Virtual Seminars

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.

Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, for instance a massive quantum system prepared in a quantum superposition of positions and sourcing a gravitational field, has attracted a lot of attention: experiments are working towards realising such a scenario in the laboratory, and measuring the gravitational field associated to a quantum source is expected to give some information about quantum aspects of gravity. However, there are still open questions concerning the precise conclusions that these experiments could draw on the nature of gravity, such as whether experiments in this regime will be able to test more than the Newtonian part of the gravitational field. In this talk, I will show that a static mass in a quantum state gives rise to effects that cannot be reproduced using the Newton potential nor with a known classical model of gravity. These effects can in principle be measured by performing an interference experiment, and are independent of graviton emission.
Identifying stronger quantum aspects of gravity than those reproducible with the Newton potential is crucial to prove the nonclassicality of the gravitational field and to plan a new generation of experiments testing quantum aspects of gravity in a broader sense than what proposed so far.

I will present a consistent theory of classical systems coupled to quantum ones via the path integral formulation. The dynamics is linear in the density matrix, completely positive and trace-preserving. We then apply the formalism to general relativity, since it’s reasonable to question whether spacetime should have a quantum nature given it’s status within quantum field theory. In the classical-quantum formalism, the measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical spacetime necessarily causes collapse of the wave-function. Any such classical quantum theory necessarily has an experimental signature which can be used to test the quantum nature of gravity. I’ll conclude with an update on the current status of the program.

Many presentations of quantum mechanics include a postulate that the state of a system undergoes an instantaneous change following a measurement. This is clearly incompatible with special and general relativity and raises questions concerning the description of measurement in quantum field theory (QFT). Attempts to extend measurement postulates to QFT by hand have produced pathologies, …

Christopher J. Fewster
University of YorkMeasurement in Quantum Field Theory: Problems and Solutions Read More »

Identical quantum particles exhibit only two types of statistics: bosonic and fermionic. Theoretically, this restriction is commonly established through the symmetrization postulate or (anti)commutation constraints imposed on the algebra of creation and annihilation operators. The physical motivation for these axioms remains poorly understood, leading to various generalizations by modifying the mathematical formalism in somewhat arbitrary …

Borivoje Dakić
University of ViennaReconstruction of Quantum Particle Statistics: Bosons, Fermions, and Transtatistics Read More »