arXiv subjects

Roman Shakhovoy

Publications and source records attributed to Roman Shakhovoy.

13 recordsLinked to original sources

arXiv2025

Optical-pumping attack on a quantum key distribution laser source

We report a new type of vulnerability in practical implementations of quantum key distribution systems. We show that it is possible to increase the pulse energy of a source laser diode not only by injection-locking it by external light near its emission wavelength of 1550 nm, but also by optically pumping it at a much shorter wavelength. We demonstrate 10% increase in pulse energy when exposing the laser diode to 1310-nm, 1.6-mW cw laser light via its fiber pigtail. This may allow an eavesdropper to steal the secret key. A possible countermeasure is to install broadband optical filters and isolators at the source's output and characterise them during the security certification.

quant-ph↗

arXiv2025

Passive state preparation for quantum key distribution with phase encoding

We propose here a method of passive state preparation for quantum key distribution with phase encoding based on the measurement of the phase difference between pulses of a gain-switched laser. The features of the optical scheme of the transmitter are discussed and the results of computer simulation are presented to demonstrate the effectiveness of the proposed method.

quant-ph↗

arXiv2025

Modulator-free transmitter for quantum key distribution in metropolitan area networks

A positive economic effect from the implementation of quantum key distribution (QKD) technology can be achieved only with significant scaling, which involves the deployment of branched metropolitan area networks. The creation of QKD systems suitable for such networks is an important task for the coming years. This paper considers a method for preparing quantum states using pulsed optical injection, which can be used as a basis for a compact modulator-free transmitter ideally suited for QKD at typical distances within a city. Considering the relative proximity between nodes of a MAN, we suggest to abandon the decoy states, which, together with the proposed method of quantum state preparation, allows making the transmitter extremely simple. We report here the results of an experiment confirming the operating principle and provide a security analysis of the three-state decoy-free QKD protocol that can be implemented using such a device.

quant-ph↗

arXiv2025

Influence of optical self-injection on statistical properties of laser-pulse interference

Pulsed optical self-injection markedly affects the emission characteristics of semiconductor lasers. In this work, we analyze its influence on the statistical properties of laser-pulse interference. We experimentally demonstrate that varying the arrival time of reflected optical pulses back into the laser cavity influences the phase-diffusion and induces an effect that, by analogy with external optical injection, we call phase locking. A comprehensive theoretical analysis of this phenomenon is also presented.

physics.optics↗

arXiv2023

Digitization of a random signal from the interference of laser pulses

In the study of quantum random number generators (QRNGs), the problem of random signal digitization is often not considered in detail. However, in the context of a standalone QRNG device, this issue is very important. In this paper, we consider the problem of digitizing laser pulses with random intensity and analyze various approaches used to estimate the contribution of classical noise. A simple method for determining the quantum reduction factor suitable for digitization with an analog-to-digital converter is proposed.

quant-ph↗

arXiv2023

Phase randomness in a semiconductor laser: Issue of quantum random-number generation

Gain-switched lasers are in demand in numerous quantum applications, particularly, in systems of quantum key distribution and in various optical quantum random number generators. The reason for this popularity is natural phase randomization between gain-switched laser pulses. The idea of such randomization has become so familiar that most authors use it without regard to the features of the laser operation mode they use. However, at high repetition rates of laser pulses or when pulses are generated at a bias current close to the threshold, the phase randomization condition may be violated. This paper describes theoretical and experimental methods for estimating the degree of phase randomization in a gain-switched laser. We consider in detail different situations of laser pulse interference and show that the interference signal remains quantum in nature even in the presence of classical phase drift in the interferometer provided that the phase diffusion in a laser is efficient enough. Moreover, we formulate the relationship between the previously introduced quantum reduction factor and the leftover hash lemma. Using this relationship, we develop a method to estimate the quantum noise contribution to the interference signal in the presence of phase correlations. Finally, we introduce a simple experimental method based on the analysis of statistical interference fringes, providing more detailed information about the probabilistic properties of laser pulse interference.

quant-ph↗

arXiv2023

Wavelength- and time-division multiplexing via pump current variation of a pulsed semiconductor laser -- a method of synchronization for quantum key distribution

The dependence of the semiconductor laser wavelength on the pump current is a well-known phenomenon, which is generally attributed to a change in the refractive index of the active layer due to carrier injection. This effect is usually considered to be a drawback as it causes frequency chirping of the pulses produced via direct current modulation. Here, we show that at high values of thermal resistance of a laser diode, the lasing wavelength is red-shifted due to the change of the refractive index caused by a thermal effect and this shift may significantly exceed (in absolute value) the blue shift related to a refractive index change induced by the carrier injection. We propose to benefit from the ability to tune wavelength by the pump current variation and use the same laser to generate qubits and synchronization optical pulses for quantum key distribution at different wavelengths. To demonstrate the proposed method of synchronization, we perform numerical simulations as well as proof-of-principle experiments.

