Quantum Key Distribution over 658 km Fiber with Distributed Vibration Sensing

Twin-field quantum key distribution (TFQKD) promises ultralong secure key distribution which surpasses the rate distance limit and can reduce the number of the trusted nodes in long-haul quantum network. Tremendous efforts have been made toward implementation of TFQKD, among which, the secure key with finite size analysis can distribute more than 500 km in the lab and in the field. Here, we demonstrate the sending-or-not-sending TFQKD experimentally, achieving a secure key distribution with finite size analysis over a 658 km ultra-low-loss optical fiber. Meanwhile, in a TFQKD system, any phase fluctuation due to temperature variation and ambient variation during the channel must be recorded and compensated, and all this phase information can then be utilized to sense the channel vibration perturbations. With our quantum key distribution system, we recovered the external vibrational perturbations generated by artificial vibroseis on both the quantum and frequency calibration link, and successfully located the perturbation position in the frequency calibration fiber with a resolution better than 1 km. Our results not only set a new distance record of quantum key distribution, but also demonstrate that the redundant information of TFQKD can be used for remote sensing of the channel vibration, which can find applications in earthquake detection and landslide monitoring besides secure communication.

Figure 1

Schematic of experimental setup. In Alice’s (Bob’s) lab, a seed laser is locked to an ultra-low-expansion (ULE) glass cavity to achieve a subhertz linewidth by using the Pound-Drever-Hall (PDH) [41, 42] technique. After PDH locking, a 500 MHz acoustic-optic modulator (AOM) with adjustable carrier frequency is inserted at Bob to eliminate the frequency difference of the two stable lasers. Then, the ultrastable light sources are split into two parts, respectively; one is used for QKD, the other is sent to the other user via a 500 km frequency calibration fiber link for heterodyne interference. Bidirectional erbium-doped fiber amplifiers (BEDFAs) are inserted every 50 km to maintain the power of the transmitted light, two AOMs with fixed carrier frequency of 40 and 70 MHz are inserted at both ends of the link to filter the reflection in the channel. PD: photodiode. In the QKD part, the light is modulated with phase modulators (PMs) and intensity modulators (IMs) and attenuated to a single photon level with an attenuator (ATT), to generate the quantum signals with the phase reference signals. The light is finally sent to Charlie via 329.3 and 329.4 km ultra-low-loss fiber spools (658.7 km) for detection. Charlie uses a dense wavelength division multiplexer (DWDM), a circulator (CIR) to filter the noises before the polarization beam splitter (PBS) and the beam splitter (BS). The interference results are detected by superconducting nanowire single-photon detectors (SNSPDs). Additionally, the fiber stretchers are inserted into the QKD channel and the wavelength calibration channel, as the artificial vibroseis. EPC: electric polarization controller; PC: polarization controller.

Figure 2

Secure key rates of the SNS TFQKD experiment. The green star indicates the experimental result over 658 km ultra-low-loss optical fibers, with the secure key rate of $R=9.22\times {10}^{-10}$. The yellow diamond, purple circle, and blue triangle indicate the experimental results of Refs. [19, 20, 22] in the lab. The black square indicates the experimental result of Ref. [23] in field. The red curve is the simulation result with the experimental parameters. The brown dotted line and cyan dotted line show the absolute and relative PLOB bound [7].

Figure 3

Vibration test results via the QKD link. The blue curve indicates the recovered relative phase variation signal. The gray dotted line indicates the modulation signal of program-controlled PZT.

Figure 4

Vibration test results via frequency calibration link. The blue curve and red curve indicate the recovered phase variation signals of Alice and Bob. The gray dotted line indicates the modulation signal of program-controlled PZT.