Conventional quantum key distribution (QKD) typically uses binary encoding based on photon polarization or time-bin degrees of freedom and achieves a key capacity of at most one bit per photon. Under photon-starved conditions the rate of detection events is much lower than the photon generation rate, because of losses in long distance propagation and the relatively long recovery times of available singlephoton detectors. Multi-bit encoding in the photon arrival times can be beneficial in such photonstarved situations. Recent security proofs indicate high-dimensional encoding in the photon arrival times is robust and can be implemented to yield high secure throughput. In this work we demonstrate entanglement-based QKD with high-dimensional encoding whose security against collective Gaussian attacks is provided by a high-visibility Franson interferometer. We achieve unprecedented key capacity and throughput for an entanglement-based QKD system because of four principal factors: Franson interferometry that does not degrade with loss; error correction coding that can tolerate high error rates; optimized time-energy entanglement generation; and highly efficient WSi superconducting nanowire single-photon detectors. The secure key capacity yields as much as 8.7 bits per coincidence. When optimized for throughput we observe a secure key rate of 2.7 Mbit s −1 after 20 km fiber transmission with a key capacity of 6.9 bits per photon coincidence. Our results demonstrate a viable approach to high-rate QKD using practical photonic entanglement and single-photon detection technologies.

We report on MoSi SNSPDs which achieved high system detection efficiency (87.1 ± 0.5% at 1542 nm) at 0.7 K and we demonstrate that these detectors can also be operated with saturated internal efficiency at a temperature of 2.3 K in a Gifford-McMahon cryocooler. We measured a minimum system jitter of 76 ps, maximum count rate approaching 10 MHz, and polarization dependence as low as 3.3 ± 0.1%. The performance of MoSi SNSPDs at 2.3 K is similar to the performance of WSi SNSPDs at < 1 K. The higher operating temperature of MoSi SNSPDs makes these devices promising for widespread use due to the simpler and less expensive cryogenics required for their operation. Sobolewski, "Response time characterization of NbN superconducting single-photon detectors," IEEE Trans. Appl. Supercond. 13(2), 180-183 (2003). 11. H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, "Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors," Nat. Photonics 1(6), 343-348 (2007

In this paper, we characterize the rotational dynamics and observe rotor-rotor interactions within a crystalline, three-dimensional array of dipolar molecular rotors. The rotating portion of each rotor molecule consists of a dipolar fluorine-substituted phenyl group. The phenyl rotors are connected by acetylene linkages to bulky triphenyl methyl groups which are held rigid in the crystal lattice. These custom synthesized rotor molecules allow control over the molecular spacing in the lattice, the dipole strength, and the rotational hindrance, thus permitting formation of systems with rapid thermal rotation and strong dipole-dipole interactions, which is of interest for studying new phases and collective phenomena. Dielectric and 2 H NMR spectroscopy measurements are used to map the rotational potential, and to explore the influence of rotor-rotor interactions. Interactions due to dipole-dipole effects are studied using a Monte Carlo simulation, while contributions from steric interactions between rotors are investigated using molecular mechanics methods. Both contributions are needed explain the dielectric spectroscopy results.

We report the results of the dynamics of a three dimensional lattice of dipolar molecular rotors where the unit cells consist of a dipolar phenylene ring rotating about an axle stabilized by stationary triphenyl groups. The molecules are synthesized such that the lattice may be customized to elicit novel and useful physical phenomena. Using dielectric spectroscopy and H-2 NMR, we demonstrate rapid thermal rotation of the molecular rotors in the solid state, and characterize the depth and asymmetry of the rotational potential. Calculations show that rotor-rotor interactions are weak in this structure, and the rotational potential is dominated by steric interactions between each rotary element and the nonrotating portions of neighboring molecules

We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.Superconducting nanowire single photon detectors (SNSPD) have been shown to have high efficiency, low dark counts, and tens of picosecond timing 1 . SNSPDs have been particularly useful in applications requiring high timing resolution and detection in the near-infrared (λ > 1µm)2 . Until recently small arrays of nanowire detectors for imaging, higher count rates, large collection areas, and photon number resolving detection have been technologically challenging to realize. The recent observation of the saturation of internal detection efficiency at ∼ 40% of the maximum bias current in SNSPDs fabricated from amorphous tungsten silicide (WSi) is key in enabling high-efficiency arrays to be constructed 3,4 . Detectors fabricated from niobium nitride (NbN), for example, have high detection efficiencies only at bias currents close to the critical current 5 . Current "cross talk" between detectors in arrays biased so close to their maximum operating current could cause other detectors to falsely fire when one detector in the array fires. A wide margin in operating bias, or "bias plateau", allows the detectors to be biased at a fraction of their respective critical currents and to remain sensitive to photons, even as other detectors in the array fire. Previously, we demonstrated a 4-pixel WSi SNSPD array with an integrated, scalable multiplexed readout 6 . In this work we extend to a free-space coupled 64-pixel (8 × 8 square) array using a slightly modified version of our multiplexed readout.To date, only a handful of experiments have demonstrated arrays of SNSPDs. Architectures where each detector has its own readout/bias line have been measured 5,7-9 . However, one critical issue to consider when scaling up to larger numbers of elements is the available cooling power of the cryogenic system. Each readout channel adds to the total thermal power budget and can quickly limit achievable base temperatures. Therefore, multiplexing schemes, where the total number of readout channels is kept to a small fraction of the number of array elements, become a necessity. Attempts at multiplexed readout have primarily been limited to single flux quantum (SFQ) logic schemes 10-14 . SFQ-logic-based readout is attractive due to the intrinsic low power consumption but the designs can be quite complex and require additional fabrication steps for the Josephson junctions. Apart from SFQ, an inductive current splitting technique where the firing pixel location was encoded onto the magnitude of the output pulse, has been demonstrated in a 4-pixel linear array 15 .Another key issue in array development is device yield. The traditional m...