Gravitationally Mediated Entanglement in Interferometers with Repeated Splitting

Abstract

Proposals to probe the quantum nature of gravity through entanglement between spatially separated masses have attracted considerable interest. In such schemes, spatial superpositions are prepared through controlled splitting operations that produce several interferometric branches. Here we propose and analyse protocols involving repeated splitting stages, each of which doubles the number of occupied paths. We show that the density matrix after $n$ splitting stages is related to that of the $(n-1)$-splitting protocol through local parity dephasing. Two of the resulting sectors reproduce the preceding state, while the other two acquire additional gravitational phases. Removing the locally generated parts gives an exact lower bound on the change in negativity, expressed in terms of the preceding state's negative partial-transpose eigenspace. For two splitting stages, this leads to an explicit condition on three gravitational phases under which the additional split cannot reduce negativity. We also show that further splitting cannot shorten the lifetime of the generated entanglement. If the local internal system preserves the full branch label, the positional entanglement is retained upon recombination. We describe an implementation using a reusable two-level system and local memories, and compare it with a readout without a memory register. For the latter, we derive an exact condition for when an additional splitting stage improves the readout, based on a $2\times 2$ Schur complement. Numerical simulations show an advantage in positional negativity over the rates considered, with an improvement surviving the restricted readout at all decoherence rates considered at the final readout time.