We experimentally demonstrate a quantum receiver based on the Kennedy scheme for discrimination between two phase-modulated weak coherent states. The receiver is assembled entirely from standard fiber-optic elements and operates at a conventional telecom wavelength of 1.55 $\mu \text{m}$. The local oscillator and the signal are transmitted through different optical fibers, and the displaced signal is measured with a high-efficiency superconducting nanowire single-photon detector. We show the discrimination error rate is two times below that of a shot-noise-limited receiver with the same system detection efficiency.

High sensitivity imaging at the level of single photons is an invaluable tool in many areas, ranging from microscopy to astronomy. However, development of single-photon sensitive detectors with high spatial resolution is very non-trivial. Here we employ the single-pixel imaging approach and demonstrate a proof-of-principle single-pixel single-photon imaging setup. We overcome the problem of low light gathering efficiency by developing a large-area microstrip superconducting single photon detector coupled to a multi-mode optical fiber interface. We show that the setup operates well in the visible and near infrared spectrum, and is able to capture images at the single-photon level.

We present evidence that in the sub-THz frequency band, human skin can be considered as an electromagnetic bio-metamaterial in which its natural emission is a product of skin tissue geometry and embedded structures. Radiometry was performed on 32 human subjects from 480 to 700 GHz. Concurrently, the subjects were exposed to stress, while heart pulse rate (PS) and galvanic skin response (GSR) were also measured. The results are substantially different from the expected blackbody radiation signal of the skin surface. PS and GSR correlate to the emissivity. Using a simulation model for the skin, we find that the sweat duct is a critical element. The simulated frequency spectra qualitatively match the measured emission spectra and show that our sub-THz emission is modulated by our level of mental stress. This opens avenues for the remote monitoring of the human state.

We report on experimental study of the effect of disorder on electronic parameters and inelastic scattering mechanisms in ultrathin superconducting NbN films, which are commonly used in single-photon detectors. An increase in disorder in the studied 2.5-nm-thick NbN films characterized by Ioffe-Regel parameter from 6.3 to 1.6 is accompanied by a decrease in the critical temperature $T_c$ from 11.5 to 3.4 K. By measuring magnetoconductance in the range from $T_c$ to $\sim 3T_c$, we extract the inelastic scattering rates of electrons, including electron-phonon (e-ph) scattering rate ${\tau }_{\text{e-ph}}^{-1}$. We observe that ${\tau }_{\text{e-ph}}^{-1}$ and their temperature dependencies are insensitive to disorder that is not described by the existing models of the e-ph scattering in disordered metals. As temperature decreases, the temperature dependence of ${\tau }_{\text{e-ph}}^{-1}$ varies from $T^3$ to $T^2$, which can be result of a decrease in the dimensionality of phonons involved in the e-ph scattering process. Our results call for further theoretical and experimental studies of the e-ph scattering in ultrathin disordered films.

We report a study of the relaxation time of the restoration of the resistive superconducting state in single crystalline boron-doped diamond using amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown on an insulating diamond substrate have a low carrier density of about $2.5\times {10}^{21}{\mathrm{cm}}^{-3}$ and a critical temperature of about $2\mathrm{K}$. By changing the modulation frequency we find a high-frequency rolloff which we associate with the characteristic time of energy relaxation between the electron and the phonon systems or the relaxation time for nonequilibrium superconductivity. Our main result is that the electron-phonon scattering time varies clearly as $T^{-2}$, over the accessible temperature range of $1.7$ to $2.2$ K. In addition, we find, upon approaching the critical temperature $T_c$, evidence for an increasing relaxation time on both sides of $T_c$.