Interplay of Gravity and Quantum Mechanical Superpositions

Lead Research Organisation: University College London
Department Name: Physics and Astronomy


The two most fundamental and well-verified theories of physics are General Relativity,
which describes gravity, and Quantum Mechanics, which describes the other three
fundamental forces known to us. A consistent quantum mechanical theory of gravity still
eludes us. The unification of quantum mechanics and gravity has been a greatly desired
prospect for some time. With the unification of quantum mechanics and special relativity
being achieved with Quantum Field Theory, it has been seen as inevitable for General
Relativity to also be, at some level, quantum in nature. To determine if this is the case
numerous tests have been proposed -- however no definitive answers have been
found. With this in mind this project will seek, among other things, to help answer whether
gravity, at least in its low energy limit, is fundamentally quantum in nature.
Some have started to question whether gravity is fundamentally a quantum entity, raising
the possibility of whether it could be a classical field/background happily co-existing with
quantum mechanics. Perhaps the most striking arena where we will suffer if gravity is
indeed classical is to predict the gravitational field when matter in peculiarly quantum
states, say in highly delocalized superpositions, acts as the source of gravity. To this end,
the broad aim of this project will be to design experiments which couple quantum
superpositions of states of mesosocpic objects to gravity, or use them as sources of gravity
and through that infer about the quantum nature of gravity. We will also investigate the
potential of these experiments to enable both precision accelerometry and gravimetry and
to enable the determination of gravitational force law and Newton's constant over short
The current project will seek to
(i) Extend the above gravitational accelerometry proposals for superpositions of
squeezed states and other engineered non-classical quantum states so as to
optimize the precision of measurements for a given investment of energy.
(ii) Extend the above proposals up to the macroscopic boundary of the mesoscopic
regime to explore the boundary between the quantum regime and gravity. In
particular, the extrapolation from nano-meter radii beads to micro-meter radii
beads, which can serve as the origin of measurable gravitational fields, will be
examined. For this purpose, a new way to split the spatial position through
inhomogeneous electric fields coupling to spin states through crystal
anisotropies, will be used.
(iii) A theoretical calculation of the decoherence of a superposition of distinct
energy-momentum states of a system in one region of space due to the presence
and fluctuations of other surrounding systems that couple gravitationally to it
will be made.
(iv) By bringing a probe mass in proximity to another mass in a highly non-classical
state, we are going to investigate the precisions to which the Newton's Constant
G, and potential corrections to Newton's law for short distances, stemming, for
example from extra dimensions, could be determined.
(v) Expand these investigations to two interferometer systems in order to include
bipartite entanglement to further explore how gravity addresses a highly
quantum, massive system. We will consider the interactions of two masses in
highly non-Gaussian states (prepared by methodologies founded earlier in the
project) to generate some form of 'loophole free' tests for the quantum nature
of gravity.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509577/1 01/10/2016 30/09/2021
1930690 Studentship EP/N509577/1 25/09/2017 24/09/2021 Ryan James Marshman