The interplay between Einstein’s theory of gravity and quantum mechanics remains one of the main open questions in physics. Each of the theories has been confirmed individually, but phenomena described by the two usually take place at very different physical regimes. Nevertheless, gravity can affect quantum systems and some aspects of the interplay can be probed in experiments. In this talk, I will discuss a series of works that stem from considering a quantum version of the “twin paradox” in which a clock is brought in superposition of being at two different gravitational potentials. I will show how this leads to new post-Newtonian effects in quantum interferometry [1], how it can cause decoherence of composite quantum systems [2,3] and how it leads to a new result in quantum gravity [4].

[1] M. Zych, F. Costa, I. Pikovski, and Č. Brukner, Nature Commun. 2, 505 (2011)
[2] I. Pikovski, M. Zych, F. Costa, and Č. Brukner, Nature Phys. 11, 668-672 (2015)
[3] I. Pikovski, M. Zych, F. Costa, and Č. Brukner, New J. Phys. 19, 025011 (2017)
[4] M. Zych, F. Costa, I. Pikovski, and Č. Brukner, Nature Commun. 10, 3772 (2019)