Variable Quantum Key Distribution Research Articles

Quantum communications in a moderate-to-strong turbulent space

Since the invention of the laser in the 60s, one of the most fundamental communication channels has been the free-space optical channel. For this type of channel, a number of effects generally need to be considered, including diffraction, refraction, atmospheric extinction, pointing errors and, most importantly, turbulence. Because of all these adverse features, the free-space optical (FSO) channel is more difficult to study than a stable fiber-based link. For the same reasons, only recently it has been possible to establish the ultimate performances achievable in quantum communications via free-space channels, together with practical rates for continuous variable (CV) quantum key distribution (QKD). Differently from previous literature, mainly focused on the regime of weak turbulence, this work considers the FSO channel in the more challenging regime of moderate-to-strong turbulence, where effects of beam widening and breaking are more important than beam wandering. This regime may occur in long-distance free-space links on the ground, in uplink to high-altitude platform systems (HAPS) and, more interestingly, in downlink from near-horizon satellites. In such a regime we rigorously investigate ultimate limits for quantum communications and show that composable keys can be extracted using CV-QKD.

On Global Quantum Communication Networking.

Research in quantum communications networks (QCNs), where multiple users desire to generate or transmit common quantum-secured information, is still in its beginning stage. To solve for the problems of both discrete variable- and continuous variable-quantum key distribution (QKD) schemes in a simultaneous manner as well as to enable the next generation of quantum communication networking, in this Special Issue paper we describe a scenario where disconnected terrestrial QCNs are coupled through low Earth orbit (LEO) satellite quantum network forming heterogeneous satellite–terrestrial QCN. The proposed heterogeneous QCN is based on the cluster state approach and can be used for numerous applications, including: (i) to teleport arbitrary quantum states between any two nodes in the QCN; (ii) to enable the next generation of cyber security systems; (iii) to enable distributed quantum computing; and (iv) to enable the next generation of quantum sensing networks. The proposed QCNs will be robust against various channel impairments over heterogeneous links. Moreover, the proposed QCNs will provide an unprecedented security level for 5G+/6G wireless networks, Internet of Things (IoT), optical networks, and autonomous vehicles, to mention a few.

Fake calibration attack using a beam sampler in a continuous variable-quantum key distribution system

A Fake Calibration Attack process for a Continuous Variable-Quantum Key Distribution system using a Beam Sampler is presented. The Fake Calibration Attack allows a calibration that balances the Standard Quantum Limit for all the optical path in the experiment (differential Standard Quantum Limit is ≈ 0.39 dB) allowing Eve to acquire ≈ 0.0671 for a particular information quadrature which establishes a Quantum Bit Error Rate ≈ 5.8%. As a final result, the balancing of the Standard Quantum Limit for both states of polarization signals allows maintaining the overall Quantum Bit Error Rate at a particular value ≈ 3%, which implies an important basis for detecting a potential spy considering the minimum Quantum Bit Error Rate.

High-Speed Free-Space Optical Continuous Variable-Quantum Key Distribution Based on Kramers–Kronig Scheme

We propose and experimentally investigate a high-speed Kramers–Kronig (KK) scheme based continuous-variable quantum key distribution (CV-QKD) system over a free-space optical (FSO) channel. The atmospheric turbulence channel is emulated by three spatial light modulators on which three randomly generated azimuthal phase patterns yielding Andrews’ spectrum are recorded. The proposed KK scheme enables a low-complexity implementation, insensitivity to laser phase noise, low detection noise, and accurate channel transmittance monitoring. In addition, eight state CV-QKD protocols and 10-channels wavelength-division multiplexing (WDM) scheme are experimentally verified to obtain a high secure key rate. In our experiment, the minimum transmittance of 0.6 is required to guarantee the secure transmission, and a total SKR of 2.1 Gb/s can be obtained at the mean transmittance in weak turbulence regime.

Adaptive low density parity check encoder for a complete free space optics/continuous variable-quantum key distribution system using commercially available off-the-shelf devices for variable throughput network considering dynamical atmospheric turbulence levels

In this paper, an adaptive LDPC encoder for complete FSO/CV-QKD system using a COTS device for emulated dynamical atmospheric turbulence levels is presented. The experimental and emulation results show the maximum and minimal final secret key rates of 105 Kbps and 10 Kbps, respectively, for minimal and maximal throughput in a commercial network, 30 Mbps and 90 Mbps, respectively. Our proposal presents an adequate performance for weak and moderate atmospheric turbulence levels and a suitable option for improve the using of QKD systems.

Quantum-to-the-Home: Achieving Gbits/s Secure Key Rates via Commercial Off-the-Shelf Telecommunication Equipment

There is current significant interest in Fiber-to-the-Home (FTTH) networks, that is, end-to-end optical connectivity. Currently, it may be limited due to the presence of last-mile copper wire connections. However, in near future, it is envisaged that FTTH connections will exist, and a key offering would be the possibility of optical encryption that can best be implemented using Quantum Key Distribution (QKD). However, it is very important that the QKD infrastructure is compatible with the already existing networks for a smooth transition and integration with the classical data traffic. In this paper, we report the feasibility of using off-the-shelf telecommunication components to enable high performance Continuous Variable-Quantum Key Distribution (CV-QKD) systems that can yield secure key rates in the range of 100 Mbits/s under practical operating conditions. Multilevel phase modulated signals (m-PSK) are evaluated in terms of secure key rates and transmission distances. The traditional receiver is discussed, aided by the phase noise cancellation based digital signal processing module for detecting the complex quantum signals. Furthermore, we have discussed the compatibility of multiplexers and demultiplexers for wavelength division multiplexed Quantum-to-the-Home (QTTH) network and the impact of splitting ratio is analyzed. The results are thoroughly compared with the commercially available high-cost encryption modules.

Detection of a PNS attack by means of the quasiprobability function of the weak coherent states with phase diffusion in a CV-QKD system

On this article, we present a technique to detect photon number splitting (PNS) attacks in the quantum channel of a continuous variable-quantum key distribution system using weak coherent states with phase diffusion by means of the S-parameterized quasiprobability function of a quantum state in steady state. Such quasiprobability function is obtained with an optoelectronic Costas loop. We compare the performances of PNS attack systems considering in first place just the attenuation and then the attenuation with the vacuum noise added due to a spy system. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1349–1352, 2015

Scheme for Robust Long-Distance Continuous Variable QuantumKey Distribution

It is shown that the configuration of phase coding for quantum keydistribution with single photon can also be used for continuous variablequantum key distribution. Therefore the robust long-distance high-speedquantum key distribution can be achieved with current technology.

Polarization insensitive phase modulator for quantum cryptosystems

In this paper, we propose a polarization-insensitive phase modulation scheme based on frequency modulation of light waves using either one or a pair of acousto-optic modulators. A stable Sagnac quantum key distribution (QKD) system employing this technique is also proposed. The interference visibility for a 40km and a 10km fiber loop is 96% and 99% respectively, at single-photon level. We ran standard BB84 QKD protocol in a simplified Sagnac setup (40km fiber loop) continuously for one hour and the measured quantum bit error rate stayed within 2%-5% range.