Beating the Fundamental Rate-Distance Limit in a Proof-of-Principle Quantum Key Distribution System

With the help of quantum key distribution (QKD), two distant peers are able to share information-theoretical secure key bits. Increasing the key rate is ultimately significant for the applications of QKD in the lossy channel. However, it has been proven that there is a fundamental rate-distance limit, called the linear bound, which restricts the performance of all existing repeaterless protocols and realizations. Surprisingly, a recently proposed protocol, called twin-field (TF) QKD, can beat the linear bound with no need for quantum repeaters. Here, we present one of the first implementations of the TF-QKD protocol and demonstrate its advantage of beating the linear bound at a channel distance of 300 km. In our experiment, a modified TF-QKD protocol that does not assume phase postselection is considered, and thus a higher key rate than the original one is expected. After controlling the phase evolution of the twin fields traveling through hundreds of kilometers of optical fibers, the implemented system achieves high-visibility single-photon interference and allows stable and high-rate measurement-device-independent QKD. Our experimental demonstration and results confirm the feasibility of the TF-QKD protocol and its prominent superiority in long-distance key distribution services.

Popular Summary

Quantum key distribution (QKD), which exploits the laws of quantum physics to share encryption keys between two remote users, is a promising technology to revolutionize information security. To maximize the probability that keys are successfully transmitted across lossy fiber-optic channels, it will be necessary to boost the rate at which users share keys. However, theory indicates that, without the aid of quantum repeaters, there is a fundamental limit to how quickly keys can be shared over any given distance. We implement a QKD system that overcomes these limits, achieving a key transmission rate 3 times higher than the predicted bound across a 300-km-long optical fiber.

Our scheme is based on a recently proposed protocol known as “twin-field QKD (TF-QKD) without phase postelection.” Two users prepare pairs of weak coherent light pulses with phase and frequency locked, encode their key bits as one of two optical phases, and send the pulses to an untrusted middle station, which measures the phase difference of the pulses in each pair. To achieve stable and high-visibility single-photon interference, we compensate for the fast phase evolution of the twin pulses traveling across hundreds of kilometers of fiber channels. Finally, we confirm the feasibility of TF-QKD and its prominent superiority in real fiber channels for the first time.

Our demonstration shows that achieving a high key rate is feasible in long-distance-fiber QKD implementations, which offers a new approach to large networks.

Figure 1

Experimental setup to implement the twin-field quantum key distribution. CW laser, continuous-wave laser; IM, intensity modulator; PM, phase modulator; VA, variable attenuator; $50/50$ BS, beam splitter with a $50/50$ splitting ratio; PC, polarization controller; SSPDs, superconducting single-photon detectors. Alice and Bob have the same setup, which mainly consists of the source, chopper, and encoder modules. The source generates the laser with a locked frequency and phase with the laser from Charlie. The chopper is used to first modulate the CW laser into a pulse train (${\mathrm{IM}}_1$) and then chop it into time-multiplexed bright reference and weak quantum parts (${\mathrm{IM}}_2$). The encoder adds code or decoy information into the quantum part (${\mathrm{IM}}_3$ and PM).

Figure 2

Characteristics of the source part. (a) The quadrature interference results recorded by an oscilloscope. (b) The quadrature interference results with a feedback PM. (c) The in-phase interference results detected by two SSPDs. (d) The in-phase interference results with a feedback PM. The cyan points correspond to the counts of the constructive interference output; the magenta points refer to the counts of the destructive interference output. The curves are smoothened by averaging every 20 adjacent points. The insets are the distributions of the constructive counts and destructive counts.

Figure 3

Characteristics of the compensation of the fast phase drift at a total distance of 300 km optical fiber.(a) The phase drift by recording the counts of D0 and D1 when the driving voltage over the feedback PMs is disconnected. (b) The results of actively compensating the phase drift by recording the counts of D0 and D1 after connecting the PM driving voltage. The insets are the distributions of the counts of two SSPDs. Here, only the counts belonging to the reference parts are recorded.

Figure 4

Key rate of the TF-QKD system. The rates are plotted against the total distance of optical fiber (from Alice to Bob) with a loss efficiency of $0.18\mathrm{dB}/\mathrm{km}$. The open dots refer to experimental points at distances of 100 km, 200 km, and 300 km, respectively.

Figure 5

Stability of the TF-QKD system over a total distance of 300 km. The blue open dots represent the interference visibility of Alice’s and Bob’s reference parts, and the red open dots are QBER of the corresponding quantum parts. Each dot corresponds to the data acquired in one second.