Gerd Leuchs

In our work, we developed an optical CVQKD system that uses polarization-based QPSK modulation designed for atmospheric quantum communication and a corresponding post-processing pipeline including error correction and privacy amplification. In a first laboratory experiment, we applied the security statement of a recently published security proof to calculate composable key rates with a total security parameter of ε = 1e-10 in the finite size regime against i.i.d. collective attacks. We also used the post-processing pipeline to study the effect of error correction and frame errors on the actual key extraction in a finite-size system – finding that the common approach of going to high frame errors to increase the ECC efficiency β does not optimize the extractable key length.Furthermore, we deployed the system over an ad-hoc atmospheric channel of 1.7 km in Mai 2023 in the city of Jena, Germany. In a first proof-of-principle study, we were able to apply the full optical and post-processing pipeline to extract pseudo-asymptotic keys and discuss the further steps necessary to move the system to the finite-size regime. To the best of our knowledge, this is the first CVQKD demonstration over a real atmospheric channel combining both the new class of DMCVQKD security proofs without Gaussian optimality and error correction steps.

Besides discrete-variable QKD, where single photon detection is used, continuous-variable (CV) protocols are using homodyne detection and are thus promising to be compatible with existing classical coherent communication technology. Originally, the research on CV QKD protocols mostly focused on Gaussian modulation (see review [1]), where one assumes that Alice can continuously displace coherent states according to a 2D Gaussian distribution. This modulation allows the security proofs to take advance of Gaussian optimality conditions, but experimental implementations can only reach this pattern up to some finite discretization. Another approach is to directly use a discrete-modulated (DM) CV QKD protocol. Here, Alice is required to prepare a finite number of displaced coherent states, aiming for a higher experimental simplicity, with the drawback of higher theoretical complexity. Recently, new security proofs such as [2] and corresponding experiments [3,4] could show the feasibility of systems using quadrature amplitude modulation (QAM) with 64 and 256 displaced states. However, the security proof was limited to the asymptotic regime and since the experiments did not use implemented error correction codes, one could only estimate the achievable key rates, but could not generate the secret key itself. In this poster, we demonstrate experiments with a protocol with a smaller constellation size of four coherent states that share the same amplitude but are shifted by 90° in phase (QPSK modulation). We exploit a recently published security proof providing tight secret key rates for collective attacks even in the finite size regime [5]. Furthermore, we show that the QPSK data is compatible with our implemented low density parity check (LDPC) codes for binary symmetric channels. This allows us to perform the full QKD protocol from experimental quantum state exchange to classical post processing and to generate a secret key shared between Alice and Bob. For this purpose, we use a laboratory system based on polarization encoding in the Stokes parameters which is equivalent to a QPSK pattern in phase space. This scheme is designed to cope with the challenges of a turbulent atmospheric channel. While the fluctuating nature of such a channel can be targeted by sub-binning the transmission channels [6], the atmosphere is in general non-birefringent, allowing for atmospheric quantum communications [7]. [1] F. Laudenbach et al., Adv. Quantum Technol. 1, 1800011 (2018) [2] A. Denys et al., Quantum 5, 540 (2021) [3] F. Roumestan et al., arXiv:2207.11702 (2022) [4] Y. Pan et al., Optics Letters 47, 3307-3310 (2022) [5] F. Kanitschar et al., arXiv:2301.08686v1 (2023) [6] V. Usenko et al., New J. Phys. 14, 093048 (2012) [7] B. Heim et al., New J. Phys. 16, 113018 (2014)

We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.

We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.

How well does a quantum channel preserve the quantum properties of the transmitted quantum states? We investigate this question in the context of a continuous-variable quantum communication system using the framework of effective entanglement. This framework allows for a quantification of the transmitted entanglement using only coherent states and the well-established double homodyne detection. Experimentally, we investigated fiber channels up to a length of 40 km for a wide range of coherent state amplitudes. Additionally, we induced phase noise to study the quantum-classical transition within the framework. From the measured parameters we are able to identify the optimal point of operation for each quantum channel with respect to the rate of transmitted entanglement. We note that the benchmarking procedure is independent of the physical implementation of the quantum channel and would therefore be a promising candidate for benchmarking of future quantum technologies.

We propose and experimentally demonstrate the tools to implement a Trojan-horse attack to break the security of the commercial quantum cryptosystem ‘Clavis2’ from ID Quantique while it is operating the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. Eve launches a bright optical pulse into Bob and analyses the back-reflections that arise from different components and interfaces in Bob. By homodyning the weak coherent state in a suitable back-reflected pulse with an appropriately delayed local oscillator, Eve can get information about Bob’s basis choice with high accuracy. Since the basis choice is essentially the raw secret key in SARG04 protocol, Eve could in principle know the whole key. However, a problem that needs to be circumvented is that of afterpulsing caused by the bright Trojan-horse pulses impinging on the avalanche photodiode based single-photon detectors in Bob. This increases the dark counts or false clicks, thus directly translating into a higher quantum bit error rate (QBER) that could expose Eve. Nonetheless, we show that there exist attack regimes that allow Eve to get a partial information of the key without being discovered, thus breaching the security of the QKD system.