artificial molecule has been demonstrated. It consists of two coupled transmons, engineered to capture photons in a high-energy ‘bright’ state before transferring them to a stable, low-energy ‘dark’ state. A driven-dissipative process, a carefully controlled method of adding and removing energy, managed the flow of energy and revealed the presence of incoming photons.

The detector achieved a cyclic detection efficiency of 0.73 and a continuous detection efficiency of 0.47 over a 5MHz bandwidth, with a temporal resolution of 1μs and a 15μs dead time. Existing cyclical detectors faced limitations that this approach overcomes. The 0.47 continuous detection efficiency achieved over a 5MHz bandwidth represents an improvement over previously limited pulsed detectors, signifying a move beyond the inherent trade-off between efficiency and continuous runtime.

By utilising the superconducting artificial molecule, the device captures incoming photons and transfers them to the stable ‘dark’ state, enabling constant monitoring without interruption. Stable operation beyond brief pulsed measurements was observed, alongside a dark count rate of 1.3kHz and a continuous detection efficiency of 0.47.

Photon events are revealed as quantum jumps in the continuously monitored dark state, enabling advancements in quantum sensing and fundamental physics. The detector employs a four-level system implemented within a superconducting circuit, where two coupled transmon qubits, tuned using flux lines, form an ‘artificial molecule’ that captures incoming microwave photons.

Initially, photons are absorbed by a ‘bright’ symmetric state before being transferred to a long-lived ‘dark’ antisymmetric state via the driven-dissipative process, a technique utilising both coherent excitation and natural relaxation. Symmetry-selective couplings and the Purcell effect, where resonators enhance decay rates for specific states, protect the dark state from unwanted signal loss. However, the team acknowledges that a strong pump signal, necessary for continuous operation, can induce heating within the detector and its surrounding environment, potentially limiting long-term durability and overall performance.

Microwave photon detector overcomes operational limits with continuous signal processing

Establishing continuous operation represents a step towards realising practical quantum technologies, finally addressing a longstanding weakness in microwave photon detection compared to its optical counterpart. a 15μs dead time still affects the detector, a period of insensitivity following each photon capture. This limitation, stemming from the need for the detector’s ‘dark’ state to reset before registering another event, could prove problematic in high-count-rate experiments, demanding careful calibration and potentially restricting the scope of certain applications.

This fifteen-microsecond dead time represents a clear area for improvement and limits immediate applicability in scenarios demanding exceptionally high photon count rates. The demonstration of continuous operation is a significant advance, overcoming a fundamental limitation of previous microwave photon detectors which operated in a start-stop fashion. The artificial molecule approach, utilising a ‘dark’ state to signal photon capture, paves the way for more sensitive quantum sensors and opens new avenues for exploring quantum thermodynamics and fundamental physics.

The researchers have created a microwave photon detector that operates without interruption, a significant improvement over previous devices. This continuous operation circumvents a longstanding trade-off between detection efficiency and runtime, previously hindering the development of practical quantum technologies.

The researchers demonstrated a continuously operated microwave single-photon detector with a cyclic detection efficiency of 0.73 and a continuous detection efficiency of 0.47 over a 5MHz bandwidth. This achievement overcomes a limitation of earlier detectors, which cycled between ‘on’ and ‘off’ states, and allows for uninterrupted signal processing. The detector uses a superconducting artificial molecule to capture photons and signals their arrival via quantum jumps in a monitored ‘dark’ state. The authors note a 15 s dead time remains, which may require consideration in experiments needing very high photon count rates.