Category: Seminar

It is possible to engineer the properties of photons in an optical medium to have an effective mass and repulsive interactions, so that they act like a gas of atoms These “renormalized photons” are called polaritons In the past decade, several experiments have demonstrated many of the canonical effects of Bose Einstein condensation and superfluidity of polaritons In this talk I will review some of the physics of polaritons and present recent results with polaritons that have very long lifetime, including our recent results on persistent circulation of a polariton condensate.

The study of strongly correlated matter is a central focus in quantum many-body physics. Despite the inherent complexity of systems with numerous interacting components and degrees of freedom, such as Fermi liquids and superfluids, these systems can often be described by relatively simple and elegant quasiparticle models. A key example of this is the polaron, which represents a mobile impurity interacting with the low-energy excitations of its host medium. Originally introduced to explain the behavior of electrons in crystals, the concept of polarons has since gained broad relevance across various fields, ranging from condensed matter physics to quantum simulations and computing. Recent experimental advances with neutral ultracold atoms have provided a powerful framework for investigating both Fermi and Bose polarons, where impurity atoms interact with a degenerate Fermi sea or a Bose-Einstein condensate (BEC).

In this talk, I will begin by reviewing the recent theoretical and experimental advancements in the quantum simulation of polarons. As a concrete example, I will discuss a recent development arising from the crossroad of cold atoms and ions, with potential applications in quantum technologies. Finally, I will connect this research to established solid-state platforms where quasiparticles also play a significant role, such as polaritons and polarons-polaritons, and explore the remaining open problems in this field.

In the fields of optoelectronics and photochemistry there is growing interest in studying the response to optical excitation of organic molecules, films and interfaces, as these systems are of fundamental relevance for the development of the next generation of environmentally sustainable optoelectronic devices and catalysts. In order to probe the charge and exciton dynamics in such materials down to the sub-nanosecond timescale we developed a setup at the ALOISA beamline of the Elettra synchrotron that exploits the chemical selectivity of X-ray absorption spectroscopy (XAS) and X-ray photoemission (XPS) in an optical pump/X-ray probe experiment. In this talk, I will present our recent results on tracking triplet dynamics in pentacene and perylene thin films. The role of the film morphology will be discussed by comparing the response of perylene molecules with different terminations. Finally, I will show how the developed setup can be used to follow a structural transition in few examples of 2D-materials.

Prime numbers play a crucial role in mathematics being the key elements for the factorization of the integers. The idea to use them for designing a quantum abacus has recently received a new support from the experimental realization of a single-particle quantum Schrodinger Hamiltonian whose eigenvalues are given by the first N prime numbers. Such an experimental set-up consists of light intensities profiles, tuned by a computer-generated holography, able to create an optical trap for ultracold atoms. The statistical properties of the primes, such as their asymptotic scaling law for the 𝑛-th prime 𝑝! ≃ 𝑛 log 𝑛, besides being the key to implement such a quantum potential, are also shared by other sequences of integers obtained by means of sieves. This is the case of the so-called “lucky numbers”, originally studied by Stan Ulam, for which there exists indeed an associated quantum Hamiltonian. Integrated with other considerations, these two examples pave the way toward the possibility to set up quantum systems able of performing arithmetic manipulations, including the factorization of integers.

Zoom link: https://zoom.us/j/98397895198?pwd=V0pDemkxaStRQjFHcUpzdCtUeW5hZz09

Diving into the fascinating realm of quantum systems, the seminar will offer a panoramic view of long-range interactions. Starting with an encompassing tour of critical phenomena in systems featuring power-law interactions 1/𝑟𝛼 at 𝛼 < 𝑑, we’ll unveil the intricate equilibrium scaling dependence on the power-law exponent α. We will then take a deep dive into the dynamic world of “strong” long-range systems with 𝛼 < 𝑑, where we’ll unravel the counterintuitive features of out-of-equilibrium scaling dynamics during sudden and gradual quenches.

Zoom link: https://zoom.us/j/95634523890?pwd=N1QrSEpScG9lU3NrU1JCU3FVbDREZz09

Testing the limits of validity of the superposition principle is of crucial importance in the foundations of quantum mechanics and the development of quantum technologies. A way to quantify possible breakdowns of the superposition principle is given by collapse models. These models modify the Schrödinger equation, by adding non-linear and stochastic terms which describe spontaneous collapse in space of the wavefunction. The effects of the non-linear terms are negligible for microscopic systems however, because of an amplification mechanism built in the models, they become dominant for macroscopic objects, providing in this way a natural solution to the measurement problem. Because in collapse models the Schrödinger equation is modified, they make different predictions compared to Quantum Mechanics, hence they can be tested in experiments. We will introduce the most relevant collapse models, the Continuous Spontaneous Localization (CSL) model, and the Diósi-Penrose (DP) model. Then, we will give a summary of the current bounds set by different experiments on their phenomenological parameters. In particular, we will focus on experiments based on the study of radiation emission from matter.

