Beyond entanglement

Is gravity classical in nature or quantum mechanical? To settle this longstanding and fundamental question, experimentalists have tried to test gravity's quantum nature in different ways. In one method, they set up a bunch of objects that interacted only via gravity to see if there was a signature of entanglement — an entirely quantum and mysterious linking of the properties of subatomic particles that causes them to act as a single entity, even when separated by large distances.

Now, two researchers from the Indian Institute of Technology (IIT) Bombay (bit.ly/Quantum-question) argue that this route is not the right one to reveal the nature of gravity; instead, they propose an alternative path. "People have been trying to say that if a system is quantum, it should have clear signatures of entanglement," says S. Shankaranarayanan, Professor of Physics. But in a regime where entanglement is negligibly small, such a probe can make a measurement of zero entanglement and thereby conclude that the system is classical, which would be wrong. They suggest that something other than entanglement be used to probe the quantumness of gravity.

"From 2018 itself, there have been suggestions in the quantum information literature regarding using information-theoretic methods to characterise gravity," he explains, stating the premise of their proposal. "Entanglement alone is not the tool that probes the dynamics of the system," says P. George Christopher, Shankaranarayanan's collaborator. "Since we have never seen the graviton, to use something like entanglement to characterise its dynamics is not the right thing to do." The graviton is a quantum of the gravitational field — just as the photon is of the electromagnetic field — but has never been "sighted" in experiments.
 

Christopher and Shankaranarayanan drop this route to test the quantumness of gravity and instead propose using a concept named dynamic fidelity susceptibility (DFS), which would make a more sensitive probe of the quantum nature of gravity. Analogous to an FM radio, which, when tuned, indicates the presence of a transmission at a specific frequency, tuning external masses whose properties are known can reveal the nature of the mediating force of gravity which links the known masses. This would unravel if the mediating force is quantum in nature or classical. DFS is analogous to the concept of thermodynamic susceptibility in classical thermodynamics, which measures a system's sensitivity to external variables such as temperature or pressure.