Eleni Diamanti

Quantum sensors are powerful tools for measuring physical quantities with high sensitivity, enabling, for instance, the mapping of Earth’s gravitational field , detecting very small changes of magnetic fields, or the passage of time. The underlying principle is to use a quantum state as a probe that interacts with the physical quantity of interest, thereby encoding relevant information into the state. Although individual quantum sensors may exhibit remarkable sensitivity, the precision of a certain measurement can be significantly enhanced when multiple probes are entangled. Distributed quantum sensing extends this further and leverages entanglement among spatially separated sensors, allowing them to function as a single, coherent system. This approach enables measurements across extended spatial regions, while surpassing the precision achievable by independent sensors. However, a significant challenge in a network setting is ensuring that sensors deployed across different parties serve as the necessary resources for the correct functioning of the target sensing task. This challenge has motivated the combination of quantum cryptography with quantum sensing. In this context, Shettell et al. introduced the notion of privacy for sensor networks, ensuring that, beyond the metrological advantage of cooperative estimation of a global function, parties can also maintain the privacy of their local information and control what data is accessible to others. In this work, we adopt this protocol and focus on a multi-user quantum sensor network framework to analyze the privacy aspects of this parameter estimation task, leveraging a high-quality four-party GHZ state source.

Authentication of quantum resources is a critical tool in the development of quantum information processing protocols. In particular, the verification of quantum states is often used as a building block for communication tasks, determining whether the communicating parties can trust the resources at hand to exchange information or whether the protocol should be aborted. Self-testing methods have been used to tackle such verification tasks in a device-independent (DI) scenario. However, these approaches commonly consider the limit of large, identically and independently distributed (IID) samples, which weakens the DI claim and poses serious challenges to their experimental implementation. To address these issues, Gocanin et al. [1] developed a protocol to certify quantum states in the few-copies and non-IID regime. In this work, we adopt their protocol to experimentally demonstrate the device-independent verification of a four-photon GHZ state, produced with our compact and high-fidelity multipartite entangled photon source.

Quantum transmission links are central elements in essentially all implementations of quantum information protocols. Emerging progress in quantum technologies involving such links needs to be accompanied by appropriate certification tools. In adversarial scenarios, a certification method can be vulnerable to attacks if too much trust is placed on the underlying system. Here, we propose a protocol in a device independent framework, which allows for the certification of practical quantum transmission links in scenarios where minimal assumptions are made about the functioning of the certification setup. We take in particular unavoidable transmission losses into account by modeling the link as a completely-positive trace-decreasing map. We also crucially remove the assumption of independent and identically distributed samples, which is known to be incompatible with adversarial settings. Finally, in view of the use of the certified transmitted states for follow-up applications, our protocol allows to estimate the quality of the state and does not certify the channel only. To illustrate the practical relevance and the feasibility of our protocol with currently available technology we provide an experimental implementation based on a state-of-the-art polarization entangled photon pair source in a Sagnac configuration and analyse its robustness for realistic losses and errors.

As in modern communication networks, the security of quantum networks will rely on complex cryptographic tasks that are based on a handful of fundamental primitives. Weak coin flipping (WCF) is a significant such primitive which allows two mistrustful parties to agree on a random bit while they favor opposite outcomes. Remarkably, perfect information-theoretic security can be achieved in principle for quantum WCF, which is impossible for a classical coin flip without computational assumptions or trusting a third party. In this work, we overcome conceptual and practical issues that have prevented the experimental demonstration of this primitive to date, and demonstrate how quantum resources can provide cheat sensitivity, whereby each party can detect a cheating opponent, and an honest party is never sanctioned. Such a property is not known to be classically achievable with information-theoretic security. Our experiment implements a refined, loss-tolerant version of a recently proposed theoretical protocol and exploits heralded single photons generated by spontaneous parametric down-conversion, a carefully optimized linear optical interferometer including beam splitters with variable reflectivities and a fast optical switch for the verification step. High values of our protocol benchmarks are maintained for attenuation corresponding to several kilometers of telecom optical fiber.

