Last Updated on June 5, 2025 by Neila
In a breakthrough demonstration, researchers at the University of Science and Technology of China (USTC) have achieved quantum key distribution (QKD) rates beyond what was previously thought possible using conventional laser sources. Published in Physical Review Letters on May 29, 2025, the study reveals how an “on-demand, bright single-photon source” enabled secure key generation at rates 79% higher than the theoretical limit set by weak coherent (laser) pulses.
The international team—affiliated with the Hefei National Research Center for Physical Sciences at the Microscale and School of Physical Sciences, USTC; the Shanghai Research Center for Quantum Science and CAS Center for Excellence in Quantum Information and Quantum Physics, USTC; and the Hefei National Laboratory, USTC—set out to overcome a fundamental bottleneck in quantum cryptography. As the paper explains, “The probability of a pulse containing exactly one photon … is inherently bounded by \(P_{<n>} (1) ≤ 1/e\) when \(<n> = 1\),” meaning laser-based QKD cannot exceed a secure key rate proportional to \(1/e\) per channel use. By contrast, their bright single-photon source delivered a single-photon efficiency of 0.71 \(\pm 0.02\)—nearly triple the \(1/e \approx 0.37\) limit of attenuated lasers—and produced almost no multi-photon pulses.
The researchers constructed a compact transmitter in which a single indium arsenide (InAs)/gallium arsenide (GaAs) quantum dot was embedded in a tunable Fabry–Perot cavity. Under resonant pulsed excitation, this setup emitted up to 54 MHz of true single photons collected into a single-mode fiber with \(71\%\) efficiency. To ensure privacy against potential eavesdroppers, they added a narrow-linewidth optical filter (5.4 GHz Fabry–Perot etalon) to isolate the quantum signal from background noise, applied active phase randomization to prevent coherence between consecutive pulses, and used a high-speed Pockels cell to randomly encode polarization states—all essential steps to conform to the BB84 QKD protocol without relying on decoy states.
In a real-world field trial, the team transmitted these carefully prepared single photons over a 209 m free-space link across USTC’s Hefei campus, experiencing an average channel loss of \(14.6 \pm 1.1\) dB. At a repetition rate of 76.13 MHz and a mean photon number of \(0.292 \pm 0.008\) per pulse, they achieved a secure key rate (SKR) of \(1.08 \times 10^{-3}\) bits per pulse—\(79\%\) higher than what any weak-coherent-state QKD system could deliver under the same conditions. In their own words, “These findings unequivocally demonstrate the superior performance of single-photon sources over weak coherent light for QKD applications, marking a pivotal stride towards realizing a global quantum internet.”
Traditional QKD systems rely on heavily attenuated laser pulses, which follow a Poisson distribution: many pulses contain zero photons, some contain one photon (the only useful case for secure key bits), and a few contain two or more photons, potentially opening the door to photon-splitting attacks. Because the fraction of single-photon pulses is capped at \(1/e \approx 37\%\) when the average photon number per pulse is one, the maximum secure key generation rate scales directly with this bound.
By contrast, a true single-photon emitter can, in principle, produce exactly one photon on demand and rarely emit multiple photons in the same pulse. The USTC team’s quantum dot source routinely achieved a near-unity single-photon emission probability—far above the laser limit—while maintaining extremely low multi-photon contamination. As channel losses increase, the performance gap only widens: below about 19 dB loss, their single-photon system consistently outperforms any laser-based approach.
This milestone confirms a long-held expectation that true single-photon emitters can break the rate ceiling imposed by attenuated lasers. As the authors note, “Our results conclusively demonstrate the superior performance of SPSs for QKD applications, paving the way for advanced single-emitter-based quantum communication and the realization of a future quantum internet.”
Looking ahead, the team identifies several avenues for improvement:
- Extending Reach: By suppressing multi-photon components even further or by integrating decoy-state theory with single-photon sources, higher channel losses could be tolerated.
- Speed Boost: Frequency doubling could push repetition rates into the gigahertz regime, matching state-of-the-art laser-based QKD experiments.
- Telecom Compatibility: Converting visible-wavelength photons to the 1.3–1.5 \(\mu\)m telecom band would allow fiber deployments spanning tens or hundreds of kilometers.
- Network Integration:Â True single photons are essential for advanced protocols, such as measurement-device-independent QKD, twin-field QKD, quantum repeaters, and teleportation-based relays, that underpin scalable quantum networks.
In practical terms, a global quantum internet built upon robust single-photon sources could offer unbreakable encryption free from the assumptions and side channels that challenge classical approaches. By demonstrating a \(79\%\) rate advantage over conventional laser-based systems in a real-world urban setting, this USTC team has turned a long-standing theoretical dream into experimental reality—and in doing so, brought us one step closer to an era of truly secure global communication.
Read the full research paper: Y. Zhang et al., Phys. Rev. Lett. 134, 210801 (2025).
I am a science enthusiast with a keen interest in exploring the latest advancements in matter wave optics and related technologies.
