Investigation of Long-Term Stability of Single-Photon Quantum Key Distribution in a Polarization Coding Scheme

Experimental results demonstrating long-term stability of the operation of our atmospheric quantum cryptography setup using the BB84 protocol and polarization coding are presented. It was shown that the “sifted” quantum key distribution rate and the quantum bit error rate in the key remained constant for 1 hour and were equal to 10 kbit/s and 6.5 %, respectively, at a distance between the transmitter and the receiver equal to 20 cm. Theoretical dependences of the secret quantum key generation rate on a quantum channel transmission coefficient for single-photon detectors, which were used in this experiment, and for new detectors with a reduced level of dark pulses are given.

Superconducting NbN thin films on various (X/Y/Z-cut) lithium niobate substrates

Lithium niobate (LN) exhibits outstanding properties in various application of photonics, electronics, and optoelectronics, showing potentials in integration. Due to the directional dependence of LN tensor properties, optical elements made up by LN favor the type of LN substrate. To introduce high-performance superconducting nanowire single-photon detectors to LN-integrated photonics chips, superconducting NbN thin films with thicknesses from 3 to 50 nm were deposited on X-cut, Y-cut, and Z-cut LN substrates using magnetron sputtering at room temperature. The different thickness dependencies of Tc, δTc, and residual resistance ratios are observed in NbN thin films on different LN substrates. NbN thin films on X-cut and Y-cut LN substrates are polycrystalline with a transition temperature (Tc) of ~6 K for a 6-nm-thick film. While NbN thin films are epitaxially textured on Z-cut LN substrates with Tc of 11.5 K for a 6-nm-thick film. NbN-SNSPD on X-cut LN substrates shows a weak saturation trend of its system detection efficiency; however, the performance of NbN-SNSPD on Z-cut LN substrates is limited. We evaluated the selection of cuts and concluded that X-cut and Y-cut LN are more suitable to be a platform of integrated LN photonic chips from the aspect of NbN-SNSPD. This study helps fabricate high-performance SNSPDs on fully integrated photonics chips on LN substrates.

Phototransduction in Anuran Green Rods: Origins of Extra-Sensitivity

Green rods (GRs) represent a unique type of photoreceptor to be found in the retinas of anuran amphibians. These cells harbor a cone-specific blue-sensitive visual pigment but exhibit morphology of the outer segment typical for classic red rods (RRs), which makes them a perspective model object for studying cone–rod transmutation. In the present study, we performed detailed electrophysiological examination of the light sensitivity, response kinetics and parameters of discrete and continuous dark noise in GRs of the two anuran species: cane toad and marsh frog. Our results confirm that anuran GRs are highly specialized nocturnal vision receptors. Moreover, their rate of phototransduction quenching appeared to be about two-times slower than in RRs, which makes them even more efficient single photon detectors. The operating intensity ranges for two rod types widely overlap supposedly allowing amphibians to discriminate colors in the scotopic region. Unexpectedly for typical cone pigments but in line with some previous reports, the spontaneous isomerization rate of the GR visual pigment was found to be the same as for rhodopsin of RRs. Thus, our results expand the knowledge on anuran GRs and show that these are even more specialized single photon catchers than RRs, which allows us to assign them a status of “super-rods”.

Photon counting statistics with imperfect detectors

Measurement of photon statistics is an important tool for the verification of quantum properties of light. Due to the various imperfections of real single photon detectors, the observed statistics of photon counts deviates from the underlying statistics of photons. Here we analyze statistical properties of coherent states, and investigate a connection between Poissonian distribution of photons and sub-Poissonian distribution of photon counts due to the detector dead-time corrections. We derive a functional dependence between the mean number of photons and the mean number of photon counts, as well as connection between higher-order statistical moments, for the pulsed or continuous wave coherent light sources, and confirm the results by numerical simulations.

Investigation of the parameters of a single photon detector for quantum communication

Nowadays the best single photon detectors from a practical view are those based on InGaAs/InP avalanche photodiodes, operating at a wavelength of 1.55 μm. The dependence of quantum efficiency and noise levels on the temperature and bias voltage of avalanche photodiodes were carried out.

Single-photon detection and cryogenic reconfigurability in lithium niobate nanophotonic circuits

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we demonstrate integration of these two key components in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform.

First-in-clinical application of a time-gated diffuse correlation spectroscopy system at 1064nm using superconducting nanowire single photon detectors

Recently proposed time-gated DCS (TG-DCS) has significant advantages compared to conventional CW-DCS, but it is still in an early stage and clinical capability has yet to be established. The main challenge for TG-DCS is the lower SNR when gating for the deeper travelling late photons. Longer wavelengths, such as 1064nm have a smaller effective attenuation coefficient and a higher power threshold in humans, which significantly increases the SNR. Here, we demonstrate the clinical utility of TG-DCS at 1064 nm in a case study on a patient with severe traumatic brain injury admitted to the neuroscience intensive care unit (NSICU). We showed a significant correlation between TG-DCS early (ρ = 0.67) and late (ρ = 0.76) gated against invasive thermal diffusion flowmetry. We also analyzed TG-DCS at high temporal resolution (50 Hz) to elucidate pulsatile flow data. Overall, this study demonstrates the first clinical translation capability of the TG-DCS system at 1064nm using superconducting nanowire single photon detector.