University of Science and Technology of China uses quantum key distribution to realize 658 km long-distance optical fiber sensing for the first time

Produced by Photon Box Research Institute

Recently, Pan Jianwei and Zhang Qiang of the University of Science and Technology of China cooperated with Wang Xiangbin and Liu Yang of Jinan Institute of Quantum Technology to realize an experimental system integrating quantum key distribution and optical fiber vibration sensing. TF-QKD) at the same time, realizes 658 kilometers long-distance optical fiber sensing, and the positioning accuracy reaches 1 kilometer, which greatly breaks through the limit of traditional optical fiber vibration sensing technology that is difficult to exceed 100 kilometers [1].

On May 2, the relevant research results were published in Physical Review Letters [2] in the form of "Editor's Recommendation", and were reported by the website "Physics" of the American Physical Society (APS) [3]. The first authors of the paper are University of Science and Technology of China doctoral students Chen Jiupeng and Zhang Chi.

The research results show that the TF-QKD network architecture can not only realize ultra-long-distance distribution of security keys, but also can be applied to ultra-long-distance vibration sensing to realize the fusion of wide-area quantum communication network and optical fiber sensor network. The redundant information of TF-QKD can be used for remote sensing of channel vibration, which can be applied to earthquake detection and landslide monitoring in addition to safety communication.

来源：墨子沙龙

Optical fiber vibration sensing uses optical fiber as a sensor for vibration sensing. By using a single optical fiber to realize vibration monitoring and signal transmission at the same time, it has the advantages of high sensitivity, fast response, simple structure, and uniform distribution. It is used in structural health monitoring, oil and gas pipeline leakage monitoring, etc. It has a wide range of application prospects in engineering fields such as perimeter protection and earthquake monitoring, so it has attracted extensive attention and research. At present, fiber-optic vibration sensing mostly uses distributed acoustic wave sensing technology, and its sensing distance is limited within 100 kilometers. An important technical challenge is how to overcome the distance limitation and realize long-distance fiber-optic vibration sensing.

Quantum Key Distribution (QKD) provides a theoretically provably secure way to distribute keys. However, since quantum signals cannot be amplified, channel loss is an unavoidable obstacle for long-distance QKD. For a transport operator η, the theoretical upper limit of the secure key rate is limited to 1.44η, which is called the Pirandola-Laurenza-Ottaviani-Bianchi (PLOB) boundary. This boundary is valid for all QKD protocols without repeaters, including the common decoy-based BB84 and measurement-device-independent QKD (MDI-QKD), which can be ignored. Measure all security vulnerabilities of your device. In the absence of practical quantum repeaters, an intermediate solution for realizing long-distance QKD networks is to set up several trusted relay nodes. Although trusted relay networks have been successfully demonstrated in the field, an increase in the number of trusted relays may increase security risks and raise costs.

Unlike conventional QKD protocols, two-field QKD (TF-QKD) can increase the secure key rate to √η without using quantum memory. This may offer a solution to reach greater distances and reduce the number of trusted relays. This joint team experiment proved the feasibility of long-distance distribution of security keys. It is worth noting that, considering the comprehensive security analysis of limited scale effect, the experiments of sending or not sending TF-QKD (sending or not sending TF-QKD, SNS-TF-QKD) have proved that more than 500 Long distance record in kilometers. Regardless of the limited size of the data, a positive key rate can even be obtained over a distance of 600 km.

In practical applications, noise such as sound and vibration along the fiber link is inevitable. Therefore, during the TF-QKD experiment, it is necessary to detect the fiber phase change caused by environmental noise in real time, and perform real-time or data post-processing compensation for it. In general, information about these phase changes is discarded after the QKD experiment is over. But in fact, this "redundant" information reflects the real-time phase changes of the transmitted light in the fiber, which may originate from vibration disturbances or temperature drifts along the fiber link. By analyzing these phase change information and combining some characteristics of vibration, the vibration information can be obtained and positioned, so as to realize ultra-long-distance optical fiber vibration sensing.

"While constructing the TF-QKD system, we put great effort into compensating for the phase fluctuations in the channel," said Qiang Zhang.

Alice

Alice and Bob used two independent ultra-stable lasers in which the relative frequency difference was eliminated. The light is modulated into a time-multiplexed pattern of single-photon-level quantum signal pulses and strong-phase reference pulses. In each 1 microsecond period, 100 signal pulses of 240 picosecond duration (1 ns = 1000 picoseconds) are sent in the first 400 nanoseconds, the phase reference pulse in the following 600 nanoseconds is sent. The signals from Alice and Bob were sent to Charlie over 329.3 km and 329.4 km (658.7 km total) of ultra-low loss fiber spools with transmission rates of 52.9 dB and 53.1 dB (106 dB total). After the perturbation of Charlie's beam splitter (BS), the signal is detected by two superconducting nanowire single photon detectors (SNSPD) (high count rate low noise single photon detectors developed by You Lixing's team of Shanghai Institute of Microsystems, Chinese Academy of Sciences) , and recorded by a time stamper. The specific experimental setup is shown in the figure below:

