Superconducting silicon-photonic chip developed for quantum communication

A superconducting silicon chip is used as an unreliable relay server for secure quantum communication. By exploiting the unique low dead time characteristic of single photon superconducting detectors integrated into the waveguide (red wires with a hairpin shape in the middle), optimal times slice encoded Bell state measurements ( represented by blue and gray waveform curves between four photons, indicated by red balls) are made. These in turn improve the secure key rate of quantum communication. Credit: MaLab, Nanjing University

Researchers solve a long-standing challenge in quantum optics: the optimal measurement of the Bell state of time-coded qubits, to improve the key rate of secure quantum communication.

Integrated Quantum Photonics (IQP) is a promising platform for realizing scalable and practical quantum information processing. So far, most demonstrations with IQP have focused on improving the stability, quality, and complexity of experiments for traditional bulk-element and fiber-optic based platforms. A more demanding question is: “Are there any possible experiences with IQP that are not possible with traditional technology?” “

This question is answered in the affirmative by a team jointly led by Xiao-Song Ma and Labao Zhang from Nanjing University, and Xinlun Cai from Sun Yat-sen University, China. As stated in Advanced photonics, the team performs quantum communication using a silicon photonics-based chip with a nanowire superconducting single-photon detector (SNSPD). The excellent performance of this chip allows them to achieve an optimal measurement of the state of Bell over time and to significantly improve the key rate in quantum communication.

The single photon detector is a key element for quantum key distribution (QKD) and highly desirable for the integration of photonic chips to realize practical and scalable quantum networks. By exploiting the unique high-speed function of the SNSPD integrated into the optical waveguide, the dead time of single photon detection is reduced by more than an order of magnitude compared to traditional SNSPD at normal incidence. This in turn allows the team to solve one of quantum optics long-standing challenges: the optimal measurement of the Bell state of time-coded qubits.

This breakthrough is important not only for the field of quantum optics from a fundamental point of view, but also for quantum communications from an application point of view. The team uses the unique advantages of the heterogeneously integrated superconducting silicon photonics platform to realize a server for the distribution of measuring device independent quantum keys (MDI-QKD). This effectively removes all possible attacks by detector side channel and thus greatly improves the security of quantum cryptography. Combined with a time division multiplexing technique, the method achieves an order of magnitude increase in the MDI-QKD key rate.

Schematic of the superconducting silicon-photonic chip

(a) Diagram of the setup of the experiment. A superconducting silicon photonic chip that performs optimal Bell condition measurements is used as a server for MDI-QKD, allowing Alice and Bob to exchange secure keys without detector side channel attacks. (b) Destructive and constructive interference in coincidences count when Alice and Bob send the same states (blue dots) or different states (red dots). (c) Key rate secured under different losses. Credit: Zheng et al., Doi 10.1117 / 1.AP.3.5.055002

By exploiting the advantages of this heterogeneously integrated system, the team achieves a high secure key rate with a clock rate of 125 MHz, which is comparable to the peak experimental results of MDI-QKD with a frequency of GHz clock. “Unlike MDI-QKD experiments at GHz clock frequency, our system does not require complicated injection locking technique, which greatly reduces the complexity of the transmitter,” says Xiaodong Zheng, doctoral student in the Ma group and first author of the Advanced photonics paper.

“This work shows that integrated quantum photonic chips not only offer a path to miniaturization, but also dramatically improve system performance compared to traditional platforms. Combined with integrated QKD transmitters, a fully chip-based, scalable, high-speed metro quantum network is expected to be realized in the near future, ”said Ma.

Reference: “Heterogeneously integrated superconducting silicon photonics platform for the distribution of independent quantum keys of measuring devices” by Xiaodong Zheng, Peiyu Zhang, Renyou Ge, Liangliang Lu, Guanglong He, Qi Chen, Fangchao Qu, LaBao Zhang, Xinlun Cai, Yanqing Lu, Shining N. Zhu, Peiheng Wu, Xiaosong Ma, October 30, 2021, Advanced photonics.
DOI: 10.1117 / 1.AP.3.5.055002


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