Nanjing University has made a breakthrough in the integration of quantum key distribution system

A Chinese research team has made an important breakthrough in the integration of quantum key distribution (QKD) system. Professor Ma Xiaosong, Academician Wu Peiheng, academician Zhu shining and Professor Lu Yanqing of Nanjing University, in conjunction with Professor Cai Xinlun of Sun Yat sen University, have successfully developed a hetero integrated superconducting silicon photonic chip for mdi-qkd (measurement device independent quantum key distribution). Relevant results were recently published in the journal Advanced Photonics. A fully chip based, scalable and cost-effective metropolitan quantum network will be realized in the near future.

Theoretically, QKD can provide information theoretic security keys for both sides of communication, but the actual QKD system is difficult to meet various idealized models assumed in the proof of security protocol.

The design and implementation of actual QKD system often leaks a certain degree of side channel information. Attacker Eve can use the side channel to obtain part of the key information, so as to reduce the security and availability of the system. Common side channel attacks include time shift attack, pseudo state attack, strong light blinding attack and so on.

In 2012, H. K. lo and others proposed the measurement device independent (MDI) QKD protocol, which can eliminate the channel vulnerability on the detector side and greatly increase the communication distance. Mdi-qkd does not rely on trusted nodes in the traditional QKD protocol, and only needs a central node (Charlie) to perform Bell state measurement (BSM). The correlation between the two senders (Alice and Bob) can be obtained from the BSM results. Importantly, even if Charlie is not trusted, as long as Charlie can project his two photons into the Bell state, the security of mdi-qkd can be guaranteed.

Mdi-QKD schematic diagram

From a hardware perspective, recent advances have been made in specific integrated photonic devices involving QKD, including on-chip encoders based on silicon modulators, on-chip transmitters including lasers, photodiodes, indium phosphide based modulators, silicon oxynitride and silica based decoders, and integrated silicon photonic chips for continuous variable (CV) QKD. The concept of MDI has also been extended to CV protocol, and can be applied to multi-party metropolitan area networks at a considerable rate.

However, most components used in QKD, including lasers, modulators and passive components (such as beam splitter (BS) and attenuator), are widely used in classical optical communication systems and are not specially designed for QKD. In addition, single photon detector is essential for discrete variable QKD system, because the sender's pulse must have an average photon number of < 1 to ensure communication security. So far, the single photon detector integrated chip platform has not been applied to mdi-qkd system. In this work, the researchers reported the implementation of hetero integrated superconducting silicon photonic chip and its application in mdi-QKD

In the mdi-QKD system in this paper, researchers use time bin qubits to encode information, which is very suitable for optical fiber based quantum communication because they are not affected by random polarization rotation in optical fiber. By using the best BSM and time division multiplexing, the key generation rate is increased by one order of magnitude compared with the system without these two technologies, which is equivalent to the experimental results of the most advanced mdi-qkd (the clock frequency is GHz).

Figure. 1 Schematic diagram of time division multiplexing mdi-qkd and star mdi-qkd networks

The team's Heterogeneous Integrated superconducting silicon photonic platform provides a server architecture for implementing a multi-user untrusted node quantum network with a fully connected bipartite graph topology.

As shown in Fig. 1 (b), the modulated weakly coherent pulse is prepared by Alice (A1, A2,..., an) and Bob (B1, B2,..., BN) and sent to the router. The two routers select a pair of Alice and Bob for communication and send their pulses to the untrusted relay server controlled by Charlie. At Charlie, a chip with multiple low dead time, low timing jitter and high efficiency detectors is combined with low loss silicon photonic devices to realize BSM. This configuration allows any user on Alice side to communicate with any user on Bob side, so as to realize a fully connected binary quantum network.

As shown in Fig. 2 (a), Alice (Bob) chopped a continuous laser with an operating wavelength of about 1536.47 nm into the required pulse sequence. The pulse width is about 370 PS, the interval is 12 ns, and the rate is 41.7 MHz (1 / 24 NS). The z-base (x-base) state is generated by chopping the laser to the | e ⟩ or (and) | L ⟩ state using an intensity modulator (IM). The average number of photons per pulse in the two bases is approximately the same. The generated pulse is sent to a phase modulator (PM) with (or without) π phase shift for generating a | − ⟩ (| + ⟩) state. The electrical signal applied to the modulator is generated by an arbitrary waveform generator (AWG). In addition, the 50:50 beam splitter (BS) combined with the power sensor (PS) is used to monitor the long-term stability of the laser power in each encoder.

Figure. 2 experimental device

These pulses are then sent to Charlie's relay server chip, which is installed on a nano locator in a closed cycle cryogenic thermostat with a base temperature of 2.1 K (271.05 ℃). Figure 2 (b) shows the U-shaped waveguide integrated superconducting nanowire single photon detector (SNSPD), in which the superconducting nanowire (80 nm wide, 80 μ M long) is highlighted in red and the silicon optical waveguide (500 nm wide) is displayed in blue. Fig. 2 (c) shows a scanning electron microscope image of a photonic crystal grating coupler, which couples light from a fiber array to a chip.

The research shows that the excellent photoelectric performance of this chip is not only conducive to the experimental high visibility Hom interference (two-photon interference) and low quantum key bit error rate (qber), but also enables researchers to perform the best BSM on time bin qubits for the first time.

The team used time division multiplexing to improve the key rate. The security key rates with different losses are shown in Figure 3. At a clock frequency of 125 MHz, a key rate of 6.166 Kbps is obtained at a loss of 24.0 dB. The chip insertion loss is about 4.5 dB, so the actual transmission loss is about 19.5 dB, corresponding to 98 km of standard optical fiber. The author of this paper said that this is the highest security key rate obtained by 20 dB transmission loss experiment in mdi-qkd, which is suitable for metropolitan quantum network environment without detector vulnerabilities. In addition, they obtained security key rates of 170 BPS and 34 BPS at a total loss of about 35.0 dB and 44.0 dB, respectively.

They stressed that the security key rate of 125 MHz clock system is very close to the optimal mdi-qkd experiment at GHz clock rate. Compared with the GHz clock rate mdi-qkd experiment, the system does not need complex injection locking technology, which greatly reduces the complexity of the transmitter.

Figure 3 key rate under different loss

Finally, this work shows that compared with the traditional platform, the integrated quantum photonic chip not only provides a way of miniaturization, but also significantly improves the system performance. Their chip based relay server can also be used for dual field quantum key distribution (tf-qkd). Tf-qkd can overcome the rate distance limit of QKD without quantum repeater, and is indispensable in long-distance intercity communication links. In addition, the chip based mdi-qkd protocol relay server proposed in this paper may be an ideal solution for scalable untrusted node metropolitan quantum networks. Using more advanced waveguide integrated SNSPD can further improve the integrated server with high detection efficiency, low timing jitter and high repetition rate.

Professor Ma Xiaosong said: "combined with photonic chip emitters, in the near future, a fully chip based, scalable and high key rate mdi-qkd metropolitan quantum network should be realized.

Link:https://www.spiedigitallibrary.org/proceedings/DownloadFiguresurl=/ContentImages/Journals/APDHC9/3/5/055002/FigureImages/AP_3_5_055002_f005.png