Free-space quantum key distribution at GHz transmission rates

Quantum key distribution (QKD) can produce unconditionally secure cryptographic key for use in symmetric cryptosystems. We have shown that telecommunications clock-recovery techniques enable the continuous operation of both free-space and fiber QKD systems at transmission rates in the GHz range, limited only by the timing resolution of the single-photon detectors. Taking advantage of improvements in detector timing resolution and FPGA performance that enable transmission rates of 2.5 GHz and higher, we discuss the performance of a free-space QKD system operating in the H$_{\alpha }$ Fraunhofer window, the classical-channel bandwidth required for post-processing, and the limitations imposed by detector recovery time. We also show that with high-repetition-rate sub-clock gating these higher-resolution detectors can reduce a free-space QKD system's exposure to solar background photons, thus reducing the quantum-bit error rate (QBER) and improving system performance.