The goal of demonstrating a quantum advantage with currently available experimental systems is of utmost importance in quantum information science. While this remains elusive for quantum computation, the field of communication complexity offers the possibility to already explore and showcase this advantage for useful tasks. Here, we define such a task, the Sampling Matching problem, which is inspired by the Hidden Matching problem and features an exponential gap between quantum and classical protocols in the one-way communication model. Our problem allows by its conception a proof-of-principle photonic implementation based on encoding in the phase of coherent states of light, the use of a fixed size linear optic circuit, and single-photon detection. This enables us to demonstrate experimentally an advantage in the transmitted information resource beyond a threshold input size, which would have been impossible to reach for the original Hidden Matching problem. Our demonstration has implications in various communication and cryptographic settings. Specifically we have used it to introduce a robust practical quantum money-scheme. Our scheme involves an honest Bank who prepares the note by independently and uniformly selecting multiple n-bit binary secret strings which are encoded into the single photon states. The note is then distributed among untrusted holders. To carry out the transaction, the note holder sends the note to the honest local verifiers of the Bank. The verifier runs the Sampling Matching scheme on some randomly selected copies of the note and forwards the classical measurement outcome to the Bank. The Bank then declares the validity of the note. Our private-key money scheme includes multiple features such as single round classical interaction of the local verifier with the Bank, optimal note re-usability (linear in the size of Bank note), linear verification circuit size, and an unconditional security against any adversary trying to forge the Bank note while tolerating the noise of up to 21.4%. The simplistic nature of our verification scheme using Sampling Matching allows for the ability to reach a maximal theoretical noise tolerance of 25%, as conjectured by Amiri et al [Phys Rev A 95, 062334].

Oblivious transfer (OT) is a fundamental primitive in cryptography, allowing the construction of general multi-party computation. Recent results have proved the possibility of quantum protocols from one-way functions, which is expected to be weaker than the assumptions needed in OT in the classical setting. In particular, a recent result by Diamanti et al. provided a quantum protocol for OT considering practical aspects of the protocol, while maintaining its composable security. In this work, we provide the first experimental implementation of a composable oblivious transfer protocol from OWF. The setup implements a weak-coherent pulses BB84 state source in polarization encoding, whose experimental parameters are employed to optimize the theoretical security bounds. The obtained security parameters are then used to perform a secure execution of the protocol, whose performances are profiled and compared with the literature benchmark.

Quantum Key Distribution (QKD) is a field with a potentially major impact on cybersecurity and telecommunications. QKD protocols allow two distant parties to share a secret key regardless of the capacities of an eavesdropper. Thanks to quantum physics laws, an attempt to measure the signal on a quantum channel will necessarily introduce a disturbance, thus an eavesdropper cannot go unnoticed. The first protocols studied used Discrete Variables (DV), but their implementation requires specific technology such as single-photon detectors. Protocols using so-called continuous variables (CV) allow for the use of standard telecommunication components. Recent studies show that high key rates can be achieved using CV-QKD. Exail coordinates the QKISS project, as part of the development of the European Quantum Communication Infrastructure (EuroQCI), together with Thales SIX, LIP6 (CNRS/Sorbonne Université) and Institut d'Optique (CNRS), with the aim of industrializing CV-QKD systems.We have explored different hardware and software configurations to reduce the excess noise and increase the secret key rate of our prototype system. Based on these studies, we have built a demonstrator, used to realize field tests. We are also inlvolved in different European projects, to concretize the European Quantum Communication Infrastructure.

In this study, we constructed a full end-to-end atmospheric channel model for a GEO quantum exchange. This model allowed to assess the performances of two MDI-QKD protocols in such conditions : MP-QKD and TF-QKD, thus setting the limits of these protocols with current and future technology on detection, emission and optic tools. We demonstrated that an untrusted GEO link between two independent parties can be achieved when using reasonable size of telescope diameter. The key rates predicted would allow transmitting up to 260 bit/s for TF-QKD and up to 180 bit/s for MP-QKD for a telescope pupil diameter of 1m at 2.5 GHz of repetition rate.

Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this work, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a non-linear waveguide pumped with the second harmonic of a femtosecond laser. We demonstrate multiparty entanglement by measuring the state’s covariance matrix. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We finally present a frequency-multiplexed quantum key distribution protocol and the expected key rates in bipartite and in multipartite scenarii.

Quantum Key Distribution (QKD) is arguably the most mature application of principles of quantum mechanics to cryptography, and several lab and field demonstrations have been realized. However the realization of QKD in deployed networks, with high distances and/or complex network architecture is still a challenge. Trusted nodes is a known solution to these issues, but requires the delegation of trust to third parties. Here, we propose a trusted node protocol where the requirements of trust delegation are lowered, with no overhead in the consumption of the key exchanged with QKD, allowing to keep the same secret key rate. This protocol is then applied to 2 links in the Parisian Quantum Network, composed of dark dedicated fibers between 8 nodes in the Parisian region, for a total fiber distance of 57 km. Our results show the overall key exchange with no degradation of the key rate.

