Transparent superconductors for single-photon detectors

Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a promising technology for quantum communication, computation, and space exploration. Currently, this technique is the most sensitive and reliable method for detection of single photons. SNSPDs work by breaking Cooper pairs in a superconducting (SC) film. Existing technology mostly utilizes thin films of superconducting metals (Nb, Al) and metal alloys (NbN, TaN, WSi, MoSi), patterned into single-thread meandering nanowires. However, metallic and metal-alloy SC films strongly reflect incident light and have very short penetration depth. In metals the majority of free electrons densely populated electronic bands below and above Fermi level are not involved in the SC state. The light absorbed by these “normal” electrons creates strong Joule heating leading to localized hot spots. These factors affect the relaxation time (detector dead time), intrinsic timing jitter, and the signal-to-noise ratio of the SNSPD. To ensure that photons interact only with Cooper pairs we propose novel transparent superconductors (TSCs) based on transparent electronic conductors (TEC), including indium-tin-oxide (ITO), non-stoichiometric titanium dioxide (TiOx), zinc oxide (ZnOx), and tungsten trioxide (WO3). When optimally intercalated or doped with alkali ions (or hydrogen), these materials exhibit superconductivity with high transparency in the visible and NIR. The SC transition temperature (Tc) can be tuned by controlling the intercalation level (from 0.3 K to 6 K in NaxITO). The high transparency (> 50% at the maximum Tc) and large optical band gap (> 3 eV) eliminate the interaction of incident photons with free electrons of conduction bands and allow selective sensing using a multilayered 3D design. Moreover, the non-stoichiometry of ITO and other TEC oxides show promising long-term stability and tolerance to harsh cosmic radiation.

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