Schematic diagram of the experimental setup. In Alice's (Bob) experiments, a seed laser was locked onto an ultra-low-expansion (ULE) glass cavity, using Pound-DreverHall (PDH) technology to achieve sub-Hz linewidths. The stabilized laser is divided into two parts: one is used to calibrate the wavelength difference between local and remote users, and the other is used as a QKD source. In the wavelength calibration section, the light is passed through Alice's (Bob) 40 MHz (70 MHz) fixed carrier frequency acousto-optic modulator (AOM) to filter noise in the channel. The light is then sent to Bob (Alice) for inter-frequency detection with a photodiode (PD). Insert an AOM with 500 MHz ± 5 MHz adjustable carrier frequency at Bob to cancel the frequency difference between Bob's and Alice's light sources. Wavelength-calibrated light travels approximately 500 kilometers from the SMF-28 spool between Alice and Bob. Bidirectional erbium-doped fiber amplifiers (biEDFAs) are inserted every 50 kilometers to maintain the power of the transmitted light. In the QKD part, the light is modulated by a phase modulator (PM) and an intensity modulator (IM) and attenuated to the single-photon level with an attenuator (ATT) to generate a quantum signal with a phase reference signal. The light was finally sent to Charlie for detection via 329.3 km and 329.4 km of ultra-low loss fiber spools (658.7 km). Charlie used a Dense Wavelength Division Multiplexer (DWDM) with a circulator (CIR) before the polarization beam splitter (PBS) and beam splitter (BS) to filter the noise. The interference results were detected by a superconducting nanowire single-photon detector (SNSPD). In addition, fiber stretchers were inserted into the QKD channel and wavelength calibration channel as artificial vibration sources. EPC: Electro-Polarization Controller. PC: Polarization Controller.

In the experiment, a 658 km G.652 ultra-low loss fiber with a total loss of 106 dB was used as a quantum channel with an average value of 0.161 dB/km including the junction; this includes Charlie's component loss and insertion loss are optimized to 1.3 dB. The team then employed a high-performance SNSPD with a detection efficiency of 82% and an effective dark count rate of 4 Hz to detect the perturbation, and set a time gate of 0.3 ns to suppress the noise. The final noise is optimized to be 6 × 10^-9 per pulse, of which about 80% comes from Rayleigh scattering.

 After calculation, the final security key rate is R = 9.22 × 10^-10, taking into account the limited key effect, which is about 0.092 bits per second (bps) considering an effective system frequency of 100 MHz. The experimental results are as follows:

The secure key rate of the SNS-TF-QKD experiment. The green star indicates the experimental results on 658 km of ultra-low frequency fiber with a security key rate of R = 9.22 × 10^-10. Yellow diamonds, purple circles and blue triangles, black squares represent experimental results in the references of existing experiments. The red curve is the simulation result with experimental parameters. The dashed brown and cyan lines show the absolute and relative PLOB boundaries.

Next, the team modulated the PZT vibration generator with a fixed frequency to simulate vibration perturbations in the channel. In the case of the PZT vibration occurring in the 658 km quantum channel, the phase drift is recovered by calculating the relative phase difference from the phase reference pulse along with it: the modulation is set to a sinusoidal signal, the frequencies chosen are 1 Hz, 10 Hz, 100 Hz and 1 kHz, which are also frequency ranges of interest in seismic and acoustic sensing. In the time domain, the recovered phase changes exactly match the actively modulated signal, ie, the vibrations externally imposed on the fiber.

The researchers experimentally demonstrated that the SNS-TF-QKD on a 658 km ultra-low-loss fiber, taking into account the limited key effect, achieved a secure key rate of 9.22×10^-10 per pulse. The team recovered the vibration disturbance of 1 Hz-1 kHz on the fiber with the phase reference and frequency locking channel, and finally realized the optical fiber dual-field quantum key distribution and fiber vibration sensing of 658 kilometers, perturbation of the artificial vibration source on the QKD link The location was located with an accuracy of better than 1 km.

This work provides a proof-of-principle that the TF-QKD architecture can be used for ultra-long-range vibration sensing while distributing secure keys. In the future, the team expects that the developed technology can expand the application of the QKD network, especially in areas such as earthquake detection, landslide monitoring, and highway traffic monitoring, which require distributed earthquake detection.

"Detecting vibrations over such long distances is impressive," said Hoi-Kwong Lo, a quantum information expert at the University of Toronto who proposed the MDI-QKD protocol, noting that similar techniques have been developed to sense vibrations along fiber optic lines. , such as a recent experiment using underwater telecom fiber optics to detect earthquakes. One of the authors of the paper, Giuseppe Marra of the UK's National Physical Laboratory, said the new QKD demonstration follows the same concepts as his and other previous work. "Future QKD links based on this technology could provide useful additional seismic information from installed fibers," he said.

Reference link：

[1]http://news.ustc.edu.cn/info/1055/79044.htm

[2]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.180502

[3]https://physics.aps.org/articles/v15/63