Towards a point-to-point CV-QKD system: Implementation challenges and perspectives

TL;DR

Realizing a practical point-to-point CV-QKD system over optical fiber requires addressing intertwined physical-layer impairments, DSP-based post-processing, and hardware implementation at scale. The paper provides a cross-layer analysis, introduces a reference framework (CV-QKD-ModSim), and presents a Brazil-focused deployment roadmap via the QuIIN initiative, linking metropolitan testbeds to future hybrid fiber/FSO and space-based links. It details DSP-driven noise mitigation, finite-size parameter estimation with robust confidence bounds, LDPC-based reconciliation including MET-LDPC and QC-LDPC structures, and large-block privacy amplification implemented on dedicated accelerators. Together, these contributions lay the groundwork for scalable, interoperable quantum-secure networks in Brazil and offer a blueprint for transitioning CV-QKD from lab demonstrations to real-world infrastructure.

Abstract

This article presents an analysis of the practical challenges and implementation perspectives of point-to-point continuous-variable quantum key distribution (CV-QKD) systems over optical fiber. The study addresses the physical layer, including the design of transmitters, quantum channels, and receivers, with emphasis on impairments such as attenuation, chromatic dispersion, polarization fluctuations, and coexistence with classical channels. We further examine the role of digital signal processing (DSP) as the bridge between quantum state transmission and classical post-processing, highlighting its impact on excess noise mitigation, covariance matrix estimation, and reconciliation efficiency. The post-processing pipeline is detailed with a focus on parameter estimation in the finite-size regime, information reconciliation using LDPC-based codes optimized for low-SNR conditions, and privacy amplification employing large-block universal hashing. From a hardware perspective, we discuss modular digital architectures that integrate dedicated accelerators with programmable processors, supported by a reference software framework (CV-QKD-ModSim) for algorithm validation and hardware co-design. Finally, we outline perspectives for the deployment of CV-QKD in Brazil, starting from metropolitan testbeds and extending toward hybrid fiber/FSO and space-based infrastructures. The work establishes the foundations for the first point-to-point CV-QKD system in Brazil, while providing a roadmap for scalable and interoperable quantum communication networks.

Figures (8)

• Figure 1: Conceptual model of a QKD protocol.
• Figure 2: Different transmission systems. (a) Transmitting Local Oscillator (TLO), in which the local oscillator is generated at Alice and co-propagates with the quantum signal through the optical fiber. A Telecom laser ($\lambda$ = 1550nm) is split such that a fraction is called quantum signal (QS), which is modulated by an I/Q modulator (IQM) driven by an arbitrary waveform generator (AWG) seeded by a quantum random number generator (QRNG). The remaining optical power is transmitted to function as a local oscillator (TLO). A variable optical attenuator (VOA) sets the appropriate signal power before transmission, and a polarization beam splitter (PBS) is employed to multiplex the optical fields. At the receiver, Bob performs coherent detection, which will be explained in Sec. (b) Local-local Oscillator (LLO) configuration, where the quantum signal and a pilot tone are generated at Alice, while the local oscillator is independently generated at Bob. The pilot tone is transmitted along with the quantum signal to enable phase and frequency recovery at the receiver. At Bob, a local laser acts as the LLO and is combined with the received signal for coherent detection.
• Figure 3: Coexistence of the quantum signal with classical C-Band ($\lambda =$ 1530-1565nm) channels. The signals are combined/separated with a dense wavelength-division multiplexer(DWDM). At the receiver, the quantum channel is demultiplexed from the classical traffic and coherently detected, with an active polarization control stage to compensate for polarization drifts induced by the optical fiber.
• Figure 4: Detection setups for CV-QKD. In both figures, the blue line represents the pilot tone, while the orange line corresponds to the quantum signal. The gray boxes indicate the balanced detectors. (a) Homodyne detection. The quantum signal is separated from the pilot tone. It is directed into a beamsplitter together with the local-local oscillator (LLO), whose phase is randomized by a phase modulator (PM) driven by a quantum random number generator (QRNG). The pilot tone, on the other hand, passes through an interferometer with the LLO, where one branch introduces a phase shift of $\pi/2$, allowing full reconstruction of both quadratures of the pilot tone. (b) Double homodyne/heterodyne. In this configuration, both the quantum signal and the pilot tone enter an interferometer with the LLO, where one branch imposes a phase difference of $\pi/2$, enabling the full measurement of both quadratures for each signal.
• Figure 5: End-to-end pipeline of a CV-QKD system, illustrating the generation, transmission, detection, and post-processing of quantum states for secret key extraction. The upper part depicts the quantum layer, from state preparation at Alice to quadrature measurement at Bob, while the lower part shows the classical and authenticated post-processing stages, including parameter estimation, information reconciliation, and privacy amplification. Public communications and private data are explicitly distinguished. Adapted from zhang2024continuous.