arXiv subjects

Nino Walenta

Publications and source records attributed to Nino Walenta.

At least 19 recordsLinked to original sources

arXiv2026

A consolidated and accessible security proof for finite-size decoy-state quantum key distribution

In recent years, quantum key distribution (QKD) has evolved from a scientific research field to a commercially available security solution, supported by mathematically formulated security proofs. However, since the knowledge required for a full understanding of a security proof is scattered across numerous publications, it has proven difficult to gain a comprehensive understanding of all steps involved in the process and their limitations without considerable effort and attention to detail. Our paper aims to address this issue by providing a rigorous and comprehensive security proof for the finite-size 1-decoy and 2-decoy BB84 protocols against coherent attacks within Renner's entropic uncertainty relation framework. We resolve important technical flaws found in previous works regarding the fixed-length treatment of protocols and the careful handling of acceptance testing. To this end, we provide various technical arguments, including an analysis accounting for the important distinction of the 1-decoy protocol where statistics are computed after error correction, along with a slight improvement of the secure-key length. We also explicitly clarify the aspect of conditioning on events, addressing a technical detail often overlooked and essential for rigorous proofs. We extensively consolidate and unify concepts from many works, thoroughly discussing the underlying assumptions and resolving technical inconsistencies. Therefore, our contribution represents a significant advancement towards a broader and deeper understanding of QKD security proofs.

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arXiv2025

Dynamic rerouting and interruption resilience of quantum communication via single-photon-based resynchronization

We present a resynchronization method for quantum key distribution (QKD) systems that enables rapid and reliable recovery from interruptions of the quantum channel and changes of its optical path length. By periodically transmitting short fixed pulse patterns over the quantum channel, our approach achieves swift clock offset recovery, typically within a few hundred milliseconds. We implemented this method in our time-bin-phase BB84 QKD system, demonstrating successful resynchronization after multi-minute channel interruptions and fiber length changes exceeding 100 km. The method can be retrofitted to existing systems via a software upgrade and without hardware changes, allowing for broad applicability. In total, the resynchronization method significantly enhances QKD system resilience and allows for reliable operation in challenging environments such as dynamically routed optical networks, i.e., software-defined networks, and free-space optical links with mobile nodes.

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arXiv2025

Exponential-recovery model for free-running SPADs with capacity-induced dead-time imperfections

Current count-rate models for single-photon avalanche diodes (SPADs) typically assume an instantaneous recovery of the quantum efficiency following dead-time, leading to a systematic overestimation of the effective detection efficiency for high photon flux. To overcome this limitation, we introduce a generalized analytical count-rate model for free-running SPADs that models the non-instantaneous, exponential recovery of the quantum efficiency following dead-time. Our model, framed within the theory of non-homogeneous Poisson processes, only requires one additional detector parameter -- the exponential-recovery time constant $τ_\mathrm{r}$. The model accurately predicts detection statistics deep into the saturation regime, outperforming the conventional step-function model by two orders of magnitude in terms of the impinging photon rate. For extremely high photon flux, we further extend the model to capture paralyzation effects. Beyond photon flux estimation, our model simplifies SPAD characterization by enabling the extraction of quantum efficiency $η_0$, dead-time $τ_\mathrm{d}$, and recovery time constant $τ_\mathrm{r}$ from a single inter-detection interval histogram. This can be achieved with a simple setup, without the need for pulsed lasers or externally gated detectors. We anticipate broad applicability of our model in quantum key distribution (QKD), time-correlated single-photon counting (TCSPC), LIDAR, and related areas. Furthermore, the model is readily adaptable to other types of dead-time-limited detectors. A Python implementation is provided as supplementary material for swift adoption.

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arXiv2024

Evaluation of quantum key distribution systems against injection-locking attacks

While ideal quantum key distribution (QKD) systems are well-understood, practical implementations face various vulnerabilities, such as side-channel attacks resulting from device imperfections. Current security proofs for decoy-state BB84 protocols either assume uniform phase randomization of Alice's signals, which is compromised by practical limitations and attacks like injection locking, or rely on a (partially) characterized phase distribution. This work presents an experimental method to characterize the phase de-randomization from injection locking using a heterodyne detection setup, providing a lower bound on the degree of isolation required to protect QKD transmitters against injection-locking attacks. The methods presented are source-agnostic and can be used to evaluate general QKD systems against injection-locking attacks.

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arXiv2024

Ad-hoc hybrid-heterogeneous metropolitan-range quantum key distribution network

This paper presents the development and implementation of a versatile ad-hoc metropolitan-range Quantum Key Distribution (QKD) network. The approach presented integrates various types of physical channels and QKD protocols, and a mix of trusted and untrusted nodes. Unlike conventional QKD networks that predominantly depend on either fiber-based or free-space optical (FSO) links, the testbed presented amalgamates FSO and fiber-based links, thereby overcoming some inherent limitations. Various network deployment strategies have been considered, including permanent infrastructure and provisional ad-hoc links to eradicate coverage gaps. Furthermore, the ability to rapidly establish a network using portable FSO terminals and to investigate diverse link topologies is demonstrated. The study also showcases the successful establishment of a quantum-secured link to a cloud server.

