Category: Seminar

I will discuss the problem of unreasonable effectiveness of random matrix theory for description of spectral fluctuations in extended quantum lattice systems. A class of locally interacting spin systems has been recently identified where the spectral form factor is proven to match with gaussian or circular ensembles of random matrix theory, and where spatiotemporal correlation functions of local observables as well as some measures of dynamical complexity can be calculated analytically. These, so-called dual unitary systems, include integrable, non-ergodic, ergodic, and generically, (maximally) chaotic cases. After reviewing the basic properties of dual unitary Floquet circuits, I will argue that correlation functions of these models are generally perturbatively stable with respect to breaking dual-unitarity, and describe a simple result within this framework.

For many years, since the beginning of the 19th century, the existence of magnetism in low dimensions has been both desired and controversial. It was long thought, that magnetic orders in low dimensional systems could not be realized at temperatures different from zero. At least, this was what the Mermin-Wagner theorem stated for isolated Heisenberg spins. The scarcity of low-dimensional materials with magnetic properties, and the partial understanding of the role of spin-anisotropy have supported this picture for several decades. In three-dimensions, magnetism has revolutionized our everyday life, enabling familiar technologies which are of common use. A few examples include computers’ memories, RAM, hard-disks, key cards, credit cards, electric batteries, light, and distance sensors. This relentless pace of development has motivated the search for magnetism in systems with increasingly smaller sizes.

With cooperation of experimental and theoretical physics, researchers discovered that spin-anisotropy can stabilize low-dimensional magnetism. In this, spin-orbit coupling plays an important role. Surface experimental probes, such as angle-resolved photoelectron spectroscopy provide researchers access to the electronic structure of solids. Despite the advances in the field, recently, new forms of surface local magnetism completely different from standard descriptions have appeared, with relevance in quantum transport information, dissipationless transport, and quantum sensing.

Here, I aim to give an overview of a new powerful methodology to uncover hidden phases of electrons, including spins, and magnetism which was so fare elusive, and that we were able to uncover for the first time.

Finding the time dependent perturbation that extracts the maximal amount of energy (a.k.a. ergotropy) from a thermally isolated quantum system is a central, solved, problem in quantum thermodynamics. Notably, the same problem has been long studied for classical systems as well, e.g., in the field of plasma physics, but a general solution is still missing there. By building on the analogy with the quantum solution, we provide the classical ergotropy extraction driving: it consists of an instantaneous quench followed by an adiabatic return. The solution is valid under the ergodic hypothesis and we also show that for quantum systems that are classically ergodic the classical expression of ergotropy emerges as the semiclassical limit of the quantum one. The presented results open new ways for practical energy recovery in the classical regime and for understanding the genuine quantum features of quantum ergotropy.

We will review the research areas currently developed at the Quantum Communication Group in Malta, with the aim of generating collaboration between the groups. Particular emphasis would be placed on novel security proof techniques and the optimization of quantum-classical channel capacity.

I will discuss the integrability property of a stochastic and quantum deformation of the Rule 54 cellular automaton: the simplest microscopic (deterministic) reversible model in 1+1 discrete space and time dimensions with strong local interactions. First, I will introduce the Rule 54 model and its two deformations: In the stochastic case, I couple the system to stochastic boundary reservoirs and show that the resulting non-equilibrium steady states can be constructed explicitly in matrix product form. In the quantum case, I explain how the model can be embedded into the Yang-Baxter integrability framework. It turns out that Yang- Baxter integrability is more common than previously thought! Based on ongoing work with T. Prosen.

Continuous-variable quantum systems enable encoding complex states in fewer modes through large-scale non-Gaussian states. Motion, as a continuous degree of freedom, underlies phenomena from Cooper pair dynamics to levitated macroscopic objects. Hence, realizing high-energy, spatially extended motional states remains key for advancing quantum sensing, simulation, and foundational tests.

In the talk, I will present the following control tasks for various nonlinear mechanical systems, including trapped atoms, levitated particles, and clamped oscillators with spin-motion coupling.
(i) Nonharmonic potential modulation: Optimal control of a particle in a nonharmonic potential enables generation of non-Gaussian states and arbitrary unitaries within a chosen two-level subspace [1].
(ii) Macroscopic quantum states of levitated particles: Rapid preparation of a particle’s center of mass in a macroscopic superposition is achieved by releasing it from a harmonic trap into a static double-well potential after ground-state cooling [2].
(iii) Phase-insensitive force sensing: For randomized phase-space displacements, quantum optimal control identifies number-squeezed cat states as optimal for force sensitivity under lossy dynamics [3].

These approaches exploit either intrinsic nonharmonicity or coherent nonlinear coupling, providing a unified framework for motion control in continuous-variable quantum systems—from levitated nanoparticles to optical and microwave resonators—paving the way toward universal quantum control of mechanical motion.

[1] PTG, H. Pichler, C. A. Regal, O. Romero-Isart, Quantum control of continuous systems via nonharmonic potential modulation, Quantum 9, 1824 (2025)
[2] M. Roda-Llordes, A. Riera-Campeny, D. Candoli, PTG, O. Romero-Isart, Macroscopic quantum superpositions via dynamics in a wide double-well potential, Phys. Rev. Lett. 132, 023601 (2024)
[3] PTG, R. Filip, Optimal phase-insensitive force sensing with non-Gaussian states, arXiv: 2505.20832 (2025)

Quantum key distribution is well known to be an essentially quantum task, which is not possible by purely classical means. After being suggested on the basis of strongly nonclassical single-photon states and direct photodetection in the discrete-variable approach, it was later extended to continuous variables, using Gaussian states and coherent detection, well known in classical optical communication. We will therefore discuss the role of nonclassicality and non-Gaussianity in quantum key distribution with discrete and continuous variables, reveal the role of nonclassical resources in security and robustness of the protocols, and consider their extension to multiple users.

The foundations of quantum physics like superposition and entanglement put our everyday understanding of physical reality into question. At the same time, they provide the basis for a host of novel quantum applications. Entanglement will be the key resource for future quantum networks, where it will be used for quantum communication but also to connect quantum sensors and quantum computers. This presentation will highlight recent developments in my laboratory to create narrowband sources of entanglement to help distributing entanglement over long distances and to couple distant quantum sensors or atomic quantum systems. At the same time, these sources promise to allow novel ways of creating macroscopic quantum superpositions to test the foundations of physics.

Within the last two decades, Quantum Technologies have made tremendous progress, from proof of principle demonstrations to real life applications, such as Quantum Key Distribution (QKD) and Quantum Random Number Generators (QRNGs). We will discuss the results that we have recently obtained in our group at the University of Padova towards the realization of secure QRNGs and mature and efficient QKD systems.