Quantum Theory and Gravity

Schematic illustration of a single quantum system in a spatial superposition, with an internal-energy superposition acting as a running quantum clock, above the Earth. One branch of the spatial superposition lies at a higher gravitational potential than the other. The curved loop indicates the recombination of the two branches to produce interference. The visibility of the interference pattern decreases in proportion to the which-path information encoded in the clock due to gravitational time dilation.

General relativity and quantum theory differ profoundly in both structure and interpretation. While each has been tested to high precision in its own regime, gravitational effects in current quantum experiments are still described by the Newtonian limit, and precision tests of general relativity remain classical. Understanding regimes in which both theories are simultaneously required is therefore of fundamental importance.

Our group investigates this interplay from several directions. We predicted novel quantum effects of relativistic time dilation in interference experiments with quantum clocks, whose internal dynamics provide an operational notion of time. For sufficiently large systems, these effects give rise to an effective decoherence mechanism, already relevant at low energies and in Earth’s gravitational field.

We also study table-top probes of phenomenological quantum-gravity models, including scenarios with modified canonical commutation relations. In addition, we have analysed gedanken experiments showing that, to avoid signaling or violations of complementarity, a quantum superposition of a Newtonian-like gravitational field must be accompanied by the quantum radiation field of gravity. This suggests that retaining the former without the latter is not consistent.