Decoy-like Quantum Key Distribution Enables Secure Communication with Imperfect Sources, Even with G^(2)(0) > 0.1

Quantum key distribution promises secure communication, but currently relies on the generation of ideal single photons, a significant experimental hurdle. Chanaprom Cholsuk, Furkan Ağlarcı, Daniel K. L. Oi, Serkan Ateş, and Tobias Vogl have developed a new protocol that overcomes this limitation, enabling the use of more readily available, imperfect single photon sources. Their research demonstrates that by employing a ‘decoy-like’ method, the system can effectively detect and mitigate attacks that exploit imperfections in photon generation, even when sources exhibit relatively high levels of multi-photon emission. This breakthrough significantly expands the range of viable sources for quantum key distribution, offering a pathway towards practical, high-performance systems applicable to both terrestrial and satellite-based communication, and importantly, does so without requiring additional, complex hardware.

Decoy States Mitigate Imperfect Photon Sources

A new protocol enhances the performance of quantum key distribution (QKD) systems by addressing the challenges posed by imperfect single photon sources. Existing QKD methods often assume ideal sources, but this research investigates how to improve security and key rates when real-world sources emit multiple photons. The team developed a protocol that introduces additional decoy states, carefully tailored to characterise and compensate for the probability of multi-photon emission, allowing for precise understanding of the source’s behaviour and optimisation of the QKD process. This approach models single photon sources as emitting photons according to a statistical distribution, enabling accurate quantification of multi-photon emissions.

This detailed modelling enables a comprehensive security analysis, rigorously proving the protocol’s resilience against collective attacks, even with imperfect sources and detectors. Numerical simulations demonstrate that the proposed protocol significantly outperforms existing methods when using realistic single photon sources, achieving higher key rates and extending the maximum secure transmission distance, particularly when signal quality is reduced. This protocol effectively addresses the limitations imposed by imperfect single photon sources, paving the way for more practical and robust QKD systems.

Sub-Poissonian Photon Sources for Quantum Key Distribution

Researchers are developing improved single photon sources for quantum key distribution using semiconductor nanostructures. The team employs molecular beam epitaxy to fabricate high-quality quantum dots embedded within a carefully engineered waveguide structure. Precise control over the quantum dot size, composition, and arrangement allows for optimisation of the single photon emission characteristics, aiming to create sources that emit photons individually, a crucial requirement for secure communication. The emitted light is characterised using a time-correlated single photon counting system, accurately measuring the photon statistics and confirming the emission of fewer than one photon per excitation cycle.

Detailed spectroscopic analysis investigates the quantum dot energy levels and identifies potential sources of decoherence. The experimental setup incorporates a cryostat to maintain low temperatures, enhancing the signal-to-noise ratio. Researchers analyse the emitted photons using a technique that measures the second-order correlation function, confirming the non-classical nature of the light source. Optimising experimental parameters maximises the single photon emission rate and minimises the probability of emitting multiple photons, crucial for secure quantum communication.

Solid-State QKD with Boron Nitride Defects

This research focuses on developing a practical quantum key distribution (QKD) system using solid-state quantum emitters, specifically defects in hexagonal boron nitride (hBN). The ultimate goal is to create a secure communication system that leverages the principles of quantum mechanics to guarantee the security of the encryption key, moving beyond traditional QKD systems towards a more compact, robust, and potentially mobile solution. The research demonstrates that defects in hBN can act as efficient single-photon emitters, making them suitable for QKD applications. Researchers have identified and characterised various defects exhibiting desirable quantum properties, putting significant effort into controlling the emission wavelength, brightness, and stability of these hBN emitters, including techniques like cavity integration and careful material engineering.

The team has successfully demonstrated QKD using hBN emitters, showing the feasibility of this approach and working towards miniaturising the QKD system, potentially enabling mobile or handheld devices. A comprehensive database of hBN defects has been created, cataloguing their properties and aiding in the selection of optimal emitters. The research explores and implements improvements to QKD protocols, such as coincidence detection and advanced data processing techniques, to enhance key rates and security.

Higher Order Correlations Enhance QKD Security

Scientists have developed a new approach to quantum key distribution (QKD) that relaxes stringent requirements for single photon sources. This research demonstrates that sources routinely achieved in experiments, but previously considered unsuitable for secure communication, can be used effectively. The team shows that variations in these correlation values can be used to detect attacks that attempt to intercept the quantum signal, a significant security concern in QKD systems, analogous to established decoy-state methods. Crucially, the protocol enables both single and two-photon pulses to contribute securely to the secret key rate, improving performance, particularly under conditions of high signal loss.

Simulations and experimental results using hexagonal boron nitride demonstrate that the proposed method outperforms the conventional framework, extending the operational range of QKD systems. The researchers determined that a relatively small number of photons must be detected to achieve stable estimation of the correlation values, a practical threshold for implementation. Future work will likely focus on optimising the detection process and exploring the protocol’s applicability to different solid-state single photon sources and satellite-based quantum communication systems. This advancement establishes a viable route toward high-performance QKD without relying on ultra-pure single photon sources, simplifying system requirements and broadening the potential for secure quantum communication.