The University of Southampton

Scientists explore low-energy approach to probe mystery of fundamental physics

Published: 7 January 2020
Professor Hendrik Ulbricht (centre) hopes the new approach could help merge two prominent theories

Physicists from the universities of Southampton and Glasgow have conducted an innovative low-energy experiment that tests the unexplained interface between quantum mechanics and general relativity.

The collaborative research studied the quantum interference effect, known as the Hong-Ou-Mandel (HOM) dip, in a rotating frame equivalent to curved space time. Their findings have been published as an Editor's Suggestion in Physical Review Letters.

Finding a consistent explanation of quantum mechanics and general relativity represents one of the greatest challenges in modern science, with many prominent experiments such as the Large Hadron Collider considering high-energy physics to unite the theories.

This new study instead looked to the low-energy regime by running an optical experiment in a non-inertial frame.

Professor Hendrik Ulbricht, Head of the Southampton Quantum, Light and Matter research group, says: "Scientists have tried for decades to find an answer to this profound fundamental physics question with no success. This experiment is a first step in a very promising direction as it questions the possible merger of the theories in a different way by looking in a completely different regime.

"The rotating HOM experiment is one possible answer to this question, but there are more and I expect this will become a fruitful research field in fundamental physics, starting from existing quantum optical experiments and turning on non-inertial effects."

Rotation is the simplest way to experimentally implement a non-inertial frame. Quantum mechanics is formulated with inertial frames in flat spacetime, with no acceleration relative between the particle and reference frame. In general relativity, gravity comes from acceleration because of curvature of spacetime. According to the theory's equivalence principle, gravity can then be mimicked in an experiment through acceleration.

Scientists installed the rotating frame by putting the HOM interferometer on a large rotating platform, with experimental parts moving with the frame while the photons were propagated in free spacetime. The experiment's observations matched the expected outcomes as defined by quantum mechanics.

The study took place in facilities at the University of Glasgow and was directed by Professors Miles Padgett and Daniele Faccio. Southampton expertise from Hendrik and Marko Toros contributed to the theory, data analysis and discussion of results.

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