Virtual Seminars

The laws of Physics are time-reversible—yet we can only go towards the future. This apparent contradiction is known as the “arrow of time problem”. Its resolution is that the future is the direction of increasing entropy. But entropy can only increase towards the future… if it was low in the past, and past low entropy is a very strong hypothesis to make, because low entropy states are so improbable!
Some authors suggest that we can do away with this “past hypothesis” within reversible dynamical laws featuring expansion à la GR. We prove that this is indeed the case in principle, within a toy model. It consists in just graphs upon which particles circulate and interact according to local reversible rules. Some rules locally shrink or expand the graph. We prove that almost all states expand and that entropy always increases as a consequence.
Interestingly, the toy universe can easily be run backwards, until “before the Big Bang”. We can therefore tell the story of “the beginnings of times” in this model. It’s enlightening… and mind-bending at the same time.
Joint work with Gilles Dowek and Amélia Durbec

We propose a special relativistic framework for quantum mechanics. It is based on introducing a Hilbert space for events. Events are taken as primitive notions (as customary in relativity), whereas quantum systems (e.g. fields and particles) are emergent in the form of joint probability amplitudes for position and time of events. Textbook relativistic quantum mechanics and quantum field theory can be recovered by dividing the event Hilbert spaces into space and time (a foliation) and then conditioning the event states onto the time part. Our theory satisfies the full Poincare’ symmetry as a `geometric’ unitary transformation, and possesses relativistic observables for space (location of an event) and time (position in time of an event).

Bell’s theorem and Bell inequality violations are one of the most iconic results in quantum physics. While certainly further from experimental realisation, experiments involving quantum control of observers—the so-called Extended Wigner’s friend scenarios—pose an even stronger challenge to our understanding of quantum theory. In this talk, after a review of the implications of Bell and Local Friendliness no-go theorems on the interpretations of quantum mechanics, I will present a recent result about the failure of causal reasoning in dealing with the predictions of quantum mechanics in extended Wigner’s friend scenarios: not just classical causal reasoning, but causal reasoning using generalised probabilistic theories fails to faithfully account for the predictions of QM.

Understanding the properties of a generic state given by the superposition of different spacetimes configuration is one of the major questions that quantum gravity aim to address. This has major implication for the physics of the early universe, where the seeds of cosmic structures are originated from quantum fluctuations of the geometry. Predicting the strength of correlations of geometrical observables in this regime gives direct access to the initial conditions of our universe, from which all matter structures later evolved.
Exploiting techniques from covariant Loop Quantum Gravity, I present an operational recipe to construct physical states of the quantum geometry, to define a cosmological interpretation for them, and compute analytically and numerically the quantities that characterize them, i.e. correlations between the volumes of spacially-separated region and the corresponding entanglement entropy. I review the results obtained and I discuss the future perspective of this research program.

The received view is that superluminal signalling is prohibited in relativistic theories and allowed in non-relativistic theories. In line with this received view, there are special circumstances under which superluminal signalling with non-selective measurements is not permitted in non-relativistic quantum mechanics (NRQM), but not a general prohibition. I will survey some examples of hybrid models that combine non-relativistic and relativistic assumptions and which permit superluminal signalling with non-selective measurements. These examples illustrate that both the relativistic dynamics and a relativistic model of local measurement operations contribute to the prohibition on superluminal signalling in relativistic quantum theories.

I discuss recent claims according to which relational understandings of quantum physics would undermine the credibility of science, or bring to a solitary solipsism. I show that they have no ground.

The conjecture of causal decompositions proposes that there is a deep link between two fundamental structures of (unitary) quantum theory: causal structure and compositional structure. It states that, for any unitary U from several inputs to several outputs, U features a certain causal structure (i.e. satisfies a certain set of no-influence relations between inputs and outputs) if and only if it can be decomposed into the composition of ‘smaller’ unitaries, along a graph that makes the causal structure evident. This would provide a very important equivalence between an empirical, operationally accessible notion (causal structure) and a powerful mathematical structure (compositional structure). This conjecture has so far been proven in cases involving a few inputs and outputs. Here, I will present a result (to appear soon) showing that causal decompositions also exist for 1D Quantum Cellular Automata (QCAs). 1D QCAs can be seen as representing lightcone-abiding dynamics in a discretised (1+1)D Minkowski spacetime. This result can be understood as showing that, in one (discretised) spatial dimension, a dynamics satisfies relativistic causality if and only if it can be broken down into a series of nearest-neighbour interactions. It also provides the first constructive form of 1D QCAs at any causality radius. On the way, I will also discuss how the proof of this result involves important conceptual and mathematical innovations, related to the use of C* algebras to model quantum subsystems, which might find application elsewhere as well.

The possibility of a future `table top’ Quantum Gravity test has been much discussed the past years. I will discuss the family of `standard’ protocols that envision entanglement production between spatially delocalised particles, the controversy surrounding the topic with respect to a `theory fixed’ vs a `theory independent’ interpretation and the related confusion on the assumption of `locality’ and the `mediating degree of freedom’. I will also discuss alternative protocols involving macroscopic quantum systems like bose einstein condensates, a genuine relativistic test involving superposition of rotations (superposition of mass), as well as these ideas may be leveraged also for the detection of Planck mass dark matter particles (which could be tiny black/white holes!).

I will sketch the toolbox and achievements of modern lattice quantum gravity, aka solving the full 4D Lorentzian gravitational path integral, using causal dynamical triangulations (CDT). It provides us with a long-sought window on physics in a near-Planckian regime, where geometry is highly nonclassical and not describable in terms of standard tensor calculus. With concrete answers to longstanding issues of how to properly implement diffeomorphism invariance, causal structure and Wick rotation, we are now focusing on interesting, approach-independent questions on the nature of nonperturbative quantum gravity and its observables. Based on results on the dynamical emergence of a 4D universe with de Sitter properties from first principles, early-universe cosmology may be our best bet in terms of quantum gravity phenomenology.

Quantum networks are the center of many of the recent advances in quantum science, not only leading to the discovery of new properties in the foundations of quantum theory but also allowing for novel communication and cryptography protocols. It is known that networks beyond that in the paradigmatic Bell’s theorem imply new and sometimes stronger forms of nonclassicality. We will review some recent experiments addressing non-classicality in different network structures.