Credit: © 2008 AIP

Superconducting-nanowire single-photon detectors are potentially a highly attractive solution for many tasks in quantum optics because of their fast recovery rate, low dark-count rate and high quantum efficiency in the infrared regime. However, there's a hitch. The quantum efficiency of conventional designs is dependent on the polarization of light, making them impractical for many applications, such as quantum key distribution.

Now, Sanders Dorenbos and co-workers from Delft University of Technology in The Netherlands, have reported two new designs of superconducting single-photon detector that eradicate the unwanted polarization sensitivity (Appl. Phys. Lett. 93, 161102; 2008). The key is to use a geometry that removes any asymmetry, resulting in uniform detection for photons of any polarization.

One detector design achieves this by being divided into two subparts. Each part contains NbN nanowires (100 nm wide and 4 nm to 6 nm thick) that, importantly, are aligned perpendicularly to the nanowires in the other subpart. Thus both axes of detection (x and y) are covered. The detector has a fill fraction of 50% and an active area of 10 μm × 10 μm. The result is identical quantum efficiency for all polarization directions.

The second detector is made of NbTiN nanowires and is designed in such a way that the wires spiral towards and then away from the centre of the detector — again serving both axes of detection. Wires with the same width and same fill fraction as those in the first detector are used for this geometry; the diameter of the spiralling detector is 10 μm.

Dorenbos and co-workers report quantum efficiencies of 2.6% for the NbN perpendicular detector and 0.6% for the NbTiN spiral detector, at a wavelength of 650 nm. The team say that although both designs are polarization insensitive, the spiral detector is more robust against misalignment.