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Journal of Physics B Atomic Molecular and Optical Physics

Security analysis of unidimensional continuous-variable quantum key distribution with discretized amplitude modulation

Yingying Li, Tianyi Wang

Published Jun 14, 2024• 5 citations
Publisher

Abstract

Abstract Unidimensional continuous variable quantum key distribution (UD-CVQKD) applies Gaussian modulation on one single quadrature of the coherent state. However, in practical experimentations, the finite resolution of modulator driving voltages will result in Gaussian modulation discretization. To investigate its impact on the security of UD-CVQKD, the discretization of amplitude modulation is modeled and analyzed, with the secret key rate formulated against entangling cloner attack. Simulation results demonstrate that despite the performance degradation, UD-CVQKD protocol with discretized amplitude modulation may relax the restriction on correlation parameters to extend the secure region under certain parameters, which can be beneficial to the practicability of UD-CVQKD.

DOI
10.1088/1361-6455/ad5891

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Abstract Unidimensional continuous variable quantum key distribution (UD-CVQKD) applies Gaussian modulation on one single quadrature of the coherent state. However, in practical experimentations, the finite resolution of modulator driving voltages will result in Gaussian modulation discretization. To investigate its impact on the security of UD-CVQKD, the discretization of amplitude modulation is modeled and analyzed, with the secret key rate formulated against entangling cloner attack. Simulation results demonstrate that despite the performance degradation, UD-CVQKD protocol with discretized amplitude modulation may relax the restriction on correlation parameters to extend the secure region under certain parameters, which can be beneficial to the practicability of UD-CVQKD.
Discrete-modulated continuous-variable quantum key distribution offers significant practical deployment advantages due to its straightforward state preparation and high compatibility with coherent optical communication systems. However, security analysis and parameter estimation of discrete-modulated protocol are different with Gaussian-modulated protocols, which could cause different practical security problems. Herein, we investigate the amplitude-boosting attack against discrete-modulated continuous-variable quantum key distribution systems and assess its impact on system performance. Our findings reveal that this attack could cause overestimation of secret key rate perceived by Alice and Bob, thereby opening a security loophole, and the vulnerability could be severer than Gaussian modulation. Additionally, we summarize defensive countermeasures, marking a crucial step towards enhancing the practical security of discrete-modulated continuous-variable quantum key distribution.
Continuous-variable quantum key distribution (CV-QKD), which uses quadratures of the electromagnetic field, enables practical quantum communication using standard telecommunication technologies. Unidimensional CV-QKD (UD-CVQKD) simplifies the implementation by restricting modulation to a single quadrature. In this work, we experimentally demonstrate a free-space Gaussian-modulated UD-CVQKD system operating under a high detector electronic-noise regime (1.4 shot-noise units). The system employs polarized coherent states with signal and local oscillator co-propagating in the same spatial mode in orthogonal polarizations, ensuring stable interference. System security is analyzed under both untrusted (UTD) and trusted (TD) detector noise models. While no positive secret key rate is obtained under the UTD model, the TD model enables secure key generation over a finite range of modulation variances, highlighting the critical role of detector trust in high-noise conditions. A maximum secret key rate of 270 kbps is achieved at an optimal modulation variance of 11.57. Furthermore, secure operation requires high-transmittance (low-loss) channels under such noise conditions. This study demonstrates the practical feasibility of free-space UD-CVQKD in realistic high electronic-noise detection constraints and highlights detector electronic noise as a key limiting factor in practical systems.
In experimental setups of Gaussian modulation continuous-variable quantum key distribution (CV-QKD), the amplitude and phase of coherent states will be modulated and discretized due to the finite resolution of voltages that drive electro-optical modulators, resulting in Gaussian modulation discretization. We demonstrate that the impact of discretization on CV-QKD can be described as a multiplicative coefficient experimentally and modeled as a preparation noise imposed on ideal Gaussian modulation theoretically. To obtain an accurate estimation of quadrature fluctuation induced by discretized polar modulation, a data-filtering process named preselection can be applied before state transmission. Numerical results show that when amplitude resolution is 0.25 and phase resolution is 0.02, discretized polar modulation enables a transmission distance of 69 km under homodyne detection, reaching 89% of ideal Gaussian modulation. As for heterodyne detection, 55 km and 80% are achieved.
We propose the continuous-variable quantum key distribution protocol based on the Gaussian modulation of a single quadrature of the coherent states of light, which is aimed to provide simplified implementation compared to the symmetrically modulated Gaussian coherent-state protocols. The protocol waives the necessity in one of the quadrature modulations and the corresponding channel transmittance estimation. The security of the protocol against collective attacks in a generally phase-sensitive Gaussian channel is analyzed and is shown achievable upon certain conditions. Robustness of the protocol to channel imperfections is compared to that of the symmetrical coherent-state protocol. The simplified unidimensional protocol is shown possible at a reasonable quantitative cost in terms of key rate and of tolerable channel excess noise.
Unidimensional (1D) Gaussian-modulated continuous-variable quantum key distribution protocols have been proposed as a way to simplify implementation and reduce costs through single-quadrature modulation, requiring only one modulator while maintaining compatibility with standard optical infrastructure. Here, we determine security bounds for a 1D discrete-modulated protocol under the Gaussian extremality assumption by extending the method of Ghorai et al. [Phys. Rev. X 9, 021059 (2019)]. We establish the appropriate symmetry arguments to extend the method to the 1D discrete-modulated case, define the physicality zone in which the protocol is allowed to operate, and derive a security bound against collective attacks in the asymptotic regime via semidefinite programming. Our analysis for uniformly distributed coherent states reveals a limitation: the Gaussian extremality assumption systematically overestimates Eve's information with increasing constellation size, yielding bounds so conservative that secure key extraction becomes increasingly limited for larger constellations, even under ideal conditions. This overestimation worsens with excess noise and restricts viable modulation amplitudes to near-vacuum values. Unlike two-dimensional (2D) protocols, where Gaussian extremality improves with constellation size, 1D protocols lack the growing phase-space isotropy required for the approximation to remain tight as the constellation grows. Our results expose these limitations and highlight the necessity of alternative methods or optimized non-uniform constellation designs for this class of protocols.
Most continuous-variable quantum key distribution schemes are based on the Gaussian modulation of coherent states followed by continuous quadrature detection using homodyne detectors. In all previous schemes, the Gaussian modulation has been carried out in conjugate quadratures thus requiring two independent modulators for their implementations. Here, we propose and experimentally test a largely simplified scheme in which the Gaussian modulation is performed in a single quadrature. The scheme is shown to be asymptotically secure against collective attacks, and considers a specific attack using asymmetric preparation and excess noise. We find that this protocol is considerably more sensitive to noise than other CVQKD schemes, as a consequence of the simplified implementation. A single-quadrature modulation approach renders the need for a costly amplitude modulator unnecessary, and thus facilitates commercialization of continuous-variable quantum key distribution, provided that the low noise requirement can be achieved.