Superconducting Single-Photon Detectors (Michiel de Dood)

Figure: (left) CAD drawing of a cryogenic scanning probe that could be used to observe the local response of superconducting nanowires. Light enters the setup through the fiber on the right and illuminates a metal tip mounted on a quartz tuning fork above the detector.
(right) Scanning electron microscope image (R. Gaudio and A. Fiore, TU/e) of a nanofabricated constriction comprising the shortest possible nanowire.

 

Superconducting single photon detectors consist of a current carrying superconducting nanowire that can become normal due to the absorption of a single photon at optical frequencies. We do optical experiments at cryogenic temperatures and investigate the physical mechanisms that are important for photon detection.

 

To unveil the physics we use different materials, different detector geometries and explore the use of a sharp metal tip to locally enhance photon absorption. In addition we use a technique called ‘quantum detector tomography’ to extract the quantum response of the detector. The recorded count rates as a function of the average power in a laser beam is inverted to a response of the detector to quantum states with exactly one, two, three etc. photons. This gives a wealth of information about the detector and allows to explore the physics of these devices.

 

The current understanding of NbN nanowires is that entry of magnetic vortices play an important role. This renders the detectors more susceptible to detection of photons at the edges. Very recently we have observed the signature of this effect in the polarization dependent response of detectors. Outstanding challenges are to directly observe the effect or to explore MoGe material where vortices can move through the material with much less dissipation.