24 October 2016 Enhancing the secure key rate in a quantum-key-distribution system using discrete-variable, high-dimensional, time-frequency states
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High-dimensional (dimension d > 2) quantum key distribution (QKD) protocols that encode information in the temporal degree of freedom promise to overcome some of the challenges of qubit-based (d = 2) QKD systems. In particular, the long recovery time of single-photon detectors and large channel noise at long distance both limit the rate at which a final secure key can be generated in a low-dimension QKD system. We propose and demonstrate a practical discrete-variable time-frequency protocol with d = 4 at a wavelength of 1550 nm, where the temporal states are secured by transmitting and detecting their dual states under Fourier transformation, known as the frequency-basis states, augmented by a decoy-state protocol. We show that the discrete temporal and frequency states can be generated and detected using commercially-available equipment with high timing and spectral efficiency. In our initial experiments, we only have access to detectors that have low efficiency (1%) at 1550 nm. Together with other component losses, our system is equivalent to a QKD system with ideal components and a 50-km-long optical-fiber quantum channel. We find that our system maintains a spectral visibility of over 99.0% with a quantum bit error rate of 2.3%, which is largely due to the finite extinction ratio of the intensity modulators used in the transmitter. The estimated secure key rate of this system is 7.7×104 KHz, which should improve drastically when we use detectors optimized for 1550 nm.
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Nurul T. Islam, Nurul T. Islam, Clinton Cahall, Clinton Cahall, Andrés Aragoneses, Andrés Aragoneses, Charles Ci Wen Lim, Charles Ci Wen Lim, Michael S. Allman, Michael S. Allman, Varun Verma, Varun Verma, Sae Woo Nam, Sae Woo Nam, Jungsang Kim, Jungsang Kim, Daniel J. Gauthier, Daniel J. Gauthier, } "Enhancing the secure key rate in a quantum-key-distribution system using discrete-variable, high-dimensional, time-frequency states", Proc. SPIE 9996, Quantum Information Science and Technology II, 99960C (24 October 2016); doi: 10.1117/12.2241429; https://doi.org/10.1117/12.2241429


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