Christoph Pacher

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.

Continuous-variable quantum key distribution offers a practical way for doing secure key exchange by means of broadband modulators and coherent detectors operating in the telecom band. Recent advances in theory and practice have improved the security and eased the system implementation. These include composable security with a finite number of distributed Gaussian-modulated coherent states and the use of pilot/reference signals and a real local oscillator for sharing the phase reference across the communicating parties. Here we report the first prepare-and-measure continuous-variable quantum key distribution experiment that can produce composable keys in the finite-size regime with security against collective attacks. Through novel improvements in the existing security proofs and a fast, yet low-noise and highly stable system operation, we obtain a secret key rate $>$5 Mbps over a 20 km long fiber channel. Our demonstration verifies the security of practical continuous-variable quantum key distribution when used for encryption or other cryptographic tasks.

The conventional point-to-point setting of a Quantum Key Distribution (QKD) protocol typically considers two directly connected remote parties that aim to establish secret keys. This work proposes a natural generalization of a well-established point-to-point discrete-modulated continuous-variable (CV) QKD protocol to the point-to-multipoint setting. We explore four different trust levels among the communicating parties and provide secure key rates for the loss-only channel and the lossy & noisy channel both in the asymptotic limit and in the finite-size regime. We experimentally demonstrate the feasibility of our protocols in an access network topology with 10 km-long access links, achieving a key rate of $7.09 \times 10^{-3}$ bits per symbol or of 0.866 Mbit/s. Our study shows that discrete-modulated CV-QKD is a suitable candidate to connect several dozens of users in a point-to-multipoint network, achieving high rates at a reduced cost, using off-the-shelf components employed in modern communication infrastructure.

The conventional point-to-point setting of Quantum Key Distribution (QKD) typically considers two directly connected remote parties that aim to establish secret keys. However, almost all digital communication tasks involve multiple nodes and complex network architectures. Thus, it is essential to adapt and integrate QKD protocols and their security analyses to accommodate these complex environments and ensure secure communication across interconnected systems. This work proposes a natural generalization of a well-established point-to-point discrete modulated (DM) continuous-variable (CV) QKD protocol to the multi-party setting. We explore four different trust levels among the communicating parties and provide secure key rates in lossy and noisy channels. Our study shows that discrete modulated CV-QKD is a suitable candidate to connect several dozens of users in a point-to-multipoint network, achieving high rates at low cost, using off-the-shelf components employed in modern communication infrastructure.

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)

In our work, we explore various multi-user scenarios for Continuous Variable Quantum Key Distribution with discrete modulation. We propose and analyse DM CV-QKD protocols for various different multi-user scenarios such as * One Alice to $n$ Bobs, where the Bobs do not trust each other, * One Alice to $n$ Bobs, where $m<n$ Bobs trust each other, * Conference Key Agreement between one Alice and $n$ Bobs. One common feature of all protocols that we study is that Alice's source does not need any additional expensive components except state-of-the-art beamsplitters, therefore we call it `cheap source'. This makes the transmitter of our proposed protocols easily implementable in experiments and demonstrations. In our work, we calculate asymptotic secret key rates for a range of parameters and different trust scenarios and show that in the asymptotic limit multi-user DM CV-QKD is possible for distances relevant for mid-sized urban area networks between at least 16 user. This highlights, that DM CV-QKD can be extended to the multi-user scenario and remains a feasible candidate also for early implementations of Quantum Key Distribution in local networks.

Advances in the security analysis of continuous-variable quantum key distribution (CVQKD) protocols with true discrete modulation aim to unlock the same performance as that obtained from `traditional' protocols based on Gaussian modulation. We report a CVQKD experiment using 4 states that utilizes a composable security proof to generate a secret key fraction of $5.6 \times 10^{-3}$ bits/symbol over 10 km channel, while providing security against collective attacks.

Continuous-variable quantum key distribution with phase-shift keying modulation is a promising candidate for practical applications of quantum cryptography due to high compatibility with existing telecommunication infrastructure. It is known that postselection, i.e., omitting those parts of the raw key where an adversary might have gained more information than the communicating parties, can improve the secure key rate significantly. We introduce a new cross-shaped postselection strategy and use a recent numerical security proof framework to compare it to other existing postselection strategies. Furthermore, we provide novel analytical results for the operators that define the respective postselection regions in phase space for each of the postselection strategies, enabling a quicker evaluation without introducing additional numerical errors. Motivated by the high computatoinal effort for the error-correction phase, we point out how postselection can be used to reduce the raw key (so, the data that has to be error-corrected) significantly without lowering the secure key rate considerably. As therefore Bob uses his measurement outcomes directly without requiring any additional computations, the cross-shaped scheme can be implemented easily both in new and existing QKD systems.

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.

See the short abstract in the attached file. In this poster, we present the finite length efficiency of some of our codes and show how it can improve the secret key rate. For this, the FER performance of some of these codes is plotted versus the efficiency and then we plot the secret key rate versus distance by replacing our codes with other existing codes in the literature.

Quantum random number generators (QRNG) are a well-studied quantum resource for information and communication technologies. Offering the randomness derived from quantum mechanical principles, QRNGs will replace current technologies of pseudo random number generation. However, prices and form factors must come down, if this technology is to feed mobile or IoT devices in the future. We present a step in this direction by realizing a QRNG in the polymer-based photonic integration platform PolyBoard.

The transition of Quantum Technologies (QT) being no longer pure basic research but touching applied fields with emerging products is companied by requests for standardization and certification. The call of the markets requesting products based on QT will require alignment of QT products to match not only the need for standards but also the proof to fulfil certification procedures. We will isolate and identify the most promising candidates for this endeavour and by listing challenges, obstacles and dependencies we will propose a strategy and envision a standardization roadmap

Here in this poster we present two new MET-LDPC codes designed for rates 0.02 and 0.01 with even higher efficiency: β = 99.2% and β = 98.7% , respectively. We will present simulation results to demonstrate their performance. The presented codes can be used by different reconciliation strategies to increase the distance of CV-QKD.

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.

We present R10, a novel release of the QKD open source software by AIT. We discuss in detail the main modification in architecture, the introduction of autonomous QKD modules that can communicate directly and no longer need a centralized supervision of a QKD node. This approach allows to get rid of performance and throughput bottlenecks and greatly simplify protocol design and implementation. We further discuss novel algorithms in R10, related to authentication as well as reconciliation and error estimation during post processing. We also present a novel QKD IPSEC integration, enabled by R10.

We analyse the statistical properties of a maximum likelihood estimator for the crossover probability of a binary symmetric channel based on the syndrome of a linear code (e.g. a Low Density Parity Check code). We derive analytical expressions for the estimator, its bias and mean squared error and perform simulations and a comparison. This estimator can be used (i) as a replacement for the sampling estimator and (ii) to optimize the reconciliation phase of discrete-variable QKD during post-processing and thus increase the final secure key rate of QKD.

We present a highly optimized version of the sum-product-algorithm (SPA) for decoding LDPC codes used in quantum cryptography. We make use of i) quasi-cyclic LDPC codes, ii) interleaved variable/check node processing, iii) different numerical representations of the LLR values, and of iv) vectorized SSE commands (performing four 32 bit operations at once). Our algorithm achieves the same decoding properties as the original SPA but offers a throughput on a quad-core CPU of around 40 Mbit/s which is one order of magnitude faster than the state-of-the-art.