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arXiv2024

Assessment of practical satellite quantum key distribution architectures for current and near-future missions

Quantum key distribution (QKD) allows the generation of cryptographic keys beyond the computational hardness paradigm and is befitting for secure data transmission requiring long-term security. The communication distance of fibre-based QKD, however, is limited to a few hundred kilometers due to the exponential scaling of signal attenuation. Satellite QKD (SatQKD) can instead leverage free-space optical links to establish long-range connections and enable global-scale QKD. In this work we review the manifold of design choices that concur to form the set of possible SatQKD architectures. These include the choice of the QKD protocol and its physical implementation, but also the satellite orbit, the optical link direction, and whether or not to use trusted-node relays. The possible SatQKD architectures are then evaluated in terms of key generation throughput, latency and maximum reachable communication distance, but also the system-level security and implementation complexity. Given the technical challenges of realising SatQKD systems it is paramount, for near-future satellite missions, to adhere to the simplest possible architecture that still allows to deliver the QKD service. We thus identify as advisable options the use of low-Earth orbit satellites as trusted nodes for prepare-and-measure discrete-variable QKD downlinks with weak laser pulses. The decoy-state version of BB84 is found to be the most promising QKD protocols due to the maturity of the security proofs, the high key generation rate and low system complexity. These findings are confirmed by the multitude of current and planned SatQKD missions that are adopting these architectural choices.

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arXiv2024

Clock offset recovery with sublinear complexity enables synchronization on low-level hardware for quantum key distribution

We introduce iQSync, a clock offset recovery method designed for implementation on low-level hardware, such as FPGAs or microcontrollers, for quantum key distribution (QKD). iQSync requires minimal memory, only a simple instruction set (e.g. no floating-point operations), and can be evaluated with sublinear time complexity, typically involving no more than a few thousand iterations of a simple loop. Furthermore, iQSync allows for a precise clock offset recovery within few seconds, even for large offsets, and is well suited for scenarios with high channel loss and low signal-to-noise ratio, irrespective of the prepare-and-measure QKD protocol used. We implemented the method on our QKD platform, demonstrating its performance and conformity with analytically derived success probabilities for channel attenuations exceeding 70 dB.

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arXiv2022

Ultrafast quantum key distribution using fully parallelized quantum channels

The field of quantum information processing offers secure communication protected by the laws of quantum mechanics and is on the verge of finding wider application for information transfer of sensitive data. To overcome the obstacle of inadequate cost-efficiency, extensive research is being done on the many components required for high data throughput using quantum key distribution (QKD). Aiming for an application-oriented solution, we report on the realization of a multichannel QKD system for plug-and-play high-bandwidth secure communication at telecom wavelength. For this purpose, a rack-sized multichannel superconducting nanowire single photon detector (SNSPD) system, as well as a highly parallelized time-correlated single photon counting (TCSPC) unit have been developed and linked to an FPGA-controlled QKD evaluation setup allowing for continuous operation and achieving high secret key rates using a coherent-one-way protocol.

quant-ph↗

arXiv2015

Random Variation of Detector Efficiency: A Countermeasure against Detector Blinding Attacks for Quantum Key Distribution

In the recent decade, it has been discovered that QKD systems are extremely vulnerable to side-channel attacks. In particular, by exploiting the internal working knowledge of practical detectors, it is possible to bring them to an operating region whereby only certain target detectors are sensitive to detections. Crucially, the adversary can use this loophole to learn everything about the secret key without introducing any error to the quantum channel. In this work, as a step towards overcoming detector blinding attacks, we focus on an experimentally convenient countermeasure, where the efficiency of the detectors is randomly varied.

quant-ph↗

arXiv2014

Free-running InGaAs single photon detector with 1 dark count per second at 10% efficiency

We present a free-running single photon detector for telecom wavelengths based on a negative feedback avalanche photodiode (NFAD). A dark count rate as low as 1 cps was obtained at a detection efficiency of 10%, with an afterpulse probability of 2.2% for 20 μs of deadtime. This was achieved by using an active hold-off circuit and cooling the NFAD with a free-piston stirling cooler down to temperatures of -110${^o}$C. We integrated two detectors into a practical, 625 MHz clocked quantum key distribution system. Stable, real-time key distribution in presence of 30 dB channel loss was possible, yielding a secret key rate of 350 bps.

