Mischa Woods

University College London
Jonathan Oppenhenim, University College London; Stephanie Werner, QuTech/Delft University of Technology

Finite dimensional Quantum Observers

Quantum mechanics is a highly successful science, and promises to bring us many important technologies such as the quantum internet and quantum computers. However, just like in everyday life experience, there are observers in quantum mechanics. The observers are necessary for many aspects of the quantum technologies which are on our doorstep. However, we still do not understand their fundamental properties and limitations. At present, they are never explicitly present in any of the theories we use. My research will attempt to address this issue, by putting them on the same footing as the quantum technologies they will be observing. Not only will it address issues such as how fallible are observers at the quantum scale, but also open the door to other questions such how do quantum effects of the observer affect their ability to make observations. Can these observers make observations forever, or will they degrade until they are no longer able to observe at all, after performing many observations?

Observers in quantum mechanics such as Alice and Bob are never modeled explicitly, only implicitly. Furthermore, they are “perfect” in the sense that they can perform any unitary or measurement on an explicitly modeled quantum system which are localised in time and space, and remember the outcome. I plan to understand how valid these assumptions about the observers really are. To do this, I will model observers and the system they make observations on explicitly, as a quantum mechanical system with no external control parameters. The observer and environment will undergo unitary dynamics governed by the Schrödinger equation. The observer will be a finite dimensional system consisting of a register in order to record observations of the environment, a quantum clock and battery in order to turn on/off interactions between the observer’s register and environment it wishes to observe. By modeling the observer explicitly in this way, fundamental questions about the observer can be asked: how fallible is it due to its finite size? Can a highly coherent observer outperform a semi-classical observer? Can observers make observations forever, or does the act of observing degrade the observer? These are just some of the questions I set out to answer.

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