Publications

Quantum key distribution and cryptography: a survey.

I will try to partially answer, based on a review on recent work, the following question: Can QKD and more generally quantum information be useful to cover some practical security requirements in current (and future) IT infrastructures ? I will in particular cover the following topics - practical performances of QKD - QKD network deployment - SECOQC project - Capabilities of QKD as a cryptographic primitive - comparative advantage with other solution, in order to cover practical security requirements - Quantum information and Side-channels - QKD security assurance - Thoughts about real Post-Quantum Cryptography

Reply to “Comment on ‘Physics without determinism: Alternative interpretations of classical physics’ ”

In this short note we reply to a Comment by Callegaro et al. [Callegaro, Pennecchi, and Bich, Phys. Rev. A 102, 036201 (2020)] that points out some weakness of the model of indeterministic physics that we proposed in our work [Del Santo and Gisin, Phys. Rev. A 100, 062107 (2019)], based on what we named ``finite information quantities'' (FIQs). While we acknowledge the merit of their criticism, we maintain that it applies only to a concrete example that we discussed in our work, whereas the main concept of FIQ remains valid and suitable for describing indeterministic physical models. We hint at a more sophisticated way to define FIQs which, taking inspiration from intuitionistic mathematics, would allow the criticisms in the preceding Comment to be overcome.

Triangle Nonlocality : genuine quantum nonlocality and quantum Finner inequality

No abstract is provided for this article.

Quantenklonen ist unmöglich

No abstract is provided for this article.

High Bit Rate "Plug and Play" Quantum Key Distribution

No abstract is provided for this article.

Quantum non-locality in two-qutrit systems

Recently a new Bell inequality has been introduced (CGLMP,KKCZO) that is strongly resistant to noise for maximally entangled states of two $d$-dimensional quantum systems. We prove that a larger violation, or equivalently a stronger resistance to noise, is found for a non-maximally entangled state. It is shown that the resistance to noise is not a good measure of non-locality and we introduce some other possible measures. The non-maximally entangled state turns out to be more robust also for these alternative measures. From these results it follows that two Von Neumann measurements per party may be not optimal for detecting non-locality. For $d=3,4$, we point out some connections between this inequality and distillability. Indeed, we demonstrate that any state violating it, with the optimal Von Neumann settings, is distillable.

Trojan-horse attacks on quantum-key-distribution systems (6 pages)

Quantum key distribution: Weak pulses versus entangled photons

No abstract is provided for this article.

Polarization mode dispersion analyses with polarization OTDRs

Testing a Bell inequality in multipair scenarios

To date, most efforts to demonstrate quantum nonlocality have concentrated on systems of two (or very few) particles. It is, however, difficult in many experiments to address individual particles, making it hard to highlight the presence of nonlocality. We show how a natural setup with no access to individual particles allows one to violate the Clauser-Horne-Shimony-Holt inequality with many pairs, including in our analysis effects of noise and losses. We discuss the case of distinguishable and indistinguishable particles. Finally, a comparison of these two situations provides insight into the complex relation between entanglement and nonlocality.

Inter laboratory measurement campaign on single-mode fibres

An interlaboratory comparison of measurements on six single-mode fibres of different designs, both conventional and dispersion modified, has been performed in the framework of COST 217 Project. This measurement campaign was aimed at the evaluation and optimisation of the various measurement techniques. The measured quantities included mode field diameter, cutoff wavelength, chromatic dispersion, geometrical parameters and refractive-index profile. The agreement was generally better for conventional than for dispersion modified fibres. Although for dispersion and geometrical parameters the comparison was quite satisfactory, for mode field diameter and cutoff wavelengths the higher discrepancies were such as to require a further refinement of the relative measurement methods, particularly in dispersion modified fibres.

Measurement of chromatic dispersion in optical fibers using pairs of correlated photons

Photon pairs generated by spontaneous parametric downconversion are used to implement a novel technique for measuring the chromatic dispersion of optical fibers. The time and energy correlation of the photons of a pair allow a measurement with a high time resolution which is only limited by the detection electronics. The device gives direct access to the differential group delays at several wavelengths around 1.3 μm. Our prototype setup is capable of measuring group delay differences with a resolution of about 5 ps, only limited by the upper delay limit of the time interval analyzer (presently 50 ns).

GHz QKD at telecom wavelengths using up-conversion detectors

We have developed a hybrid single photon detection scheme for telecom wavelengths based on nonlinear sum-frequency generation and silicon single-photon avalanche diodes (SPADs). The SPAD devices employed have been designed to have very narrow temporal response, i.e. low jitter, which we can exploit for increasing the allowable bit rate for quantum key distribution. The wavelength conversion is obtained using periodically poled Lithium niobate waveguides (W/Gs). The inherently high efficiency of these W/Gs allows us to use a continuous wave laser to seed the nonlinear conversion so as to have a continuous detection scheme. We also present a 1.27GHz qubit repetition rate, one-way phase encoding, quantum key distribution experiment operating at telecom wavelengths that takes advantage of this detection scheme. The proof of principle experiment shows a system capable of MHz raw count rates with a QBER less than 2% and estimated secure key rates greater than 100 kbit/s over 25 km.

