Virtual Seminars

Robert Oeckl
Universidad Nacional Autónoma de México

What is Quantum Theory?

Abstract: The invention of quantum theory in the 1920s represented a paradigm shift in our approach to describing the natural world. The focus on the object as a primitive shifted to the observation as a primitive. At the time, the first applications of interest came with a classical description in the language of Hamiltonian evolution, canonical variables and states. Staying close to this particular language lead to the development of the quantum formalism of Hilbert spaces, operators, Schrödinger equation and Born rule. Somewhat unfortunately, this standard formulation has come to dominate our understanding of what quantum theory is. While it was successfully employed in describing the micro-structure of matter and its relevant interactions, describing the dynamics of spacetime itself is outside of its scope. With the present talk I want to promote the idea that quantum theory is much more general than this standard formulation. I aim to clarify the essence of the paradigm shift that lies at the heart of the transition from classical to quantum theory. On this basis I then review the derivation from first principles of a more fundamental formulation of quantum theory, the positive formalism, and the recovery of the standard formulation as a special case.

Časlav Brukner
University of Vienna

Quantum superposition of processes with opposing thermodynamic arrows of time

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.

Lucien Hardy
Perimeter Institute

A Time Symmetry in Operational Theories

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.

Anna Pearson with Natalia Ares
Oxford University

Testing gravitational decoherence through the heating of a mechanical resonator

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.

Nick Huggett, with Tushar Menon and Fedele Lizzi
University of Illinois

Missing the Point in Noncommutative Geometry

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.

Carlo Rovelli
Center for Theoretical Physics at Aix-Marseille University

Why can we influence the future?

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.

Sougato Bose
University College London

A Table-top Testing of the Quantum Nature of Gravity: Assumptions, Implications and Practicalities of a Proposal

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.

Eugenio Bianchi
Penn State University

Entanglement and the Architecture of Spacetime

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.