Fast single-photon detectors and real-time key distillation enable high secret-key-rate quantum key distribution systems

Abstract

Quantum key distribution has emerged as the most viable scheme to guarantee information security in the presence of large-scale quantum computers and, thanks to the continuous progress made in the past 20 years, it is now commercially available. However, the secret key rates remain limited to just over 10 Mbps due to several bottlenecks on the receiver side. Here we present a custom multipixel superconducting nanowire single-photon detector that is designed to guarantee high count rates and precise timing discrimination. Leveraging the performance of the detector and coupling it to fast acquisition and real-time key distillation electronics, we remove two major roadblocks and achieve a considerable increase of the secret key rates with respect to the state of the art. In combination with a simple 2.5-GHz clocked time-bin quantum key distribution system, we can generate secret keys at a rate of 64 Mbps over a distance of 10.0 km and at a rate of 3.0 Mbps over a distance of 102.4 km with real-time key distillation.

Keywords: Quantum information; Quantum optics.

Fig. 1. The SNSPD with 14 interleaved pixels.

Image taken with a scanning electron microscope (SEM). The detector covers an area with a diameter of ~15.5 μm, which corresponds to an overlap of 99.7% (6σ) with the mode of the SMF-28 fibre. The width of the nanowires is 100 nm with a fill factor of 50%, and the interleaved design ensures uniform illumination of the pixels.

Fig. 2. Timing resolution of detections.

Histogram of the arrival times measured with one pixel (for a total count rate of 15.3 Mcps), which is the result of a convolution of the laser pulse shape and the jitter of the detector including readout electronics. Detections falling in the central green time bin t₀ are correct, detections falling in the red time bins t₋₁ and t₁ lead to errors, whereas events in the grey time bins t_nd are discarded to lower the QBER. In other words, jitter leads both to loss (t_nd bins) or errors (t₋₁ and t₁ bins).

Fig. 3. SDE of the multipixel detector versus count rate.

At low count rates (below 10 Mcps) we measure a maximum SDE of 82%. As the count rate increases, the SDE begins to drop as more photons are reaching the detector within the dead time of each pixel. The crosses show the operating points for the performed key exchanges. At a distance of 10 km, the detector operates at 320 Mcps with SDE of 64% and temporal jitter of 58 ps, and at a distance of 100 km it operates at 15.0 Mcps with SDE of 81% and temporal jitter of 36 ps. The contributions to the jitter come both from the detector and the readout electronics.

Fig. 4. QKD set-up.

Schematic representation of the QKD set-up with all the key components. CFD, constant fraction discriminator; DAC, digital-to-analogue converter; DCF, dispersion-compensating fibre; FM, Faraday mirror; FPGA, field-programmable gate array; IM, intensity modulator; PC, polarization controller; PPM, piezo-electric phase modulator; SNSPD, superconducting nanowire single-photon detector; ULL SMF, ultra-low-loss single-mode fibre; VA, variable attenuator. The dashed boxes are temperature-stabilized. FPGA I controls the state preparation, FPGA II is used for error correction, and FPGA III acquires the detection events. The sifting is done between FPGAs I and III. Both Michelson interferometers exhibit an imbalance of 200 ps.

Fig. 5. States prepared by Alice.

Each state consists of two time bins. The two pulses of one state have a fixed phase relation, but pulses of different states have a random phase relation. Alice chooses the mean photon number μ₀ or μ₁ for each state at random. The green boxes are the detection time windows of Bob, each with a duration of 100 ps.

Fig. 6. Stability of the key exchange.

Measured secret and sifted key rate, QBER in the Z basis (QBER Z) and phase error rate (ϕ_Z) over consecutive privacy amplification blocks of 134 Mbits over 10 km of ULL fibre. The average acquisition time per block was 0.84 s.