arXiv subjects

Mario Stipčević

Publications and source records attributed to Mario Stipčević.

At least 19 recordsLinked to original sources

arXiv2026

Dynamic Entanglement Distribution for Multi-User and Multi-Protocol Quantum Networking

Dynamic entanglement distribution is a key requirement for scalable, multi-user and multi-protocol quantum networks. We demonstrate a metropolitan-scale entanglement-based quantum communication network enabled by a quantum reconfigurable optical add-drop multiplexer (q-ROADM), which dynamically distributes polarisation-entangled photon pairs from a broadband source to six users over deployed campus and metropolitan fibre. The network supports programmable full-mesh, partial-mesh and sliced sub-network configurations, enabling flexible allocation of entanglement resources according to link condition and service requirement. We demonstrate stable six-user full-mesh operation over more than 150 hours, compare full-mesh and time-shared partial-mesh strategies under different source and detector conditions, and realise quantum network slicing with optional/additional interconnection links. We also show that the same infrastructure can support different quantum protocols by showcasing Secure Inaugural Authentication-Transfer (SIAT) combined with Network flooding over multiple paths to improve the security of onboarding a new user. These results demonstrate a q-ROADM-enabled entanglement distribution architecture as a novel route towards reconfigurable, service-oriented quantum networking over optical fibre infrastructure.

quant-ph↗

arXiv2024

Randomness: what is it and why does it matter?

Randomness is a crucial resource for a broad range of important applications, such as Monte Carlo simulation and computation, generative artificial intelligence and cryptography. But what is randomness? A widely accepted definition has eluded researchers thus far, yet without one any work that relies on notion of randomness lacks scientific rigor and its results are questionable. Here, I propose an information-theory-based definition of randomness which, unlike previous attempts, does not list desired properties of the generated number sequence, but rather focuses on the physical process of random number generation itself. This approach results in a definition which complies with our intuitive perception of randomness. It is demonstrated to be non-empty and verifiable. Moreover, a new quantity named "randomness deviation" allows for a practical measure of quality of a random number generating process or a device. An expression for it is derived for a Markovian process, which is frequently encountered in practice. Finally, a process-oriented definition of a random number sequence completes the toolbox needed for understanding, proving, and practical use of randomness.

quant-ph↗

arXiv2023

Scalable authentication and optimal flooding in a quantum network

The global interest in quantum networks stems from the security guaranteed by the laws of physics. Deploying quantum networks means facing the challenges of scaling up the physical hardware and, more importantly, of scaling up all other network layers and optimally utilising network resources. Here we consider two related protocols, their experimental demonstrations on an 8-user quantum network test-bed, and discuss their usefulness with the aid of example use cases. First, an authentication transfer protocol to manage a fundamental limitation of quantum communication -- the need for a pre-shared key between every pair of users linked together on the quantum network. By temporarily trusting some intermediary nodes for a short period of time (<35 min in our network), we can generate and distribute these initial authentication keys with a very high level of security. Second, when end users quantify their trust in intermediary nodes, our flooding protocol can be used to improve both end-to-end communication speeds and increase security against malicious nodes.

quant-ph↗

arXiv2023

Quantum holography with single-photon states

The retrieval of the phase with single-photon states is a fundamental and technical challenging endeavor. Here we report the first experimental realization of hologram recordings with heralded single-photon illumination and continuous observation of photon statistics. Thereby, we demonstrate the basic principle of holography with single-photon states which cannot be described with the classical wave theory. Under conditions with illumination more than 200 times weaker than the noise of the detector, a hologram (interferogram) recorded with a heralded single-photon source revealed an object not visible with non-heralded illumination and slightly higher intensity. The dramatic improvement in retrieval of amplitude and phase information achieved with the heralded single-photon source can be explained by the strong suppression of noise due to the nonclassical temporal correlation between twin photons and the small coincidence time window. The method could be useful for recording and retrieving of amplitude and phase information in the presence of strong noise, for covert imaging, and for imaging of photosensitive biological and material samples.

