[Paper Review] High-rate intercity quantum key distribution with a semiconductor single-photon source

Jingzhong Yang, Zenghui Jiang+14 more|arXiv (Cornell University)|Aug 30, 2023

Quantum Information and Cryptography64 references4 citations

TL;DR

This paper demonstrates the first intercity quantum key distribution (QKD) using a semiconductor quantum dot single-photon source emitting in the telecom C-band. Operating over a 79 km fiber link with 25.49 dB loss, it achieves a secret key rate of 4.8×10⁻⁵ bits per pulse and an asymptotic maximum tolerable loss of 28.11 dB, outperforming existing single-photon source-based QKD systems and approaching decoy-state QKD with weak coherent pulses.

ABSTRACT

Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 * 10^{-5} with an average quantum bit error ratio of ~ 0.65 % are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.

Motivation & Objective

• To demonstrate high-rate, long-distance QKD using a deterministic semiconductor single-photon source in a real-world intercity fiber network.
• To overcome limitations of conventional weak coherent pulse sources, such as vulnerability to photon number splitting attacks and low secret key rates.
• To validate the feasibility of semiconductor quantum dots as scalable, high-performance sources for future quantum communication networks.
• To assess the performance of deterministic single-photon sources in practical, long-haul QKD deployments under real-world channel conditions.

Proposed method

• The experiment employs a semiconductor quantum dot embedded in a circular Bragg grating structure to generate on-demand, high-brightness single photons at 1550 nm in the telecom C-band.
• A BB84 protocol with polarization encoding is implemented using the single-photon source and a free-space beam splitter for state preparation and measurement.
• The system operates over a 79 km deployed fiber link between Hannover and Braunschweig, Germany, with a total loss of 25.49 dB.
• Quantum bit error rate (QBER) is measured at 0.65% on average, indicating high signal fidelity and low noise.
• Secret key rate (SKR) is evaluated in both asymptotic and finite-key regimes, with optimization of source efficiency, photon purity, and system dark counts.
• Theoretical modeling predicts an asymptotic maximum tolerable loss of 54.1 dB, corresponding to 276.58 km in standard fiber, under feasible future improvements.

Experimental results

Research questions

• RQ1 Can a deterministic semiconductor single-photon source enable high-rate QKD over intercity fiber links with practical loss budgets?
• RQ2 How does the secret key rate of a quantum dot-based QKD system compare to state-of-the-art decoy-state QKD using weak coherent pulses?
• RQ3 What is the maximum tolerable loss for a single-photon source-based QKD system in a real-world deployment?
• RQ4 To what extent can system parameters such as source efficiency, photon purity, and dark count rate influence the secret key rate and loss tolerance?
• RQ5 Can semiconductor quantum dots serve as viable building blocks for measurement-device-independent and quantum repeater-based quantum networks?

Key findings

• The experiment achieved a secret key rate of 4.8×10⁻⁵ bits per pulse over a 79 km fiber link with 25.49 dB total loss.
• The average quantum bit error ratio (QBER) was measured at 0.65%, indicating high signal quality and low noise in the system.
• An asymptotic maximum tolerable loss of 28.11 dB was experimentally demonstrated, corresponding to a theoretical channel length of 143.71 km in standard fiber.
• The system outperforms all previously reported QKD implementations using single-photon sources in terms of secret key rate.
• Theoretical modeling suggests that with feasible improvements in source efficiency, photon purity, and dark count suppression, a maximum tolerable loss of 54.1 dB (276.58 km) could be achieved.
• The results show that semiconductor quantum dot sources can match or exceed the performance of established decoy-state QKD with weak coherent pulses, even without further optimization.