Maria Papageorgiou
IQOQI Vienna
January 2024
Jonathan Oppenheim
University College LondonA post quantum theory of classical gravity
I will present a consistent theory of classical systems coupled to quantum ones via the path integral formulation. The dynamics is linear in the density matrix, completely positive and trace-preserving. We then apply the formalism to general relativity, since it’s reasonable to question whether spacetime should have a quantum nature given it’s status within quantum field theory. In the classical-quantum formalism, the measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical spacetime necessarily causes collapse of the wave-function. Any such classical quantum theory necessarily has an experimental signature which can be used to test the quantum nature of gravity. I’ll conclude with an update on the current status of the program.
Remote sensing of a levitated superconductor with a flux-tunable microwave cavity
We present a cavity-electromechanical system comprising a superconducting quantum interference device which is embedded in a microwave resonator and coupled via a pick-up loop to a 6 $mu$g magnetically-levitated superconducting sphere. The motion of the sphere in the magnetic trap induces a frequency shift in the SQUID-cavity system. We use microwave spectroscopy to characterize the system, and we demonstrate that the electromechanical interaction is tunable. The measured displacement sensitivity of $10^{-7} , mathrm{m} / sqrt{mathrm{Hz}}$, defines a path towards ground-state cooling of levitated particles with Planck-scale masses at millikelvin environment temperatures.
Measurement events relative to temporal quantum reference frames
The Page-Wootters formalism is a proposal for reconciling the background-dependent, quantum-mechanical notion of time with the background independence of general relativity. However, there has been much discussion regarding the physical meaning of the framework. In this work, we compare two consistent approaches to the Page-Wootters formalism to clarify the operational meaning of evolution and measurements with respect to a quantum temporal reference frame. The so-called “twirled observable” approach implements measurements as operators that are invariant with respect to the Hamiltonian constraint. The “purified measurement” approach instead models measurements dynamically by modifying the constraint itself. While both approaches agree in the limit of ideal clocks, a natural generalization of the purified measurement approach to the case of non-ideal, finite-resource clocks yields a radically different picture. We discuss the physical origin of this discrepancy and argue that they describe operationally distinct situations. Moreover, we show that, for non-ideal clocks, the purified measurement approach yields time non-local, non-unitary evolution and implies a fundamental limitation to the operational definition of the temporal order of events. Nevertheless, unitarity and definite temporal order can be restored if we assume that time is discrete.
Quantum-enhanced screened dark energy detection
We propose an experiment based on a Bose-Einstein condensate interferometer for strongly constraining fifth-force models. Additional scalar fields from modified gravity or higher dimensional theories may account for dark energy and the accelerating expansion of the Universe. These theories have led to proposed screening mechanisms to fit within the tight experimental bounds on fifth-force searches. We show that our proposed experiment would greatly improve the existing constraints on these screening models by many orders of magnitude, entirely eliminating the remaining parameter space of the simplest of these models.
