photonnumberresolving

Photon-number resolving detection refers to the ability of a detector to determine the exact number of photons in a light pulse, rather than merely signaling the presence or absence of light. This capability is essential for characterizing quantum states of light, such as Fock states, and for performing precise photon-statistics measurements in quantum optics and quantum information experiments.

Technologies and approaches

True photon-number resolution is most robust in transition-edge sensors (TES), which are cryogenic superconducting calorimeters that convert absorbed photon energy into a measurable temperature rise. TES devices can resolve multiple photons per pulse with high quantum efficiency, but require complex cryogenics and typically have slower response times. For more scalable or faster systems, photon-number information with detectors like superconducting nanowire single-photon detectors (SNSPDs) is achieved through multiplexing. Spatial multiplexing uses arrays of detectors, while time multiplexing splits a pulse into multiple time bins with delays. In both cases, the distribution of detection events across channels or time bins provides an estimate of the photon number, though true, intrinsic resolution is detector-dependent.

Limitations and performance

Performance is governed by detection efficiency, dark counts, dead time, and readout bandwidth. TES offers high energy resolution but limited speed and operational complexity. Multiplexed SNSPD approaches trade off simplicity and speed for limited dynamic range and calibration requirements. While capable of counting multiple photons, the accuracy improves with more channels and careful calibration.

Applications

Photon-number resolving detectors enable quantum state tomography, characterization of nonclassical light, heralded single-photon sources, and measurements in quantum metrology and imaging. They also support security in quantum key distribution by enabling better assessment of photon-number statistics.