Quantum clocks and thermodynamics

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

Recent work in quantum thermodynamics has shown a quantitative connection between our ability to measure time precisely via a physical clock, and the thermodynamic arrow of time (in terms of increasing entropy). It would be interesting to explore this connection further, going beyond the particular model studied initially, as well as to investigate the broader implications of using explicit quantum clocks to measure time (such as the effective dynamics relative to the clock-time). Two additional related questions are:

How are equilibration and thermalisation affected if time is discrete, such as in a quantum cellular automata (a local translationally invariant model of quantum computation)?

Could we use a quantum clock to create a superposition of two different time-orderings of measurements, such that the results could not be explained by a classical mixture of time-orderings?

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509619/1 01/10/2016 30/09/2021
1942515 Studentship EP/N509619/1 01/01/2018 30/07/2021 Thomas Purves
 
Description We have looked into causal inequality violation beyond standard quantum theory. Causality in the normal world is described by well-ordered events (for example, I put on my socks, and they I put on my shoes) or a probabilistic mixture of well ordered events (I toss a coin and either put my socks inside my shoes first then put on my socks and shoes, or I put my socks on and then my shoes - in principle both of these processes have the same result!). In quantum theory, and beyond into quantum gravity, this may not be the case and we allow for the possibility for both events to happen in superposition with each other. Casual inequality violation is a hallmark that something non-classical has gone one. We have found that, contrary to previously held beliefs, it is possible to violate a causal inequality and maintain a local causal narrative, where future and past are distinct. We published these results in PRA in 2019.

Further to that, we have now completed a general proof that quantum theory does not violate a causal inequality. More specifically, although causal indefiniteness is present in the mathematical description of quantum theory we can simulate the outcomes of causally indefinite quantum processes by making use of a classical (probabilistic) causal order. This result was quite surprising, and has been prepared as a pre-print available on the arXiv, and is currently in prep for submission to a journal.

In other avenues, we have examined thermodynamics in physics. We have provided a calculation for the work associated to the quantum measurement procedure, and how post-selection can change this value. We have done this my making use of a formalism for thermodynamics, the explicit battery formalism - where the first law of energy conservation holds through the modelling of a battery being charged and discharged (as opposed to the implicit framework, where work is left defined as the very thing that makes the first law hold).
Exploitation Route We want to further our results into claims about quantum theory in general. Our example causal inequality violation can only be realized in quantum theory by post-selection and so is non-physical. Is this true of quantum theory in general? Is causality 'protected' in the circuit model of quantum theory. We are aiming to answer these questions as of now, with initial results promising.
Sectors Other
URL https://scholar.google.com/citations?user=JsJgtLAAAAAJ&hl=en