The physical properties of many complex Quantum Materials (QM), like transition metal oxides, are the results of a complex interplay among electrons, phonons, and magnons. This complexity makes the properties of QM highly susceptible to external factors such as pressure, doping, magnetic fields, or temperature. This leads to the intricate phase diagrams found in many families of compounds and, in turn, provides the means to switch between completely different macroscopic functionalities by a fine tuneing of “control parameters”, such as temperature or pressure. The same susceptibility also renders QM an ideal platform for designing experiments where tailored electromagnetic fields interacting with matter can lead to the emergence on ultrafast timescales of novel, occasionally exotic, physical properties. This aspect has been explored in time domain studies [1] and has led to different proofs that ultrashort mid-IR light pulses can “force” the formation of quantum coherent states in matter. Those findings disclose a new regime of physics where photo- excitation can be used to dynamically “sustain” quantum coherence, thermodynamic-limits may be bridged and quantum effects can, in principle, appear at ambient temperatures, where thermal fluctuations normally inhibits them at equilibrium. In this presentation, I will review our recent results in archetypal strongly correlated cuprate superconductors and demonstrate the feasibility of a light-based control of quantum phases in real materials [2,3,4]. I will then introduce our new approaches to time domain spectroscopy going beyond mean photon number observables [5-10] and show that the statistical features of light can provide information on superconducting fluctuations beyond standard linear and non-linear optical spectroscopies [11]. Finally, I will elaborate on our current research effort to use cavity electrodynamics to control the onset of quantum coherent states in complex materials [12].

[1] Advances in physics 65, 58-238 (2016)

[2] Science 331, 189-191 (2011)

[3] Phys. Rev. Lett. 122, 067002 (2019)

[4] Nature Physics 17, 368–373 (2021)

[5] Phys. Rev. Lett. 119, 187403 (2017)

[6] New J. Phys. 16 043004 (2014)

[7] Nat. Comm. 6, 10249 (2015)

[8] PNAS March 19, 116 (12) 5383-5386 (2019)

[9] J. of Physics B 53, 145502 (2019)

[10] Optics Letters 45, 3498 (2020)

[11] Light: Science & Applications 11, 44 (2022)

[12] Nature 622, 487–492 (2023)

https://zoom.us/j/99542736244?pwd=U1YwVVZTVWtOOXE3QjFJbWlrQ1AyQT09

In addition to the usual projective measurements, quantum mechanics allows for alternative ways to extract information from a quantum system. Some of these lead to a quasi-probability distribution for the observable measured which are not positively defined. In perfect analogy with the Wigner quasiprobability distribution, the presence of negative regions in the distribution can be used to spot pure quantum behaviors of the system or the dynamics. I will present a particular scheme, called quantum non-demolition, where quasi-probability distribution arises naturally. It exploits an additional quantum detector coupled to the system to be measured which allows us to gain important information about the wave-function of the system. I will discuss what are the advantages and disadvantages of this approach with some practical examples: the measure of the work done on a quantum system driven by an external field and the calculation of the derivative of a quantum operator. In the last part of the talk, I will discuss the connection with the violation of the Leggett-Garg inequalities, and how this approach identifies pure quantum effects and quantum-to-classical transition due to the interaction with an environment.

https://zoom.us/j/91331596512?pwd=RFlwdTA5OTNYRmFaNGxjYUphclpFUT09

As the 21st century unfolds, quantum physics and information theory continue to increase their impact on science and modern technology. Today, a frontier of our current knowledge is made by Complex Quantum Systems: many-body, out-of-equilibrium, open quantum systems interacting with highly structured environments. From gauge theories to complex molecules and quantum devices, understanding and controlling their information processing capabilities is a formidable task that lies at the interface of both technological progress and fundamental science. Therein lie several challenges, old and new, that will benefit from a new perspective and a brand-new set of tools. In this Colloquium I will introduce Conditional Ensembles, a new way to study open quantum systems that includes microscopic information about their environment. After introducing and discussing the significance of this new approach, I will give a bird’s-eye-view of the recent results achieved using this framework, and outline some future research work, aimed at synthesizing a new understanding of how quantum information is structured and processed in complex quantum systems.

Zoom link: https://zoom.us/j/97893050441?pwd=UDdFZkNuaHhYd3A3MWplcDVOajdvdz09

Recent experiments with quantum simulators and noisy intermediate-scale quantum devices have demonstrated unparalleled capabilities of probing many-body wave functions, via directly probing them at the single quantum level via projective measurements. However, very little is known about how to interpret and analyze such huge datasets. This represents a fundamental challenge for theory to understand experimental data, that is also relevant to other fields where similarly large data sets are routinely explored – from classical simulations of gauge theories, to observatory studies of many-body ensembles.

In this talk, I will show how it is possible to provide such characterisation of quantum hardware via direct and assumption-free data mining. The core idea of this programme is the fact that snapshots of many body systems can be construed as a very high-dimensional manifold. Such a manifold can be characterized via basic topological concepts, in particular, by their intrinsic dimension, and by advanced theoretical tools from network theory and non-parametric, unsupervised learning.

This new approach to the many-body problem opens up a cornucopia of methods to connect physical properties to a stochastic sampling of the system wave function. I will focus here on two specific applications. Firstly, I will discuss theoretical results for both classical and quantum many-body spin systems that illustrate how data structures undergo structural transitions whenever the underlying physical system does, and display universal (critical) behavior in both classical and quantum mechanical cases. These results pave the way for a systematic understanding of field theory aspects in data space, a topic of current interest in particle and statistical physics. Secondly, I will discuss how our methods allow to track Kolmogorov complexity in quantum simulators and quantum computers, providing novel insights into the working of such systems, in terms of both practical and fundamental aspects – including cross-certification of quantum devices, a grand challenge in the field.

Zoom link: https://zoom.us/j/93164444063?pwd=S25EbkVBUFdicjUwemxCL01taTgvZz09