quant-ph↗

arXiv2022

A 3.3 Gbps SPAD-Based Quantum Random Number Generator

Quantum random number generators are a burgeoning technology used for a variety of applications, including modern security and encryption systems. Typical methods exploit an entropy source combined with an extraction or bit generation circuit in order to produce a random string. In integrated designs there is often little modelling or analytical description of the entropy source, circuit extraction and post-processing provided. In this work, we first discuss theory on the quantum random flip-flop (QRFF), which elucidates the role of circuit imperfections that manifest themselves in bias and correlation. Then, a Verilog-AMS model is developed in order to validate the analytical model in simulation. A novel transistor implementation of the QRFF circuit is presented, which enables compensation of the degradation in entropy inherent to the finite non-symmetric transitions of the random flip-flop. Finally, a full system containing two independent arrays of the QRFF circuit is manufactured and tested in a 55 nm Bipolar-CMOS-DMOS (BCD) technology node, demonstrating bit generation statistics that are commensurate to the developed model. The full chip is able to generate 3.3 Gbps of data when operated with an external LED, whereas an individual QRFF can generate 25 Mbps each of random data while maintaining a Shannon entropy bound > 0.997, which is one of the highest per pixel bit generation rates to date. NIST STS is used to benchmark the generated bit strings, thereby validating the QRFF circuit as an excellent candidate for fully-integrated QRNGs.

cs.CR↗

arXiv2022

Deploying an inter-European quantum network

Around forty years have passed since the first pioneering works have introduced the possibility of using quantum physics to strongly enhance communications safety. Nowadays Quantum Cryptography, and in particular, Quantum Key Distribution (QKD) exited the physics laboratories to become commercial technologies that increasingly trigger the attention of States, military forces, banks, and private corporations. This work takes on the challenge of bringing QKD closer to a consumer technology: optical fibers deployed and used by telecommunication companies of different States have been used to realize a quantum network, the first-ever connecting three different countries. This pushes towards the necessary coexistence of QKD and classical communications on the same infrastructure, which currently represents a main limit of this technology. Our network connects Trieste to Rijeka and Ljubljana via a trusted node in Postojna; a key rate of over 3 kbps has been achieved in the shortest link, and a 7-hour long measurement has demonstrated the system stability and reliability. Finally, the network has been used for a public demonstration of QKD at the G20 Digital Ministers' Meeting in Trieste. The reported experimental results, together with the significant interest that one of the most important events of international politics has attracted, showcase the maturity of the QKD technology bundle, placing it in the spotlight for consumer applications in the near term.

quant-ph↗

arXiv2022

Unconditionally secure digital signatures implemented in an 8-user quantum network

The ability to know and verifiably demonstrate the origins of messages can often be as important as encrypting the message itself. Here we present an experimental demonstration of an unconditionally secure digital signature (USS) protocol implemented for the first time, to the best of our knowledge, on a fully connected quantum network without trusted nodes. Our USS protocol is secure against forging, repudiation and messages are transferrable. We show the feasibility of unconditionally secure signatures using only bi-partite entangled states distributed throughout the network and experimentally evaluate the performance of the protocol in real world scenarios with varying message lengths.

quant-ph↗

arXiv2022

Improved circuits for a biologically-inspired random pulse computer

We present improved circuits intended for building a universal computer based on Random Pulse Computing (RPC) paradigm, a biologically-inspired way of computation in which variable is represented by a frequency of a Random Pulse Train (RPT) rather than a logic state. The RPC we mention here is also known as "stochastic unipolar computation" in newer literature. Unlike in previous art, where randomness is obtained from electronics noise or a pseudorandom shift register while processing circuitry is deterministic, in our approach both variable generation and signal processing rely on the random flip-flop (RFF) whose randomness is derived from a fundamentally random quantum process. This offers advantage in better precision and faster calculation.

cs.ET↗

arXiv2021

Scalable quantum random number generator for cryptography based on the random flip-flop approach