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arXiv2013

Concise Security Bounds for Practical Decoy-State Quantum Key Distribution

Due to its ability to tolerate high channel loss, decoy-state quantum key distribution (QKD) has been one of the main focuses within the QKD community. Notably, several experimental groups have demonstrated that it is secure and feasible under real-world conditions. Crucially, however, the security and feasibility claims made by most of these experiments were obtained under the assumption that the eavesdropper is restricted to particular types of attacks or that the finite-key effects are neglected. Unfortunately, such assumptions are not possible to guarantee in practice. In this work, we provide concise and tight finite-key security bounds for practical decoy-state QKD that are valid against general attacks.

quant-ph↗

arXiv2013

Quantum Metropolitan Optical Network based on Wavelength Division Multiplexing

Quantum Key Distribution (QKD) is maturing quickly. However, the current approaches to its application in optical networks make it an expensive technology. QKD networks deployed to date are designed as a collection of point-to-point, dedicated QKD links where non-neighboring nodes communicate using the trusted repeater paradigm. We propose a novel optical network model in which QKD systems share the communication infrastructure by wavelength multiplexing their quantum and classical signals. The routing is done using optical components within a metropolitan area which allows for a dynamically any-to-any communication scheme. Moreover, it resembles a commercial telecom network, takes advantage of existing infrastructure and utilizes commercial components, allowing for an easy, cost-effective and reliable deployment.

quant-ph↗

arXiv2013

A fast and versatile QKD system with hardware key distillation and wavelength multiplexing

We present a 625 MHz clocked coherent one-way quantum key distribution (QKD) system which continuously distributes secret keys over an optical fibre link. To support high secret key rates, we implemented a fast hardware key distillation engine which allows for key distillation rates up to 4 Mbps in real time. The system employs wavelength multiplexing in order to run over only a single optical fibre and is compactly integrated in 19-inch 2U racks. We optimized the system considering a security analysis that respects finite-key-size effects, authentication costs, and system errors. Using fast gated InGaAs single photon detectors, we reliably distribute secret keys with rates up to 140 kbps and over 25 km of optical fibre, for a security parameter of 4E-9.

quant-ph↗

arXiv2013

A high-speed multi-protocol quantum key distribution transmitter based on a dual-drive modulator

We propose a novel source based on a dual-drive modulator that is adaptable and allows Alice to choose between various practical quantum key distribution (QKD) protocols depending on what receiver she is communicating with. Experimental results show that the proposed transmitter is suitable for implementation of the Bennett and Brassard 1984 (BB84), coherent one-way (COW) and differential phase shift (DPS) protocols with stable and low quantum bit error rate. This could become a useful component in network QKD, where multi-protocol capability is highly desirable.

quant-ph↗

arXiv2012

Sine gating detector with simple filtering for low-noise infra-red single photon detection at room temperature

We present and analyze a gated single photon avalanche detector using a sine gating scheme with a simple but effective low-pass filtering technique for fast low-noise single photon detection at telecom wavelength. The detector is characterized by 130 ps short gates applied with a frequency of 1.25 GHz, yields only 70 ps timing jitter and noise probabilities as low as 7E-7 per gate at 10% detection efficiency. We show that the detector is suitable for high rate quantum key distribution (QKD) and even at room temperature it could allow for QKD over distances larger than 25 km.

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arXiv2010

Practical private database queries based on a quantum key distribution protocol

Private queries allow a user Alice to learn an element of a database held by a provider Bob without revealing which element she was interested in, while limiting her information about the other elements. We propose to implement private queries based on a quantum key distribution protocol, with changes only in the classical post-processing of the key. This approach makes our scheme both easy to implement and loss-tolerant. While unconditionally secure private queries are known to be impossible, we argue that an interesting degree of security can be achieved, relying on fundamental physical principles instead of unverifiable security assumptions in order to protect both user and database. We think that there is scope for such practical private queries to become another remarkable application of quantum information in the footsteps of quantum key distribution.

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arXiv2010

2.23 GHz gating InGaAs/InP single-photon avalanche diode for quantum key distribution

We implement an InGaAs/InP single-photon avalanche diode (SPAD) for single-photon detection with the fastest gating frequency reported so far, of 2.23 GHz, which approaches the limit given by the bandwidth of the SPAD - 2.5 GHz. We propose a useful way to characterize the afterpulsing distribution for rapid gating that allows for easy comparison with conventional gating regimes. We compare the performance of this rapid gating scheme with free-running detector and superconducting single-photon detector (SSPD) for the coherent one-way quantum key distribution (QKD) protocol. The rapid gating system is well suited for both high-rate and long-distance QKD applications, in which Mbps key rates can be achieved for distances less than 40 km with 50 ns deadtime and the maximum distance is limited to ~190km with 5 $μ$s deadtime. These results illustrate that the afterpulsing is no longer a limiting factor for QKD.

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arXiv2009

Quantum key distribution and 1 Gbit/s data encryption over a single fibre

We perform quantum key distribution (QKD) in the presence of 4 classical channels in a C-band dense wavelength division multiplexing (DWDM) configuration using a commercial QKD system. The classical channels are used for key distillation and 1 Gbps encrypted communication, rendering the entire system independent from any other communication channel than a single dedicated fibre. We successfully distil secret keys over fibre spans of up to 50 km. The separation between quantum channel and nearest classical channel is only 200 GHz, while the classical channels are all separated by 100 GHz. In addition to that we discuss possible improvements and alternative configurations, for instance whether it is advantageous to choose the quantum channel at 1310 nm or to opt for a pure C-band configuration.

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