April 2021

Abstract: Fundamental laws of physics are generally time-symmetric. The directionality of time is then often explained with the thermodynamic arrow of time: the entropy of an isolated system increases during a process, and it is constant only if the process is reversible. In this talk, I will consider a quantum superposition between two processes with opposing thermodynamic arrows of time. How is a definite arrow of time established for such a superposition? I will show that a quantum measurement of entropy change (for values larger than the thermal fluctuations) can be accountable for this. In particular, while the individual result of the measurement is random, once the value of the entropy variation has been observed, the system continues its evolution according to a definite arrow of time. Furthermore, for entropy variations lower than (or of the order of) the thermal fluctuations, interference effects can cause entropy changes describing more or less (ir)reversible processes than either of the two constituents, or any classical mixture therefrom.

Abstract: The standard framework for probabilistic operational theories is time asymmetric. The fact that future choices cannot affect the probability of earlier outcomes is mathematised by the statement that the deterministic effect is unique. However, deterministic preparations are not unique and, correspondingly, earlier choices can influence later probabilities of outcomes. This time asymmetry is rather strange because abstract probability theory knows nothing of time. Furthermore, the Schoedinger equation is time symmetric and, additionally, measurement situations can be treated by very simple models (without invoking the Second Law at all). In this talk I will outline how it is possible to give a time symmetric treatment of operational probabilistic theories with particular application to Quantum Theory. In so doing, we will see that the usual formulation of operational quantum theory is, in some sense, missing half of the picture.

Abstract: The theory of classical channel gravity models gravitational interactions as classical measurement channels. These channels are a source of decoherence even if the results of the measurements are never recorded in a lab and thus the gravitational interaction can be thought of as having the same effect as an observer. This leads to two potentially observable effects – decoherence in the position basis and a density dependent heating effect. We have set up an experiment to test for the latter, using a cavity optomechanical setup at cryogenic temperatures to measure the mode heating of a silicon nitride membrane.

Abstract: Noncommutative geometries generalize standard smooth geometries, parametrizing the noncommutativity of dimensions with a fundamental quantity with the dimensions of area. The question arises then of whether the concept of a region smaller than the scale makes sense in such a theory. We argue that it does not, in two interrelated ways. In the context of Connes’ spectral triple approach, we show that arbitrarily small regions are not definable in the formal sense. While in the scalar field Moyal-Weyl approach, we show that they cannot be given an operational definition. We conclude that points do not exist, and that continuous spacetime is an appearance, in such geometries.

Abstract: The two main components of the QISS community –quantum information and quantum gravity– have opposite views on the arrow of time. This is generally taken as foundational in the dominant instrumentalist approach of the fist; while is often considered to be only a contingent aspect of the macroworld in the second. I show how to reconcile the two perspectives. This requires two steps: a careful analysis of the arrow of implication; and an understanding of the physical source of the time orientation of the agent. A number of papers have recently addressed these issues offering a compelling solution to the apparent disagreement.

Abstract: After briefly discussing constructor theory (a recently proposed generalisation of the quantum theory of computation), I will present the general argument that provides a robust theoretical underpinning for the recently proposed experiments to witness quantum effects in gravity.

Abstract: A lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Motivated by this, I will present a feasible idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, is also provided and can be measured through simple spin correlations. Further, I clarify the assumptions underpinning the above proposal such as our reasonable definition of “classicality”, as well as the crucial aspect of the locality of physical interactions. The role of off-shell processes is also highlighted to clarify what the mediators actually are according to the standard theory of quantum gravity. How the experiment sits within relativistic quantum field theory is clarified. Lastly, a list of practical challenges are noted.

Abstract: The quantum field vacuum is highly entangled, even in causally disconnected regions. In contrast, the state of a quantum geometry of space can be unentangled, resulting in an uncorrelated network of elementary quanta of space. In this talk I discuss how the architecture of spacetime emerges from entanglement between these elementary quanta. I will focus on loop quantum gravity, causal structures and the primordial universe.