Quantum Key Distribution Mitigates Phase Correlations with Path-selection Modulation at 1GHz Repetition Rates

Quantum key distribution promises secure communication, but vulnerabilities remain in practical implementations. Amita Gnanapandithan, Li Qian, and Hoi-Kwong Lo from the University of Toronto now demonstrate a previously under-explored weakness stemming from phase correlations in systems using standard encoding methods. Their work reveals that these correlations, arising at repetition rates up to the GHz level, create potential security loopholes. To address this critical issue, the team proposes and experimentally validates a novel “path-selection modulation” source, which eliminates the need for active phase modulation altogether and achieves robust phase randomization using gain-switching, representing a significant step towards truly secure quantum communication networks.

Correlated Light Sources, QKD Security Analysis

This document presents supplemental material detailing research into the security of quantum key distribution (QKD) systems, focusing on correlations introduced by the light sources used. The research investigates both actively modulated and partially passive sources, aiming to minimize vulnerabilities and enhance system performance. Scientists meticulously characterized potential correlations, a crucial step towards building more secure communication networks. The team investigated actively modulated light sources, revealing that even with precise control, residual correlations can arise. These correlations, potentially exploitable by eavesdroppers, were quantified through experimental measurements and detailed simulations.

Complementary investigations focused on partially passive sources, designed to inherently reduce these problematic correlations. The goal is to develop light sources that minimize vulnerabilities without compromising performance. Experiments involved specialized setups utilizing arbitrary waveform generators, radio frequency amplifiers, intensity modulators, photodiodes, and high-speed oscilloscopes. Researchers carefully controlled the modulation of light, generating pseudorandom pulse trains to observe and analyze phase and intensity fluctuations. Data analysis focused on quantifying the relationship between successive pulses, revealing the extent of correlations present. Simulations complemented the experimental work, verifying results and extending the analysis to higher frequencies. This combined approach provides a comprehensive understanding of correlation behavior and its impact on QKD security.

Phase Correlation Analysis at 3GHz Repetition Rates

Scientists meticulously characterized phase correlations, a potential vulnerability in quantum key distribution, through both experimental and simulated methods, extending analysis up to repetition rates of 3GHz. The study employed an electro-optic intensity modulator functioning as a phase modulator, converting phase changes into measurable intensity variations. Researchers randomly generated one of three distinct phase shifts, allowing them to observe the resulting modulation. Experiments were performed at 500MHz and 1GHz, with simulations extending to 2GHz and 3GHz, allowing for a broad assessment of correlation behavior.

The team established a linear relationship between applied voltage and phase change, enabling precise control and measurement of phase modulation. Researchers investigated non-nearest-neighbor correlations, extending the analysis beyond immediate pulse relationships to uncover more subtle vulnerabilities. To address these vulnerabilities, scientists engineered a novel path-selection modulation source, eliminating the need for active phase modulation altogether. This source utilizes multiple optical paths, each corresponding to a specific encoded state, and randomly selects between them. Phase randomization is achieved through gain-switching, a technique that rapidly alters the optical gain, effectively scrambling the phase information.

The source was demonstrated at a 1GHz repetition rate using single-mode fiber, and requires no additional post-selection or complex detector arrangements. Intensity modulators are used solely for pulse suppression, ensuring negligible intensity correlations at GHz repetition rates, a finding further verified through experimental characterization. The team rigorously characterized the generated polarizations, the level of phase randomization achieved, the indistinguishability of the generated polarization states, and the intensity correlations, demonstrating the source’s effectiveness and stability.

GHz Phase Correlations in Electro-Optic Encoding

This work details a comprehensive characterization of phase correlations arising from electro-optic phase encoding, extending to repetition rates up to the GHz level. Researchers conducted both experimental and simulated investigations to quantify these correlations, revealing key insights into potential vulnerabilities. Experiments were performed at modulation rates of 500MHz and 1GHz, while simulations extended to 2GHz and 3GHz, allowing for a broad assessment of correlation behavior. The experimental setup employed an arbitrary waveform generator, a radio frequency amplifier, an intensity modulator, a photodiode, and an oscilloscope.

By driving the intensity modulator to randomly produce phase changes, the team collected and analyzed intensity modulation events at each repetition rate. Analysis focused on quantifying how the phase of a pulse differed based on the phase of the preceding pulse, considering six possible phase transitions. Results demonstrate that at 1GHz, nearest-neighbour correlations reach approximately 0. 04π, while at 500MHz, they are roughly 0. 02π.

Notably, correlations at 1GHz only begin to level out around six pulses, compared to three at 500MHz. The data confirms a predictable relationship, where correlations observed at 1GHz over a given number of pulses correspond to those at 500MHz over half that number. To complement the experimental work, researchers developed a detailed simulation of the phase modulation process. This model incorporated the bandwidth limitations of key components, using Bessel filters to accurately represent their frequency responses. The simulation accurately reproduced the waveforms obtained experimentally, validating the model’s accuracy. The simulation allowed for extending the analysis beyond the experimentally achievable frequencies, providing a more complete understanding of correlation behavior at higher repetition rates. These findings are crucial for developing secure communication systems and mitigating potential vulnerabilities associated with phase encoding.

Phase Correlation Vulnerability in Quantum Key Distribution

This research presents a detailed characterization of phase correlations introduced by active phase modulators, a vulnerability in high-speed quantum key distribution (QKD) systems. Scientists demonstrated that at repetition rates up to the GHz level, these correlations can be significant, reaching levels on the order of 0. 01π. The team conducted both experimental and simulated analyses to quantify these effects, revealing potential security weaknesses in current QKD implementations. To address this vulnerability, researchers proposed and characterized a novel path-selection modulation source. This source eliminates the need for active phase modulation by randomly selecting between multiple optical paths, each representing a different encoded state. Through the use of gain-switching and careful control of intensity modulation, the team achieved negligible intensity correlations and a high degree of phase randomization, with visibility levels below 10−4.