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IEEE International Conference on Advanced Networks and Telecommunications Systems

Impact of BB84 QKD Transmitter’s Wavelength Mismatch on Secure Key Generation Rate

Pragya Kushwaha, Adarsh Jain, Santanu Sinha, Prashant Varma, Punam Pradeep Kumar, Apurba N. Bhattacharya

Published Dec 18, 2022• 2 citations
Publisher

Abstract

In principle BB84 protocol supports secure quantum communication by assuming transmitter and receiver devices to be ideal. However, practically it is very challenging to build an ideal single photon source and detector. These imperfections in practical devices (i.e., wavelength mismatch, full width at half maximum (FWHM) pulse width mismatch and different arrival times of photon) leads to side channel attacks. In this work, we have analyzed the wavelength mismatch issue associated with four laser diode based practical BB84 transmitter. An asymptotically achievable rate of extraction of secure key (also known as key generation rate) is estimated from side-channel leakage calculation between transmitter (Alice) and adversary (Eve).

DOI
10.1109/ants56424.2022.10227726

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In principle BB84 protocol supports secure quantum communication by assuming transmitter and receiver devices to be ideal. However, practically it is very challenging to build an ideal single photon source and detector. These imperfections in practical devices (i.e., wavelength mismatch, full width at half maximum (FWHM) pulse width mismatch and different arrival times of photon) leads to side channel attacks. In this work, we have analyzed the wavelength mismatch issue associated with four laser diode based practical BB84 transmitter. An asymptotically achievable rate of extraction of secure key (also known as key generation rate) is estimated from side-channel leakage calculation between transmitter (Alice) and adversary (Eve).
A typical implementation of BB84 protocol for quantum communication uses four laser diodes for transmitting weak coherent pulses, which may not have the same characteristics. We have characterized these lasers for mismatch in various parameters such as spectral width, pulse width, spatial mode, peak wavelength, polarization and their arrival times at the receiver. This information is utilized to calculate possible information leakage through side channel attacks by evaluating mutual information between source and eavesdropper. Based on our experimental observations of cross correlation between parameter values for different laser diodes, we suggest methods to reduce information leakage to Eve.
The absence of real single-photon sources is a significant limitation to the experimental implementation of the BB84 quantum key distribution (QKD) protocol. When substantially attenuated laser pulses are substituted for ideal sources in practical systems, the outcome is weak coherent states where each pulse has a nonzero probability of having multiple photons rather than pure single-photon emission. To evaluate quantum bit error rate (QBER), key generation rate and overall system security it is essential to describe the exact impact of the optical and physical system on such pulses. This practical tradeoff presents additional issues. It is crucial to understand how QBER is impacted by attenuation, loss, dispersion, depolarisation and detector properties particularly in adversarial scenarios such as photon number splitting (PNS) and beam-splitting attacks. Although a significant amount of research has been conducted on algorithmic simulations of BB84, there are few complete frameworks that can physically explain these effects and their impact on QBER. This work models the BB84 protocol at the physical level by developing a Python-based simulation platform that takes into account all significant channel impairments and system inefficiencies. The framework supports both theoretical and practical optimisation of QKD systems by further isolating the effects of different optical losses and eavesdropping techniques on QBER and secure key generation.
In prepare-and-measure quantum key distribution systems, careful preparation of quantum states within the transmitter device is a significant driver of both complexity and cost. Moreover, the security guarantees of such systems rest on the correct operation of high speed quantum random number generators (QRNGs) and the high-fidelity modulation of weak optical signals by high-speed optoelectronic devices, all of which must be hardened against a variety of known side-channel attacks. A fully passive state preparation approach elegantly resolves these problems by combining state preparation and QRNG stages into a single optical instrument. By using pairs of optical pulses from a gain-switched laser diode as ready-to-use qubits, the QKD transmitter can be radically simplified, eventually comprising a single laser and local phase tomography stage. We demonstrate our simplified transmitter by establishing a QKD link over a 10 km fiber, generating a secret key rate 110 bits/s, sufficient for practical deployment in "last mile" urban quantum networks. Our results show promise in making QKD simpler and more accessible, closing a critical technology gap in building a secure quantum communication infrastructure.
One of the challenges in practical quantum key distribution is dealing with efficiency mismatch between different threshold single-photon detectors. There are known bounds for the secret key rate for the BB84 protocol with detection-efficiency mismatch provided that the eavesdropper sends no more than one photon to the legitimate receiver. Here we improve these bounds and give tight bounds for the secret key rate with a constant detection-efficiency mismatch under the same single-photon assumption. We propose a method based on the analytical minimization of the relative entropy of coherence, which can be used in other problems in quantum key distribution. Also we propose an adaptation of the decoy state method to prove the security in the case of weak coherent pulses on the source side.
Side-channel attacks on practical quantum key distribution systems compromise its security. Although some of these attacks can be taken into account, the general recipe of how to eliminate all side-channel flaws is still missing. In this work, we propose a method for estimation of the total passive side-channel information leakage from the Alice's light source. The method relies on Hong-Ou-Mandel interference between different signals emitted by Alice, which reveals their overall mode mismatch without the necessity to measure all individual degrees of freedom independently. We include experimental values of interference visibility in the security proof for the decoy-state BB84 protocol, and lower-bound the secure key rate for realistic light sources. The obtained results provide a tool that can be used for certification of the current QKD systems and pave the way towards the loophole-free design of the future ones.
Subject of study. The effect of different quantum efficiencies of single-photon detectors in quantum key distribution (QKD) systems implementing the BB84 protocol with passive basis choice is investigated. Aim of study. The aim of this study is to determine the secret key generation rate and security of the final key in quantum cryptography systems as functions of detector efficiency mismatch. Additionally, an analytical equation is derived for estimating the secret key generation rate in quantum communication systems with passive basis choice while accounting for different detector efficiencies. Method. The secret key is calculated by dividing the received measurement data according to the measurement bases, followed by performing separate post-processing for each dataset, and finally summing the resulting estimates of the secret key. Main results. An analytical method for estimating the secret key generation rate in systems with passive basis choice and mismatched detector efficiencies is developed. A satellite-to-ground quantum communication channel is modeled using the parameters of the Zvenigorod Observatory in Russia. The proposed theory for accounting for the detector efficiency mismatch is applied to a model of a satellite QKD system, and the determined length of the secret key generated during a single communication session with the satellite is found to be 300 kbit. Practical significance. The obtained results account for the effect of detector efficiency mismatch when estimating the length of the secret key in QKD systems, thereby increasing their reliability in practical implementations.