Continuous Variable (CV) Quantum Key Distribution (QKD) has the advantage of high raw bit rates due to the high efficiency and high bandwidth of homodyne detection and ease of integration with existing communications infrastructure. CV protocols that employ Post-Selection (PS) [1] - a classical filtering of the measurement results - enjoy additional advantages in terms of versatility and reconciliation efficiency.
Continuous Variable (CV) Quantum Key Distribution (QKD) has the advantage of high raw bit rates due to the high efficiency and high bandwidth of homodyne detection and ease of integration with existing communications infrastructure. CV protocols that employ Post-Selection (PS) [1] - a classical filtering of the measurement results - enjoy additional advantages in terms of versatility and reconciliation efficiency.
The performance of a continuous-variable quantum key distribution (CVQKD) protocol depends on the efficiency of the post-processing of measurement results. The post-processing methods extract statistical information from the raw data, establish the mutual knowledge between the parties, and produce a final key that provides absolute security. The post-processing phase is a bottleneck in CVQKD with crucial importance to the efficiency and protocol attributes. Post-processing uses the raw data of the parties generated by the quantum-level transmission and a classical authenticated channel to generate a secret key between the parties. The current reconciliation procedures require high-complexity coding with moderate resulting efficiency. Here we define an optimization method for post-processing in continuous-variable quantum key distribution. The reconciliation method achieves additive Gaussian noise on the random secret for arbitrarily low dimensional blocks. The model consumes all information from the raw data blocks to provide maximal efficiency and security via standard operations. The results can be realized by generic Gaussian coding schemes, allowing an easily implementation for experimental CVQKD protocols.
Most quantum key distribution (QKD) protocols can be classified as either a discrete-variable (DV) protocol or continuous-variable (CV) protocol, based on how classical information is being encoded. We propose a protocol that combines the best of both worlds – the simplicity of quantum state preparation in DV-QKD together with the cost-effective and high-bandwidth of homodyne detectors used in CV-QKD. Our proposed protocol has two highly practical features: (1) it does not require the honest parties to share the same reference phase (as required in CV-QKD) and (2) the selection of decoding basis can be performed after measurement. We also prove the security of the proposed protocol in the asymptotic limit under the assumption of collective attacks. Our simulation suggests that the protocol is suitable for secure and high-speed practical key distribution over metropolitan distances.
Quantum information processing uses the laws of quantum physics to perform tasks that are infeasible or impossible classically. Within this broader programme, quantum communication has emerged as one of the most practically mature sub-fields of quantum information science, with several proposed protocols having already been demonstrated over deployed optical fibre and free-space links. Of the various platforms available for encoding quantum information for communication, the continuous-variable (CV) framework encodes information in the quadratures of the electromagnetic field and is therefore directly compatible with the existing machinery of classical coherent optical communications. The most prominent CV protocol is continuous-variable quantum key distribution (CV-QKD), which establishes an information-theoretically secure cryptographic key between two spatially separated parties. Because CV-QKD reuses much of the hardware of classical coherent telecommunications, including off-the-shelf lasers, modulators, and balanced detectors, it is a leading candidate for the deployment of quantum-secured communication over standard optical fibre networks. The development of CV-QKD has typically proceeded with the underlying physics dictating the engineering. Security proofs, often relying on the Gaussian optimality of collective attacks, set the structure of the protocol, and the practical system is then built to satisfy the assumptions on which those proofs depend. Contributions of this thesis adopt the opposite view, taking the practical engineering of high-rate CV-QKD systems as the starting point and developing the theoretical machinery required to support it. The first contribution is the formalisation of one-round sliced reconciliation with guard-band post-selection as a key distillation scheme designed for real-time, high-throughput operation. Originally introduced experimentally as a means to stabilise bit error rates and enable the use of fixed-rate error correction codes at high symbol rates, guard-band post-selection is shown here to identify the guard-band span as a free parameter that can be tuned to hold the quantum bit error rate approximately constant across a range of channel conditions. However, conditioning on accepted data produces non-Gaussian post-selected statistics that lie outside the reach of standard Gaussian-optimality security arguments, which motivates the remaining two contributions. The second contribution is an asymptotic security analysis of guard-band post-selected CV-QKD against arbitrary collective attacks, no longer restricted to Gaussian adversaries. By using a quantum channel estimate as side information and by lifting reduced-state distances to purification distances via entropic continuity bounds, security is certified against the class of channels lying close to a Gaussian thermal-loss reference, with a key-rate penalty that scales with the energy-constrained diamond distance to that reference. The third contribution is the experimental basis required to apply this framework in practice. A lightweight three-stage protocol for quantum channel estimation is proposed and demonstrated, which repurposes the Gaussian probe constellation already present in the CV-QKD signal and requires no additional hardware or modifications to the underlying protocol. Over single-mode optical fibre at distances from 10 km to 50 km, the protocol produces physically valid process tensor estimates whose energy-constrained diamond distances to a Gaussian reference channel remain on the order of 10^(-3) throughout. Together, these three contributions provide a treatment of post-selected Gaussian-modulated CV-QKD in which the engineering, the security theory, and the channel characterisation are designed to operate together within a deployable system.
Abstract Gaussian-modulated coherent-state (GMCS) continuous-variable quantum key distribution (CV-QKD) offers a practical path to quantum-secured communication owing to its compatibility with existing optical fibre networks and use of mature security proofs. Achieving high secret key rates in real-time systems, however, requires post-processing schemes that are both information-theoretically efficient and computationally scalable. In previous experimental work 1 , we have demonstrated record real-time key rates using a one-round sliced reconciliation scheme with guard-band post-selection. Here, we present the corresponding theoretical framework and provide an asymptotic security analysis that considers arbitrary collective attacks, a much broader class of attacks than the usually assumed Gaussian collective attacks. The security analysis applies the entropic continuity of quantum channels justified by a quantum channel estimation procedure. In conditions of high noise, our scheme uses 500 times less classical communication than multidimensional reconciliation.
Continuous-variable (CV) quantum key distribution (QKD) employs the quadratures of a bosonic mode to establish a secret key between two remote parties, and this is usually achieved via a Gaussian modulation of coherent states. The resulting secret key rate depends not only on the loss and noise in the communication channel, but also on a series of data processing steps that are needed for transforming shared correlations into a final string of secret bits. Here we consider a Gaussian-modulated coherent-state protocol with homodyne detection in the general setting of composable finite-size security. After simulating the process of quantum communication, the output classical data are postprocessed via procedures of parameter estimation, error correction, and privacy amplification. In particular, we analyze the high signal-to-noise regime, which requires the use of high-rate (nonbinary) low-density parity check codes. We implement all these steps in a python-based library that allows one to investigate and optimize the protocol parameters to be used in practical experimental implementations of short-range CV-QKD.
The performance of a continuous-variable quantum key distribution (CVQKD) protocol depends on the efficiency of the post-processing of measurement results. The post-processing methods extract statistical information from the raw data, establish the mutual knowledge between the parties, and produce a final key that provides absolute security. Here we define an optimization method for post-processing in continuous-variable quantum key distribution. The optimization framework allows significant performance improvements in the post-processing phase compared to other existing reconciliation approaches.