Due to their compatibility to existing telecom technology, continuous variable (CV) weak coherent state protocols are promising candidates for a broad deployment of quantum key distribution (QKD) technology. We demonstrate how an existing simulation framework for modelling classical optical systems can be utilized for simulations of weak-coherent CV-QKD links. The quantum uncertainties for the measured characteristics of coherent signals are modelled in the electrical domain by shot noise, while a coherent signal in the optical domain is described by its quadrature components. We simulate various degradation effects such as attenuation, laser RIN, Raman noise (from classical channels in the same fibre), and device imperfections and compare the outcome with analytical theory. Having complemented the physical simulation layer by the post-processing layer (reconciliation and privacy amplification), we are able to estimate secure key rates from simulations, greatly boosting the development speed of practical CV-QKD schemes and implementations.
Due to their compatibility to existing telecom technology, continuous variable (CV) weak coherent state protocols are promising candidates for a broad deployment of quantum key distribution (QKD) technology. We demonstrate how an existing simulation framework for modelling classical optical systems can be utilized for simulations of weak-coherent CV-QKD links. The quantum uncertainties for the measured characteristics of coherent signals are modelled in the electrical domain by shot noise, while a coherent signal in the optical domain is described by its quadrature components. We simulate various degradation effects such as attenuation, laser RIN, Raman noise (from classical channels in the same fibre), and device imperfections and compare the outcome with analytical theory. Having complemented the physical simulation layer by the post-processing layer (reconciliation and privacy amplification), we are able to estimate secure key rates from simulations, greatly boosting the development speed of practical CV-QKD schemes and implementations.
Due to their compatibility to existing telecom technology, continuous variable (CV) weak coherent state protocols are promising candidates for a broad deployment of quantum key distribution (QKD) technology. We demonstrate how an existing simulation framework for modelling classical optical systems can be utilized for simulations of weak-coherent CV-QKD links. Having complemented the physical simulation layer by the post-processing layer (reconciliation and privacy amplification), we are able to estimate secure key rates from simulations, greatly boosting the development speed of practical CV-QKD schemes and implementations.
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.
Continuous variable-quantum key distribution (CV-QKD) based on coherent detection has potential to be a cost-effective solution to deploy secure keys over optical networks. This is due to its compatibility with mature optical communication technologies, such as homodyne and heterodyne detection schemes. Nevertheless, realistic implementations of CV-QKD systems suffer from imperfections in the practical devices used to generate and detect the coherent states. Those imperfections deteriorate the performance of the CV-QKD system and can be exploited by an eavesdropper to steal key information without being detected. In this work, we discuss the impact of practical device imperfections on the excess noise of the CV-QKD and on the achievable transmission distance. We also analyze the impact of detection imperfections on the measurement of the shot noise, and its impact on CV-QKD system security.
In this letter, we report on the advances toward the integration of our developed continuous variables (CV) quantum key distribution (QKD) system in existing optical infrastructure and wavelength division multiplexed (WDM) networks. First, we investigate the tolerance of the CV-QKD system to spontaneous Raman scattering (SRS) noise, the latter being the most dominant impairment in a WDM co-existence environment for QKD. In particular, we investigate by experiment the impact of a WDM10×10Gb/s ON–OFF-keying signal in terms of induced SRS noise in the QKD channel. The spontaneous SRS noise influence is assessed for different transmission scenarios, i.e., for various optical launch powers of the WDM signal, and for different transmission links of 20, 40, 60, and 80 km. Based on the experimental data and on the measured system’s parameters, we estimate the key rates and reach capabilities of the proposed CV-QKD system. The scheme supports a key rate of 90 kbit/s over 20 km, for an ideal QKD system multiplexed with 2-mW optical power. A step further, we demonstrate for first time the use of the aforementioned CV-QKD system to encrypt a 10GE client service over deployable optical transport network legacy equipment over 20 km. Our results prove the feasibility of the integration of our proposed scheme with legacy telecom equipment in existing WDM optical networks.
Previously, we reported a framework capable of simulating classical transmission systems and QKD based on weak-coherent prepare-and-measure protocols. The framework’s modular architecture provides a rich library of models for various system components (e.g., lasers, modulators, fibers). QKD components include singlephoton detectors, transmitter- and receiver designs, and performance estimators. For realistic system designs, component models can account for various imperfections (e.g., laser model with linewidth, RIN, side modes). Here, we present further developments of the framework and its applications. We applied our simulation tool to investigate a satellite-based BB84 system in the downlink scenario. For fiber-based applications in a coexistence scenario, we studied the usage of multicore fibers to better separate classical and quantum channels, and various PON configurations. The system performance was estimated by analyzing the QBER dependence on the classical channel power due to Raman scattering. Furthermore, we discuss the simulation results of a Gaussian-modulated CV-QKD system with a realistic model of a true local oscillator. We studied how the utilization of DSP techniques improves the system’s performance. We evaluated the secret key rate for different transmission distances.
Fast and reliable telecom components could potentially speed up quantum communication of continuous variable (CV) quantum states. These quantum states are generated by a (strong) attenuation of typical telecom signals and are therefore determined by the methods and technological limitations of generation and measurement of the latter. Simulation is an important step in identifying key parameters that are critical for changing the operation mode from the classical to the quantum regime. Moreover, only detailed considerations of the overall system design including the reconciliation phase during post-processing will generate conclusive outcomes on the required system components. Specifically, the stringent quantum-technical requirements on the optical system and the electronics for electric-optical conversion shift the selection of components not towards current high-speed telecom ones but to specific high-quality components that are characterized by high resolution and low-noise.