Improvements in gigahertz free-space quantum-key distribution

Information-processing and communications systems based on quantum-mechanical dynamics can perform functions that cannot be accomplished with classical systems.¹ Pulses of light consisting of single photons are a convenient and robust means of realizing quantum communication. However, single-photon operation places such systems in an extreme regime of signal-to-noise ratios (S/N).

The S/N is particularly poor for free-space quantum-key distribution (QKD),² where a single-photon channel between two telescopes is used to generate cryptographic keys whose secrecy is based on the system's ability to sense the interfering presence of an eavesdropper (like a wax seal on an envelope). The key-production rate over a given QKD link depends on both the system's transmission rate and—in free space—our ability to filter out background photons originating from, e.g., the sun.

We developed a free-space QKD setup using telecommunications clock-recovery techniques to maximize the quantum channel's transmission rate.³ The clock-recovery system synchronizes the quantum-channel transmitter and receiver over an adjacent 1.25Gb/s classical channel, and we transmit on the single-photon channel every clock cycle. The system runs continuously with dedicated signal-processing hardware and high-speed error-correction and post-processing algorithms, and can support key-production rates on the order of 1Mb/s.

Figure 1. Production rate of error-corrected and privacy-amplified (EC&PA) bit keys as a function of optical link losses. Application of 200ps postselection gating reduces the system's exposure to the background level (with a count rate of 70,000/s), increasing the signal-to-noise ratio and extending the system's range. The measured link-loss uncertainty is 0.25dB (see error bars).

To reduce the system's exposure to background light, we use single-photon avalanche-diode (SPAD) detectors with modified circuitry for improved timing resolution.⁴ The timing jitter is as low as 156ps (full width at half maximum), well below the 800ps clock period. We take advantage of this resolution by employing a gigahertz discrete-logic system to achieve strong temporal gating to the detector output. This gate selects events that occur in a narrow 200ps period during each clock cycle—most likely due to photons from the transmitter—and it rejects events that occur in the relatively ‘signal-poor’ period that remains. This reduces the error rate on the quantum channel and enables our system to operate over link losses and in noise regimes that would otherwise make key production impossible.

The system performance is demonstrated in Figure 1, where we show the bit-production rate as a function of link loss. The background count rate is 70,000/s, and with minimal optical link losses the system produces error-corrected and privacy-amplified keys at bit rates exceeding 600,000/s. Beyond roughly −8dB, the S/N becomes too poor for the ungated system to produce any usable key. Application of 200ps postselection gating (see Figure 1) improves the S/N, resulting in an operational range extended to −13dB.

We have thus demonstrated an experimental system that takes advantage of improved timing resolution in SPADs to realize a QKD system operating with transmission rates in the gigahertz range and with subnanosecond temporal gating. The gating system can realize gates as short as 45ps, and further improvements in SPAD timing resolution will enable operation of single-photon communication systems over greater link losses and under more severe noise conditions. Our future research will focus on the benefits to quantum-optics research and the associated data-processing systems of improved timing resolution and high-speed signaling.