Recycling nonlocality in a quantum network

The nonlocality of an entangled qubit pair can be recycled for several Bell experiments. Here, we go beyond standard Bell scenarios and investigate the recycling of nonlocal resources in a quantum network. First, we show that the ability to recycle nonlocality can be independent of the size of the network. Then, we realize a photonic quantum 3-branch star network in which three sources of entangled pairs independently connect three outer parties with a central node. After measuring, each outer party respectively relays their system to an independent secondary measuring party. We experimentally demonstrate that the outer parties can perform unsharp measurements that are strong enough to violate a network Bell inequality with the central party, but weak enough to maintain sufficient entanglement in the network to allow the three secondary parties to do the same. Moreover, the violations are strong enough to exclude any model based on standard projective measurements on the EPR pairs emitted in the network. Our experiment brings together the research program of recycling quantum resources with that of Bell nonlocality in networks.

Exploring the local landscape in the triangle network

Characterizing the set of distributions that can be realized in the triangle network is a notoriously difficult problem. In this work, we investigate inner approximations of the set of local (classical) distributions of the triangle network. A quantum distribution that appears to be nonlocal is the elegant joint measurement (EJM) [], which motivates us to study distributions having the same symmetries as the EJM. We compare analytical and neural-network-based inner approximations and find a remarkable agreement between the two methods. Using neural network tools, we also conjecture network Bell inequalities that give a trade-off between the levels of correlation and symmetry that a local distribution may feature. Our results considerably strengthen the conjecture that the EJM is nonlocal. Published by the American Physical Society 2025

Diluting quantum information: An analysis of information transfer in system-reservoir interactions

We design a universal quantum homogenizer, which is a quantum machine that takes as an input a system qubit initially in the state $\ensuremath{\rho}$ and a set of N reservoir qubits initially prepared in the same state $\ensuremath{\xi}.$ In the homogenizer the system qubit sequentially interacts with the reservoir qubits via the partial swap transformation. The homogenizer realizes, in the limit sense, the transformation such that at the output each qubit is in an arbitrarily small neighborhood of the state $\ensuremath{\xi}$ irrespective of the initial states of the system and the reservoir qubits. This means that the system qubit undergoes an evolution that has a fixed point, which is the reservoir state $\ensuremath{\xi}.$ We also study approximate homogenization when the reservoir is composed of a finite set of identically prepared qubits. The homogenizer allows us to understand various aspects of the dynamics of open systems interacting with environments in nonequilibrium states. In particular, the reversibility vs irreversibility of the dynamics of the open system is directly linked to specific (classical) information about the order in which the reservoir qubits interacted with the system qubit. This aspect of the homogenizer leads to a model of a quantum safe with a classical combination. We analyze in detail how entanglement between the reservoir and the system is created during the process of quantum homogenization. We show that the information about the initial state of the system qubit is stored in the entanglement between the homogenized qubits.

Family of Bell-like Inequalities as Device-Independent Witnesses for Entanglement Depth

We present a simple family of Bell inequalities applicable to a scenario involving arbitrarily many parties, each of which performs two binary-outcome measurements. We show that these inequalities are members of the complete set of full-correlation Bell inequalities discovered by Werner-Wolf-Żukowski-Brukner. For scenarios involving a small number of parties, we further verify that these inequalities are facet defining for the convex set of Bell-local correlations. Moreover, we show that the amount of quantum violation of these inequalities naturally manifests the extent to which the underlying system is genuinely many-body entangled. In other words, our Bell inequalities, when supplemented with the appropriate quantum bounds, naturally serve as device-independent witnesses for entanglement depth, allowing one to certify genuine k-partite entanglement in an arbitrary n≥k-partite scenario without relying on any assumption about the measurements being performed, or the dimension of the underlying physical system. A brief comparison is made between our witnesses and those based on some other Bell inequalities, as well as quantum Fisher information. A family of witnesses for genuine k-partite nonlocality applicable to an arbitrary n≥k-partite scenario based on our Bell inequalities is also presented.

Continuous quantum jumps and infinite-dimensional stochastic equations

From a mathematical point of view, a class of infinite-dimensional stochastic differential equations describing continuous spontaneous localization in quantum dynamics will be studied. Existence and uniqueness of weak and strong solutions of respective equations are proven via Cameron–Martin–Girsanov transformation. The case of Gaussian initial states is explicitly solved.