The Nuclear Physics Division of the European Physical Society has awarded Giuliano Giacalone, a former student of IPhT, its PhD prize.
This prize is awarded every third year, and the ceremony took place during the EuNPC 2022 conference in Santiago de Compostela, Spain, in October 2022.
Giuliano, now a postdoc at the University of Heidelberg, completed his PhD thesis "Observing the shape of nuclei at high-energy colliders" in November 2020 under the supervision of Jean-Yves Ollitrault. His thesis has previously been honoured with an award of the French Physical Society.
Figure caption: The rules governing the dynamics of the balls in the "box ball system" can be illustrated with the help of a carrier that loads and unloads the balls by running through the system from left to right at each time step.
Understanding how macroscopic laws and large-scale properties emerge from the microscopic interactions between the constituents of a system is a major problem in statistical physics. One can for instance consider the appearance of hydrodynamic behaviors or the way currents react to external forces. It is generally very challenging to connect the microscopic rules to these large-scale properties, and there are relatively few systems for which exact results can be obtained.
The “Box-ball system” (BBS), a particular cellular automaton, is such an example. The BBS consists of “balls” occupying “boxes” arranged on a line, with at most one ball per box. Although the balls move at each time step according to simple deterministic rules (see figure/animation), the BBS presents a rich physics due to the fact that we can identify some objects called "solitons" (a train of k consecutive balls) of all size and whose number are preserved over time.
Using the integrability of the model a recent study of G. Misguich and V. Pasquier, researchers at the IPhT, together with A. Kuniba (Tokyo U.)  has shown how to determine several quantities related to currents and their fluctuations. These include the probability distribution of the number of balls passing through the origin during a time t, the long-time persistent current generated by a perturbation -- called Drude weight --, and some correlation functions associated with soliton currents.
 “Current correlations, Drude weights and large deviations in a box–ball system”, A. Kuniba, G. Misguich and V. Pasquier, J. Phys. A: Math. Theor., 55 244006 (2022). (https://doi.org/10.1088/1751-8121/ac6d8c) See also J. Phys. A: Math. Theor. 53 404001 (2020). (https://doi.org/10.1088/1751-8121/abadb9)
Many natural systems remain far from thermodynamic equilibrium by exchanging matter, energy or information with their surroundings. As these transfers, or fluxes, break timereversal invariance, such processes are beyond the realm of traditional thermodynamics and their statistical fluctuations do not follow the principles of equilibrium statistical mechanics.
Though a fully general theory of non-equilibrium systems still remains to be constructed, a physical principle describing the macroscopic behaviour of diffusive systems far from equilibrium has been proposed by G. Jona-Lasinio and his collaborators in 2001: this is the Macroscopic Fluctuation Theory (MFT). In the MFT framework, macroscopic fluctuations far from equilibrium are determined by two coupled non-linear hydrodynamic equations. However, for a long time, the MFT equations have remained intractable.
In a recent work that has appeared in Physical Review Letters, K. Mallick (IPhT), H. Moriya and T. Sasamoto (Tokyo Tech.) have discovered an exact analytic solution for the time-dependent MFT equations for the symmetric exclusion process, a paradigmatic model of non-equilibrium statistical physics. The MFT was solved by using Inverse Scattering Theory, originally invented to study dispersive waves such as solitons in hydrodynamics or in optical guides. This classic method of non-linear physics is strikingly relevant to establish exact results in non-equilibrium thermodynamics and allows us to predict quantitatively the appearance of rare events and dynamical fluctuations. The application of soliton theory to non-equilibrium statistical mechanics might have far reaching consequences.