For globally connected devices like smart phones, personal computers and Internet-of-things devices, the ability to generate random numbers is essential for execution of cryptographic protocols responsible for information security. Generally, a random number generator should be small, robust, utilize as few hardware and energy resources as possible, yet provide excellent randomness at a high enough speed (bitrate) for a given purpose. In this work we present a quantum random number generator (QRNG) which makes use of a photoelectric effect in single-photon avalanche diodes (SPADs) as a source of randomness and is scalable to any desired bitrate. We use the random flip-flop method in which random bits are obtained by periodic sampling of a randomly toggling flip-flop. For the first time we investigate this method in detail and find that, out of two main imperfections, bias is due only to hardware imperfections while autocorrelation predominantly resides with the method itself. SPADs are integrated on a silicon chip together with passive quenching and digital pulse-shaping circuitry, using a standard 180 nm CMOS process. A separate FPGA chip derives random numbers from the detection signals. The basic QRNG cell, made of only two SPADs and a few logic circuits, can generate up to 20 Mbit/s that pass NIST statistical tests without any further postprocessing. This technology allows integration of a QRNG on a single silicon chip using readily available industrial processes.

quant-ph↗

arXiv2020

Experimental implementation of secure anonymous protocols on an eight-user quantum network

Anonymity in networked communication is vital for many privacy-preserving tasks. Secure key distribution alone is insufficient for high-security communications, often knowing who transmits a message to whom and when must also be kept hidden from an adversary. Here we experimentally demonstrate 5 information-theoretically secure anonymity protocols on an 8 user city-wide quantum network using polarisation-entangled photon pairs. At the heart of these protocols is anonymous broadcasting, which is a cryptographic primitive that allows one user to reveal one bit of information while keeping her identity anonymous. For a network of $n$ users, the protocols retain anonymity for the sender, given less than $n-2$ users are dishonest. This is one of the earliest implementations of genuine multi-user cryptographic protocols beyond standard QKD. Our anonymous protocols enhance the functionality of any fully-connected Quantum Key Distribution network without trusted nodes.

quant-ph↗

arXiv2020

A trusted-node-free eight-user metropolitan quantum communication network

Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here we present a fully connected quantum communication network on a city wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of 8 users. Our novel network topology is easily scalable to many users, allows traffic management features and minimises the infrastructure as well as the user hardware needed.

quant-ph↗

arXiv2019

A circuit for precise random frequency synthesis via a frequency locked loop

Frequency synthesis (FS) is a technique vital for all kinds of radio frequency (RF) communications, such as: mobile phones, Bluetooth, Wi-Fi, radio, TV and satellite, and in other equipment requiring periodic signals of stable and programmable frequency. In this work, we present a generalization of the FS technique to random, non-periodic signals, whose main use is in the new area of stochastic neuromorphic computing and, information security and instrumentation. Since conventional FS circuits cannot work with random signals, we introduce a novel random frequency ratio detector, that works both with random and periodic signals.

eess.SP↗

arXiv2018

Space QUEST mission proposal: Experimentally testing decoherence due to gravity

Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum properties, such as entanglement, may exhibit entirely different behavior to purely classical systems. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph and coworkers [T C Ralph, G J Milburn, and T Downes, Phys. Rev. A, 79(2):22121, 2009, T C Ralph and J Pienaar, New Journal of Physics, 16(8):85008, 2014], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's (ESA) Space QUEST (Space - Quantum Entanglement Space Test) mission, and study the feasibility of the mission schema.

quant-ph↗

arXiv2017

Comparative study of afterpulsing behavior and models in single photon counting avalanche photo diode detectors

Single-photon detectors, like Avalanche Photo Diodes (APDs), have a great importance in many fields like quantum key distribution, laser ranging, florescence microscopy, etc. Afterpulsing is a typical non ideal behavior of APDs, operated in Geiger mode, which adversely affects any application which measures the number or timing of detection events. Several conflicting studies have tried to model afterpulsing behavior and link it to fundamental semiconductor physics. Here we describe experiments we performed on at least 2 different detectors from 3 manufacturers, all commonly used in quantum optics setups. We report on the timing structure of the counts. By fitting different models to these data, we found statistically significant differences between the detectors used. Furthermore, we report on the presence of high order afterpulses which are not accounted for in any of the standard models of afterpulse behavior. This might have significant implication e.g. in quantum key distribution systems, as the modeling of the devices in use will have to be part of the security analysis for some systems. Significant differences between manufacturers and even between seemingly identical detectors of the same make and batch suggest that every individual detector needs to be calibrated. We describe a reliable procedure to perform such a calibration and present our findings on three different commercial products.

