I agree with Chiara Marletto and Vlatko Vedral that we need to test quantum gravity experimentally (Nature 547, 156–158; 2017). However, the idea of measuring quantum superpositions of the gravitational field, as Richard Feynman described, is hardly new.

The problem is that, with current measurement precision, we can detect either the gravitational field of an object or its quantum properties, but not both. The gap is closing slowly — a main topic at the Experimental Search for Quantum Gravity meeting last year in Frankfurt, Germany. This conference series, which, as Marletto and Vedral advocate, “bring[s] together the quantum- and gravity-physics communities, as well as theorists and experimentalists”, has in fact been running since 2007.

Austrian physicist Markus Aspelmeyer and his group have published promising work in this area (see, for instance, J. Schmöle et al. Class. Quantum Grav. 33, 125031; 2016). Several independent teams are studying whether it is possible to distinguish classical from quantum correlations in the cosmic microwave background. Others are trying to quantify whether quantum entanglement that is mediated by gravity can be used to pin down the quantum nature of space and time. (For a summary, see also go.nature.com/2woh6sz.)

Marletto and Vedral reply — We proposed that quantum effects in the gravitational field could be witnessed by probing the field using two masses, with each one in a superposition of two locations (Nature 547, 156–158; 2017). Earlier proposals, from Richard Feynman and from Markus Aspelmeyer's group, instead use a single superposed mass and a classical test mass as a probe.

Our proposal depends on detecting entanglement between the two masses, generated by gravitational interaction (C. Marletto and V. Vedral Preprint at http://arxiv.org/abs/1707.06036; 2017). A related proposal has emerged from the quantum-information community (S. Bose et al. Preprint at http://arxiv.org/abs/1707.06050; 2017). More-detailed predictions, based on current quantum-gravity approaches, are needed for these schemes. We would therefore welcome an interdisciplinary meeting involving the communities that work on quantum information, quantum foundations and quantum gravity.