Information encoded in high-dimensional quantum states can achieve ultrahigh rates over metropolitan distances.

No abstract

Quantum key distribution (QKD) allows two remote users to establish a secret key in the presence of an eavesdropper. The users share quantum states prepared in two mutually-unbiased bases: one to generate the key while the other monitors the presence of the eavesdropper. Here, we show that a general d-dimension QKD system can be secured by transmitting only a subset of the monitoring states. In particular, we find that there is no loss in the secure key rate when dropping one of the monitoring states. Furthermore, it is possible to use only a single monitoring state if the quantum bit error rates are low enough. We apply our formalism to an experimental d = 4 timephase QKD system, where only one monitoring state is transmitted, and obtain a secret key rate of 17.4 ± 2.8 Mbits/s at a 4 dB channel loss and with a quantum bit error rate of 0.045 ± 0.001 and 0.037 ± 0.001 in time and phase bases, respectively, which is 58.4% of the secret key rate that can be achieved with the full setup. This ratio can be increased, potentially up to 100%, if the error rates in time and phase basis are reduced. Our results demonstrate that it is possible to substantially simplify the design of high-dimensional QKD systems, including those that use the spatial or temporal degrees-of-freedom of the photon, and still outperform qubit-based (d = 2) protocols.

We investigate experimentally a cascade of temperature-compensated unequal-path interferometers that can be used to measure frequency states in a high-dimensional quantum distribution system. In particular, we demonstrate that commercially-available interferometers have sufficient environmental isolation so that they maintain an interference visibility greater than 98.5% at a wavelength of 1550 nm over extended periods with only moderate passive control of the interferometer temperature (< ±0.50 • C). Specifically, we characterize two interferometers that have matched delays: one with a free-spectral range of 2.5 GHz, and the other with 1.25 GHz. We find that the relative path of these interferometers drifts less than 3 nm over a period of one hour during which the temperature fluctuates by < ±0.10 • C. The error in our measurement is largely dominated by the small drift in the frequency and power of the stabilized laser used to perform the measurement. When we purposely heat the interferometers over a temperature range of 20-50 • C, we find that the temperature sensitivity is different for each interferometer, likely due to slight manufacturing errors during the temperature compensation procedure. Over this range, we measure a path-length shift of 26 ± 9 nm/ • C for the 2.5 GHz interferometer. For the 1.25 GHz interferometer, the path-length shift is nonlinear and is locally equal to zero at a temperature of 37.1 • C and is 50 ± 17 nm/ • C at 22 • C. With these devices, we realize a cascade of 1.25 GHz and 2.5 GHz interferometers to measure four-dimensional classical frequency states created by modulating a stable and continuous-wave laser. We observe a visibility > 99% over an hour, which is mainly limited by our ability to precisely generate these states. Overall, our results indicate that these interferometers are well suited for realistic time-frequency quantum distribution protocols. arXiv:1610.04947v1 [quant-ph]

Two low-lying neutron-unbound excited states of 24 O, populated by proton-knockout reactions on 26 F, have been measured using the MoNA and LISA arrays in combination with the Sweeper Magnet at the Coupled Cyclotron Facility at the NSCL using invariant mass spectroscopy. The current measurement confirms for the first time the separate identity of two states with decay energies 0.51(5) MeV and 1.20(7) MeV, and provides support for theoretical model calculations, which predict a 2 + first excited state and a 1 + higher energy state. The measured excitation energies for these states, 4.70(15) MeV for the 2 + level and 5.39(16) MeV for the (1 + ) level, are consistent with previous lower-resolution measurements, and are compared with five recent model predictions.