physics.optics↗

arXiv2023

Gain-switched vcsel as a quantum entropy source: the problem of quantum and classical noise

We consider the problem of quantum noise extraction from polarization swapping in a gain-switched VCSEL. The principle of operation of a quantum random number generator is based on the generation of laser pulses with one of two orthogonal polarization states, followed by digitization of polarization-resolved pulses with a comparator. At intensity values of laser pulses close to the threshold value of the comparator, the contribution of the classical noise of the photodetector will have a crucial role in making a decision on the choice of a logical zero or one. We show how to evaluate the contribution of classical noise and how to calculate the quantum reduction factor required for post-processing.

quant-ph↗

arXiv2021

Direct phase modulation via optical injection: theoretical study

Direct phase modulation via optical injection is a newly developed method for coding the phase of a gain-switched laser, which meets high requirements placed on transmitters for quantum key distribution: compactness, low losses, compatibility with CMOS technologies, and the absence of undesirable effects leading to the side-channel information leakage. Despite the successful implementation and good prospects for the further development of this system, there is still a lack of theoretical investigations of this scheme in the literature. Here, for the first time, we perform its theoretical analysis. We study the influence of the spontaneous emission noise, examine the role of the gain non-linearity and consider the effect of the temperature drift. The results obtained reveal that these phenomena significantly affect the system performance. We have tried to formulate here practical instructions, which will help to take these features into account when elaborating and employing the optical-injection-based phase modulator.

quant-ph↗

arXiv2020

Phase randomness in a gain-switched semiconductor laser: stochastic differential equation analysis

We performed theoretical analysis of the phase randomness in a gain-switched semiconductor laser in the context of its application as a quantum entropy source. Numerical simulations demonstrate that phase diffusion r.m.s. exhibits non-linear dependence on the bias current, which could be of significant practical importance, particularly, in application to high-speed optical quantum random number generators. It is shown that phase diffusion between laser pulses cannot always be assumed to exhibit required efficiency, particularly, at high pulse repetition rates. It was also revealed that the gain saturation significantly affects the r.m.s. value of the phase diffusion and, in essence, determines the degree of non-linearity of its dependence on the pump current.

quant-ph↗

arXiv2020

Influence of chirp, jitter and relaxation oscillations on probabilistic properties of laser pulse interference

Interference of laser pulses is an essential ingredient of quantum randomness; therefore, probabilistic properties of laser pulses gains new relevance. Here, we consider in detail the combined influence of the three effects - chirp, jitter and relaxation oscillations - on the probability density function of the interference of pulses from a gain-switched semiconductor laser. We develop a rigorous model based on laser rate equations and demonstrate that only consideration of all these three effects together allows describing the interference statistics properly. We supplement our theoretical calculations with corresponding measurements at various pump currents. Experimental results demonstrate perfect agreement with predictions of the model and are well reproduced by Monte-Carlo simulations.

quant-ph↗

arXiv2020

Certified Quantum Random Numbers from Untrusted Light

A remarkable aspect of quantum theory is that certain measurement outcomes are entirely unpredictable to all possible observers. Such quantum events can be harnessed to generate numbers whose randomness is asserted based upon the underlying physical processes. We formally introduce, design and experimentally demonstrate an ultrafast optical quantum random number generator that uses a totally untrusted photonic source. While considering completely general quantum attacks, we certify and generate in real-time random numbers at a rate of $8.05\,$Gb/s with a rigorous security parameter of $10^{-10}$. Our security proof is entirely composable, thereby allowing the generated randomness to be utilised for arbitrary applications in cryptography and beyond. To our knowledge, this represents the fastest composably secure source of quantum random numbers ever reported.

quant-ph↗

arXiv2020

Quantum noise extraction from the interference of laser pulses in optical quantum random number generator

We propose a method for quantum noise extraction from the interference of laser pulses with random phase. Our technique is based on the calculation of a parameter, which we called the quantum reduction factor, and which allows determining the contributions of quantum and classical noises in the assumption that classical fluctuations exhibit Gaussian distribution. To the best of our knowledge, the concept of the quantum reduction factor is introduced for the first time. We use such an approach to implement the post-processing-free optical quantum random number generator with the random bit generation rate of 2 Gbps.

quant-ph↗