Jürg Fröhlich (ETH Zürich)

The Road Towards Heisenberg’s Discovery of Matrix Mechanics

I sketch some key ideas that led to the discovery of Matrix Mechanics by Heisenberg and Dirac (1925). My account may not be accurate, historically, but hopefully captures the main lines of Heisenberg’s reasoning. Some of the technical / mathematical insights and advances in the evolution of Matrix Mechanics to what is now called Quantum Mechanics are outlined.

Matteo Paris (Università degli Studi di Milano)

Is this Theory not Even Wrong?

We review the concepts and methodologies of quantum estimation theory and discuss its applications to assessing physical theories in terms of their falsifiability.

Flavio Del Santo (Université de Genève)

Towards a Measurement Theory in QFT: "Impossible" Quantum Measurements are Possible but not Ideal

Naive attempts to put together relativity and quantum measurements lead to signaling between space-like separated regions. In QFT, these are known as impossible measurements. We show that the same problem arises in non-relativistic quantum physics, where joint nonlocal measurements (i.e., between systems kept spatially separated) in general lead to signaling, while one would expect no-signaling (based for instance on the principle of no-nonphysical communication). This raises the question: Which nonlocal quantum measurements are physically possible? We review and develop further a non-relativistic quantum information approach developed independently of the impossible measurements in QFT, and show that these two have been addressing virtually the same problem. The non-relativistic solution shows that all nonlocal measurements are localizable (i.e., they can be carried out at a distance without violating no-signaling) but they (i) may require arbitrarily large entangled resources and (ii) cannot in general be ideal, i.e., are not immediately reproducible. These considerations could help guide the development of a complete theory of measurement in QFT.

Veronika Baumann (Università di Vienna)

Relational Dynamics and Causality

Relational quantum dynamics aims to treat time in a way similar to the way space is usually treated in quantum theory, by associating a Hilbert space with time, which is then interpreted as a quantum clock. The dynamics of a quantum system are then obtained by considering an extended system including the clock and solving a Wheeler-DeWitt-like equation with a Hamiltonian constraint operator. This ideas have been extended to multiple clock systems in order to describe scenarios and features central to the theory of relativity, such as time dilation and changes of reference frames. Relational quantum dynamics with multiple clock systems, however, also allows for describing uniquely quantum features associated with time, like frame dependent temporal localizability or the lack of an overall well defined temporal ordering of operations. The latter corresponds instances of indefinite casual order. The notion of causality employed for indefinite causal order relies on the idea of agency and effects of applying some quantum operation on outcomes obtained when applying another quantum operation, i.e.operational causality. In my talk I will present a selected overview on ideas and approaches around relational dynamics with multiple clocks and the possibility of investigating causality and potentially indefinite causal order within this timeless formulation of quantum theory.

Nicola Piovella (Università degli Studi di Milano)

Subradiance in Disordered and Ordered Arrays of Atoms in a Single-Excitation Configuration

Dicke superradiance has been observed in many systems and is based on constructive interferences between many scattered waves. The counterpart of this enhanced dynamics, subradiance, is a destructive interference effect leading to the partial trapping of light in the system. In contrast to the robust superradiance, subradiant states are fragile, and spurious decoherence phenomena hitherto obstructed the observation of such metastable states. Up to now, subradiance has been observed experimentally in a dilute clouds or in ordered arrays of cold atoms, prepared in the single-quantum excitation configuration. We analyze both these systems, discussing how subradiance may be generated.

Luca Molinari (Università degli Studi di Milano)

A Journey in Random Matrix Theory

After a general introduction, I present two topics that characterized my research: 1/N expansion and exact solutions of statistical models on random surfaces; banded matrices, transfer matrices and eigenvector localisation.

Anita Buckley (Università della Svizzera Italiana)

Quantum Networks

Quantum networks connect quantum capable nodes in order to achieve capabilities that are impossible only using classical information. Their fundamental unit of communication is the Bell pair, which consists of two entangled quantum bits. Unfortunately, Bell pairs are fragile and difficult to transmit directly, necessitating a network of repeaters, along with software and hardware that can ensure the desired results. Challenging intrinsic features of quantum networks, such as dealing with resource competition, motivate formal reasoning about quantum network protocols. To this end, we develop a novel specification language for quantum networks based upon Kleene algebra, which has sound and complete equational theory, allowing us to verify network protocols.

Luigi Galgani (Università degli Studi di Milano)

Progress Along the Lines of the Einstein Classical Program

The Einstein Classical Program, enunciated in the occasion of his 75-th birthday, consists in trying to reproduce the results of Quantum Mechanics, admittedly the correct theory, through procedures having a realistic character. Here some results are described where the realistic theory is just Classical Physics (Mechanics and Electrodynamics).
Two results are
1) The infrared spectrum of a crystal (LiF) and
2) The chemical bond in the simplest possible case (the ion of the Hydrogen Molecule).

Andrea Carati (Università degli Studi di Milano)

The Relevance of the Wheeler-Feynman Identity for Statistical Physics

In 1945, Wheeler and Feynman published a paper in Review of Modern Physics in which an identity was conjectured, and four argument was put forward to justify it. While in the literature this paper is usually known for the so called "absorber theory", here one wants to show the relevance of the Wheeler-Feynman identity for the explanation of concrete physical phenomena, as the dispersion of light in matter. Also, two way of proving such identity will be discussed.

Ämin Baumeler (Università della Svizzera Italiana)

Porting Quantum Research to Relativity — The Möbius Test

Our fundamental understanding of quantum mechanics improved dramatically through the formulation and use of quantum information: the abstract treatment of information encoded in quantum-mechanical systems. Relativity theory did not undergo any similar transformation. In this talk, I will present the Möbius inequality, an example of porting the Bell methodology from quantum theory to relativity. While a quantum-mechanical violation of a Bell inequality certifies a source of entanglement, a relativistic violation of a Möbius inequality certifies the back-action of matter to the spacetime.

Bassano Vacchini (Università degli Studi di Milano)

Quantum Description of Memory Effects

We discuss recent developments in the characterization of the dynamics of open quantum systems which allow for a possible definition of quantum non-Markovianity. The notion of non-Markovianity has recently been defined and quantified in terms of the underlying quantum dynamical map, using either its divisibility properties or the behaviour of the trace distance between pairs of initial states. We investigate and compare these definitions, further discussing how these notions are connected to the definition of Markovian process in the classical setting.