We present the implementation of a new simulation-secure quantum oblivious transfer protocol based on one-way functions. The protocol allows an efficient and noise-tolerant experimental realization, surpassing prior works' performances in terms of required quantum and classical resources. We provide the complete integration of a software and an experimental source, achieving a black-box implementation of the quantum oblivious transfer primitive, to be leveraged in the future for secure multiparty computation.

We present a new simulation-secure quantum oblivious transfer (QOT) protocol based on one-way functions in the plain model. With a focus on practical implementation, our protocol surpasses prior works in efficiency, promising feasible experimental realization. We address potential experimental errors and their correction, offering analytical expressions to facilitate the analysis of the required quantum resources. Technically, we achieve our results by achieving simulation security for QOT through an equivocal and relaxed-extractable quantum bit commitment.

Mistrustful cryptography includes important tasks like bit commitment, oblivious transfer, coin flipping, secure computations, position authentication, digital signatures and secure unforgeable tokens. Practical quantum implementations presently use photonic setups. In many such implementations, Alice sends photon pulses encoding quantum states and Bob chooses measurements on these states. In practice, Bob generally uses single photon threshold detectors, which cannot distinguish the number of photons in detected pulses. Also, losses and other imperfections require Bob to report the detected pulses. Thus, malicious Alice can send and track multi-photon pulses and thereby gain information about Bob's measurement choices, violating the protocols' security. Here, we provide a theoretical framework for analysing such multi-photon attacks, and present known and new attacks. We illustrate the power of these attacks with an experiment, and study their application to earlier experimental demonstrations of mistrustful quantum cryptography. We analyse countermeasures based on selective reporting and prove them inadequate. We also discuss side-channel attacks where Alice controls further degrees of freedom or sends other physical systems.

We present a composably secure protocol allowing n parties to test an entanglement generation resource controlled by a possibly dishonest party. The test consists only in local quantum operations and authenticated classical communication once a state is shared among them and provides composable security, namely it can be used as a secure subroutine by n honest parties within larger communication protocols to test if a source is sharing quantum states that are at least Ɛ-close to the GHZ state. This claim comes on top of previous results on multipartite entanglement verification where the security was studied in the usual game-based model. Here, we improve the protocol to make it more suitable for practical use in a quantum network and we study its security in the Abstract Cryptography framework to highlight composability issues and avoid hidden assumptions. This framework is a top-to-bottom theory that makes explicit any piece of information that each component (party or resource) gets at every time-step of the protocol. Moreover any security proof, which amounts to showing indistinguishability between an ideal resource having the desired security properties (up to local simulation) and the concrete resource representing the protocol, is composable for free in this setting. This allows us to readily compose our basic protocol in order to create a composably secure multi-round protocol enabling honest parties to obtain a state close to a GHZ state or an abort signal, even in the presence of a noisy or malicious source. Our protocol can typically be used as a subroutine in a Quantum Internet, to securely share a GHZ state among the network before performing a communication or computation protocol.

Establishing an information-theoretic secret key between the two communicating parties of a quantum key distribution (QKD) system is only possible when an accurate characterization of the quantum channel and proper device calibration routines are combined. Indeed, security loopholes caused by inappropriate calibration routines have been shown for discrete-variable QKD. Here, we propose and provide experimental evidence of an attack targeting the local oscillator calibration routine of a continuous-variable QKD system. The attack targets the classical local oscillator pulses during the QKD run in order to modify the trigger pulses used at the detection stage. This allows the eavesdropper to bias the shot noise estimation usually performed using a calibrated relationship. This loophole can be used to perform an undetected intercept-resend attack. We characterize the loophole and suggest possible countermeasures.

We experimentally implement a quantum coin flipping protocol that guarantees a strictly better security than classically possible against an all powerful adversary over a distance suitable for communication in metropolitan area networks. The implementation is based on a practical plug’n'play system, originally designed for quantum key distribution. Furthermore, we show that our protocol can be combined with quantum coin flipping protocols that provide almost perfect security against adversaries with limited resources and hence enhance them with a level of unconditional security. Our results offer a powerful theoretical and experimental toolbox for future secure quantum communications.