### Abstract

Results in atomic physics show that mass–energy equivalence plays a crucial role in energy and momentum conservation for atom–light interactions: absorption or emission of field quanta must also change the atom’s rest mass by an equivalent energy. Though the Unruh–DeWitt (UDW) detector model of a quantum particle interacting with an external environment is powerful in its simplicity, the dominant model—which assigns the detector a classical trajectory and treats only the internal state as a quantum degree of freedom—cannot capture the above mentioned effects.

Recent models upgrading the UDW model to include more realistic quantum descriptions of the centre of mass have described the detector as either moving in superposition along classical trajectories, or dynamically evolving under a non-relativistic Hamiltonian. These have led to interesting results relating to themalisation and entanglement harvesting, but they too are unable to capture the mass-energy effects we desire.

Here I will discuss how we addressed this problem, and describe a new detector model which leverages the simplicity of the UDW model while also incorporating quantisation of the detector’s mass-energy to allow mass changes due to emission/absorption. I show
that these relativistic effects persist even at low energies and cannot be ignored unless all centre of mass dynamics is ignored. I will also show how our new model compares to the previous models with classical CoM and quantum CoM, as well as the detector in a superposition of trajectories, and discuss particular effects that arise due to mass–energy equivalence. I will then present a further step we have taken, in which such a detector with a variable mass has ground and excited states in superposition, producing a model where the detector can be interpreted as a quantum clock weakly interacting with its environment.

Date

Jul 16, 2023 — Jul 20, 2023

Location

RQI-North, Chania, Greece

###### Postdoctoral Scientist

Carolyn Wood is a postdoctoral researcher at the University of Queensland, in Brisbane, Australia focusing on quantum machine learning and physics at the interface between quantum mechanics and general relativity.