Interferometry tests Schrödinger-Newton theory and mechanical operation of quantum systems.

The fundamental incompatibility between quantum mechanics and general relativity remains one of the most significant challenges in modern physics. Researchers continually seek experimental avenues to probe the quantum behaviour of gravity, often focusing on the subtle interplay between quantum systems and gravitational fields. A new proposal details an interferometric protocol designed to test the semi-classical Schrödinger-Newton (SN) theory, which predicts a state-dependent gravitational interaction between objects. This theory suggests that the gravitational force experienced by a test mass is influenced by its quantum state, a deviation from classical Newtonian gravity.

Yubao Liu, Yanbei Chen, Kentaro Somiya, and Yiqiu Ma, collaborating across institutions including the Huazhong University of Science and Technology, the California Institute of Technology, and the Tokyo Institute of Technology, present their findings in a paper entitled “Testing the quantum nature of gravity through interferometry”. Their work demonstrates the potential for conclusive testing of the SN theory using a Michelson interferometer, assisted by squeezed light states, and achievable within a practical timeframe and temperature.

Researchers propose a novel protocol utilising a Michelson interferometer to investigate the validity of the semi-classical Schrödinger-Newton (SN) theory, a theoretical framework predicting state-dependent gravitational effects on test masses. The experiment centres on deliberately inducing asymmetry between the interferometer’s arms, creating interaction between the common and differential motion of the test masses. This carefully engineered interaction imprints a distinct binary signature onto the correlated measurements of the output light fields. By analysing the correlation between these light fields, scientists aim to detect this signature, indicative of SN theory’s predictions, and circumvent challenges associated with isolating and measuring extremely weak gravitational forces.

The experiment demonstrates the feasibility of conclusively testing SN theory within a 1 Kelvin temperature environment, a crucial parameter for minimising thermal noise, and achieves this with just three hours of aggregated measurement data. Employing squeezed states – input states with reduced quantum noise – significantly enhances the signal-to-noise ratio, with the protocol achieving sufficient sensitivity utilising 10 decibel squeezed states. A Michelson interferometer functions by splitting a beam of light into two paths, reflecting them off mirrors, and then recombining them to create an interference pattern. Changes in the path length or properties of the light affect this pattern, allowing for precise measurements.

Successful detection of this binary signature would provide evidence supporting the mechanical operation of quantum systems, challenging purely unitary descriptions of quantum evolution. Current quantum mechanics describes evolution via unitary transformations, preserving probability and implying no energy dissipation. However, the SN theory suggests a mechanical interaction between quantum systems and gravity, potentially leading to decoherence – the loss of quantum coherence – and observable non-unitary effects. This implies that gravity, according to SN theory, isn’t merely a geometric property of spacetime, but an active force capable of inducing mechanical interactions at the quantum level.

Future work focuses on refining the sensitivity of the interferometer and exploring the limits of the SN theory’s predictions. This includes investigating the impact of environmental noise and developing advanced data analysis techniques to further isolate the binary signal. Expanding the experimental setup to incorporate higher-frequency mechanical modes and exploring alternative interferometer configurations could also enhance the protocol’s sensitivity and broaden its applicability. Theoretical investigations will concentrate on refining the predictions of the SN theory and exploring its implications for other areas of physics.