Abstract: Achievement of the ultimate sensitivity along with a high spectral resolution is one of the frequently addressed problems, as the complication of the applied and fundamental scientific tasks being explored is growing up gradually. In our work, we have investigated performance of a superconducting nanowire photon-counting detector operating in the coherent mode for detection of weak signals at the telecommunication wavelength. Quantum-noise limited sensitivity of the detector was ensured by the nature of the photon-counting detection and restricted by the quantum efficiency of the detector only. Spectral resolution given by the heterodyne technique and was defined by the linewidth and stability of the Local Oscillator (LO). Response bandwidth was found to coincide with the detector’s pulse width, which, in turn, could be controlled by the nanowire length. In addition, the system noise bandwidth was shown to be governed by the electronics/lab equipment, and the detector noise bandwidth is predicted to depend on its jitter. As have been demonstrated, a very small amount of the LO power (of the order of a few picowatts down to hundreds of femtowatts) was required for sufficient detection of the test signal, and eventual optimization could lead to further reduction of the LO power required, which would perfectly suit for the foreseen development of receiver matrices and the need for detection of ultra-low signals at a level of less-than-one-photon per second.

Abstract: We present a novel concept of photon number resolving detector based on 120-nm-wide superconducting stripes made of 4-nm-thick NbN film and connected in parallel (PNR-SSPD). The detector consisting of 5 strips demonstrate a capability to resolve up to 4 photons absorbed simultaneously with the single-photon quantum efficiency of 2.5% and negligibly low dark count rate.

Abstract: Superconducting single-photon detector (SSPD) is a planar nanostructure patterned from 4-nm-thick NbN film deposited on sapphire substrate. The sensitive element of the SSPD is 100-nm-wide NbN strip. The device is operated at liquid helium temperature. Absorption of a photon leads to a local suppression of superconductivity producing subnanosecond-long voltage pulse. In infrared (at 1550 nm and longer wavelengths) SSPD outperforms avalanche photodiodes in terms of detection efficiency (DE), dark counts rate, maximum counting rate and timing jitter. Efficient single-mode fibre coupling of the SSPD enabled its usage in many applications ranging from single-photon sources research to quantum cryptography. Recently we managed to improve the SSPD performance and measured 25% detection efficiency at 1550 nm wavelength and dark counts rate of 10 s-1. We also improved photon-number resolving SSPD (PNR-SSPD) which realizes a spatial multiplexing of incident photons enabling resolving of up to 4 simultaneously absorbed photons. Another improvement is the increase of the photon absorption using a λ/4 microcavity integrated with the SSPD. And finally in our strive to increase the DE at longer wavelengths we fabricated SSPD with the strip almost twice narrower compared to the standard 100 nm and demonstrated that in middle infrared (about 3 μm wavelength) these devices have DE several times higher compared to the traditional SSPDs.