Simultaneous action of a single photon at two remote places

Motivated by Einstein's thought experiment that a single quantum particle diffracted after a pinhole could in principle produce an action in two or several places on a hemispherical imaging screen, here we explore theoretically the possibility to simultaneously detect the action of a single photon at two remote places. This is considered in a cascade quantum system composed of two spatially distant cavities each coupled to a qubit in the ultrastrong coupling regime. We show that a single-photon pulse incident on the two cavities can simultaneously excite the two remote qubits and lead to two subsequent detection events even when the separation between them is comparable to the spatial length of the photon pulse. Our results not only uncover new facets of quantum mechanics at a fundamental level but also have practical applications, such as the generation of remote entanglement through a dissipative channel which is otherwise unattainable in the strong-coupling regime.