Abstract

Quantum Key Distribution (QKD) is one of the most mature applications of the principles of quantum physics to technological purposes. It is one of the two path towards secure encryption in a world with quantum computers, as it allows two trusted users, usually named Alice and Bob, to exchange a secret key (i.e. a random string of bits) with information-theoretic security, hence replacing the computationally-secure layer of asymmetric encryption key exchange. While many demonstrations, and even commercial systems, have been done in the past few years, the QKD field is not without practical challenges, such as increasing the key rate, going to higher distances, reducing the size and cost of systems, being compatible with classical communication infrastructures, and co-propagated with classical data. This practical challenges show that even if QKD is well understood on a theoretical level, research and engineering work remains for a potential larger scale deployment of QKD systems.  In this tutorial, we will start by an overview of QKD protocols, their finality and their security, and how they should be used, starting from the basic principles of quantum physics. The tutorial will continue on a discussion on how QKD systems are implemented in practice. Then we will then present state of the art systems from the academic and industrial world, and we will have a focus on the practical challenges associated with QKD, and directions for solving them. A particular focus will be done on Continuous-Variable Quantum Key Distribution that allows cryostat-free, room temperature key exchanges at high rates and with off-the-shelf telecommunication devices, and on photonic integration that will play an important role for a potential medium scale or large scale deployment of QKD systems. The tutorial will close on a perspective of future interesting QKD protocols such as Twin-Field, MDI or Mode Pairing.

Bibliography

[1] Vidick, T., & Wehner, S. (2023). Introduction to Quantum Cryptography. Cambridge University Press.

[2] Scarani, V., Bechmann-Pasquinucci, H., Cerf, N., Dušek, M., Lütkenhaus, N., & Peev, M. (2009). The security of practical quantum key distribution. Reviews of Modern Physics, 81(3), 1301–1350.

[3] Pirandola, S., Andersen, U., Banchi, L., Berta, M., Bunandar, D., Colbeck, R., Englund, D., Gehring, T., Lupo, C., Ottaviani, C., Pereira, J., Razavi, M., Shamsul Shaari, J., Tomamichel, M., Usenko, V., Vallone, G., Villoresi, P., & Wallden, P. (2020). Advances in quantum cryptography. Advances in Optics and Photonics, 12(4), 1012.

[4] Piétri, Y., & Diamanti, E. (2025). Communications sécurisées avec des variables quantiques continues. Photoniques(130), 49-54.

[5] Wang, J., Sciarrino, F., Laing, A., & Thompson, M. (2020). Integrated photonic quantum technologies. Nature Photonics, 14(5), 273-284.

Abstract

In classical physics, particles are fully described in phase space where each point represents a position and momentum. For quantum particles such as electrons, this is problematic because of Heisenberg’s uncertainty principle. In the 1930s, E. Wigner proposed a function W(r,p) to describe electrons quantum mechanically in phase space. This function calculates probability densities and mean values of quantum observables. Although popular in quantum optics, no experimental image of W(r,p) for electrons in molecular crystals has ever been obtained. The main reason is that no single experimental technique can directly measure this function. Like in tomography, multiple measurements from different angles of phase space must be combined to reconstruct W(r,p), which acts as a quasi-probability distribution. The challenges and methods of this innovative reconstruction will be illustrated with our latest results.

Bibliography

[1] « The connubium between crystallography and quantum mechanics », P. Macchi (2020) , Crystallography Reviews26, 4, 209-268

[2] « Quantum Crystallography: Current Developments and Future Perspectives » A. Genoni et al (2018) 24, 43, 10881-10905

[3] « On the quantum correction for thermodynamic equilibrium » E. Wigner (1932) Physical Review 40, 5, 749-759

[4] « Quantum-Mechanical Distribution Functions Revisited » E. Wigner (1971) in Perspectives in Quantum Theory, (MIT Press) p25-36

[5] « Intrusion of quantum crystallography into classical lands »  S. Yu & J-M Gillet (2025) Acta Crystallographica B81, 2, 168-180

Abstract

Rydberg atoms are atoms excited to levels with a high principal quantum number. In such an atomic state, the size of the electron wavefunction can become very large, leading to exaggerated properties of the system [1]. These peculiar systems have been attracting the attention of scientists for more than 50 years, first as a subject of fundamental research [2] or for the development of quantum simulators [3], and also recently for their potential as sensors for the detection and imaging of electromagnetic fields [4]. Compared to metallic dipole antennas currently used for this kind of applications, the dielectric cells containing Rydberg atoms at room temperature allow to consider wavelength-independent, precise measurements with a very high sensitivity and dynamics, and with intrinsic stability and self- calibration [5]. In addition, the sensitive part of the sensor is purely dielectric, reducing the modifications of the measured field. We will describe the specific properties of Rydberg atoms that make them appealing for applications to the sensing of RF field, and we will explain the basic principles of quantum RF sensors exploiting these properties.

Bibliography

[1] T. F. Gallagher, “Rydberg Atoms,” Cambridge University Press (1994).

[2] S. Haroche, Rev. Mod. Phys. 85, 1083 (2013).

[3] A. Broaweys and T. Lahaye, Nature Physics 16, 132 (2020).

[4] J. A. Sedlacek, A. Schwettmann, H. Kübler, R. Löw, T. Pfau, and J. P. Shaffer, Nature Physics 8, 819 (2012).

[5] C. L. Holloway, J. A . Gordon, S. Jefferts, A. Schwarzkopf, D. A. Anderson, S. S. Miller, and G. Raithel, IEEE Transactions on Antennas and Propagation 62, 6169 (2014).