Quantum Key Distribution Achieves 120km Range with Novel Single Photon Source.

Quantum key distribution utilising deterministic single photon sources achieves secure communication over a 120km fibre optic link. The system employs time-bin encoding, offering resilience to fibre impairments, and a quantum dot source at telecom wavelengths, demonstrating exceptional stability over six hours and a record key rate for such systems.

The secure transmission of information remains a paramount concern in an increasingly interconnected world, driving research into quantum key distribution (QKD) as a potential solution. QKD leverages the principles of quantum mechanics to guarantee secure communication, with recent efforts focusing on deterministic single-photon sources to enhance system performance and practicality. Researchers from Leibniz Universität Hannover, University of Stuttgart, and Nanjing University, led by Jipeng Wang, Joscha Hanel, and Xiao-Yu Cao et al, report successful QKD over a 120 kilometre fibre optic link utilising a time-bin encoded system and a quantum dot source operating at telecommunications wavelengths. Their work, entitled “Time-bin encoded quantum key distribution over 120 km with a telecom quantum dot source”, demonstrates enhanced stability and a notably high key rate, representing a significant step towards realising robust and scalable quantum communication networks based on solid-state technology. Time-bin encoding, a method of representing quantum information, offers resilience against fibre impairments, unlike polarisation encoding, and this implementation validates its compatibility with deterministic single-photon sources.

Quantum key distribution (QKD) represents a paradigm shift in secure communication, utilising the laws of quantum mechanics to guarantee secure key exchange. Recent developments focus on translating theoretical promise into practical, robust systems, and a new implementation demonstrates a 120km fibre-optic link utilising time-bin encoded QKD with a high-brightness quantum dot (QD) single-photon source operating at a telecommunication wavelength. This wavelength, around 1550nm, is crucial as it coincides with the lowest loss window for standard optical fibre, maximising transmission distance.

The system deliberately avoids polarisation encoding, a common QKD approach, because it is vulnerable to impairments within fibre networks. These include birefringence, a phenomenon where different polarisation states travel at different speeds, and polarisation-dependent loss, where signal strength varies with polarisation. Instead, it employs time-bin encoding, a method where quantum information is encoded in the precise arrival time of photons. This approach offers inherent stability against fibre-induced distortions.

A key component of this implementation is the deterministic single-photon source. Single-photon sources emit, ideally, one photon at a time. Deterministic sources, unlike heralded sources which rely on probabilistic emission and subsequent detection to confirm single-photon emission, produce a single photon with each trigger, simplifying the system’s complexity and increasing efficiency. Quantum dots, nanoscale semiconductor crystals, serve as the source, exhibiting properties that allow for the controlled emission of single photons.

The experiment confirms the viability of integrating a QD single-photon source with time-bin encoding within a telecom-band QKD system. The achieved key rate, a measure of how quickly a secure key can be established, exceeds that of other time-bin QKD systems utilising single-photon sources. This improvement stems from the source’s brightness, its ability to generate photons efficiently, and the stability of the time-bin encoding scheme. The solid-state nature of quantum dots also offers potential for miniaturisation and cost reduction, crucial factors for widespread adoption.

Ongoing research explores alternative single-photon source technologies, including those based on gallium nitride, alongside more complex encoding schemes. Multi-level time-bin encoding, for example, aims to increase key generation rates by encoding more information per photon. Furthermore, investigations into measurement-device-independent QKD protocols address potential security vulnerabilities arising from imperfections in the detectors used to receive the quantum signals. Complementary work on free-space QKD, utilising lasers to transmit photons through the atmosphere, tackles the challenges of atmospheric turbulence and signal depolarisation, offering an alternative to fibre-optic transmission and contributing to a more resilient and secure communication infrastructure.