physics.ins-det↗

arXiv2017

An advanced active quenching circuit for ultra-fast quantum cryptography

Commercial photon-counting modules based on actively quenched solid-state avalanche photodiode sensors are used in a wide variety of applications. Manufacturers characterize their detectors by specifying a small set of parameters, such as detection efficiency, dead time, dark counts rate, afterpulsing probability and single-photon arrival-time resolution (jitter). However, they usually do not specify the range of conditions over which these parameters are constant or present a sufficient description of the characterization process. In this work, we perform a few novel tests on two commercial detectors and identify an additional set of imperfections that must be specified to sufficiently characterize their behavior. These include rate-dependence of the dead time and jitter, detection delay shift, and "twilighting." We find that these additional non-ideal behaviors can lead to unexpected effects or strong deterioration of the performance of a system using these devices. We explain their origin by an in-depth analysis of the active quenching process. To mitigate the effects of these imperfections, a custom-built detection system is designed using a novel active quenching circuit. Its performance is compared against two commercial detectors in a fast quantum key distribution system with hyper-entangled photons and a random number generator.

quant-ph↗

arXiv2015

An On-Demand Optical Quantum Random Number Generator with In-Future Action and Ultra-Fast Response

Random numbers are essential for our modern information based society e.g. in cryptography. Unlike frequently used pseudo-random generators, physical random number generators do not depend on complex algorithms but rather on a physical process to provide true randomness. Quantum random number generators (QRNG) do rely on a process, which can be described by a probabilistic theory only, even in principle. Here we present a conceptually simple implementation, which offers a 100% efficiency of producing a random bit upon a request and simultaneously exhibits an ultra low latency. A careful technical and statistical analysis demonstrates its robustness against imperfections of the actual implemented technology and enables to quickly estimate randomness of very long sequences. Generated random numbers pass standard statistical tests without any post-processing. The setup described, as well as the theory presented here, demonstrate the maturity and overall understanding of the technology.

quant-ph↗

arXiv2015

Commercially available Geiger mode single-photon avalanche photodiode with a very low afterpulsing probability

Afterpulsing is one of the main technological flaws present in photon counting detectors based on solid-state semiconductor avalanche photodiodes operated in Geiger mode. Level of afterpulsing depends mainly on type of the semiconductor, doping concentrations and temperature and presents an additional source of noise, along with dark counts. Unlike dark counts which appear randomly in time, aterpulses and are time-correlated with the previous detections. For measurements that rely on timing information afterpulsing can create fake signals and diminish the sensitivity. In this work we test a novel broadband sensitive APD that was designed for sub-Geiger avalanche gain operation. We find that this APD, which has a reach-through geometry typical of single-photon detection photodiodes, can also operate in Geiger mode with usable detection sensitivity and acceptable dark counts level while exhibiting uniquely low afterpulsing. The afterpulsing of tested samples was systematically less than 0.05 percent at 10V excess voltage.

physics.ins-det↗

arXiv2014

Precise Monte Carlo Simulation of Single-Photon Detectors

We demonstrate the importance and utility of Monte Carlo simulation of single-photon detectors. Devising an optimal simulation is strongly influenced by the particular application because of the complexity of modern, avalanche-diode-based single-photon detectors.. Using a simple yet very demanding example of random number generation via detection of Poissonian photons exiting a beam splitter, we present a Monte Carlo simulation that faithfully reproduces the serial autocorrelation of random bits as a function of detection frequency over four orders of magnitude of the incident photon flux. We conjecture that this simulation approach can be easily modified for use in many other applications.

quant-ph↗