Thomas Grafenauer

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 report on a portable quantum communication platform and its application in quantum key distribution over a terrestrial free-space link. We outline on the complete chain from an efficient field-ready entangled photon source and custom-made mirror telescopes with adaptive optics for efficient link transmission to autonomous timing synchronization of detection events and subsequent secure key extraction.

Since 2004 AIT has developed a software suite for QKD post processing and key routing in trusted repeater networks. This software provides a set of building blocks to integrate sifting, error estimation, error correction, confirmation, privacy amplification and information-theoretically secure message authentication. Accompanying the QKD post-processing is the Quantum Point-to-Point Protocol (Q3P) node which enforces information-theoretically secure network peer-to-peer communication for classical applications. We already reported on the support for different DV and CV-QKD protocols, and high-performance error correction using GPUs for terrestrial QKD. Here we will discuss the following new capabilities of the software * a low dependency footprint for future use on satellites, * hibernation of QKD post-processing pipelines and switching between them for key establishment with different ground terminals and satellites, * CoAP interfaces to cover demands of the control and management plane in today’s network environment, and * the port to ARM/FPGA SoC boards. The CoAP interfaces allow rapid scripting of QKD modules or AIT QKD based applications with Python or even Bash. Utilities and tools, as well as a boilerplate setup for QKD module coding projects, also support the creation of new QKD post-processing modules or QKD based user applications. The AIT QKD software is bundled with management tools, partly GUI oriented. The software is available under different license options and AIT welcomes suggestions from academic groups or industry to co-operate.