Vacuum-Induced Quantum Gate

We demonstrate a postselected protocol in which a uniformly accelerated two-level Unruh–DeWitt detector, coupled to the Minkowski vacuum for a finite time with a smooth envelope, implements an observer-local $Z$ flip \emph{only} upon a heralded success. The finite-time coupling acts as a co-accelerated spectral filter that fixes the causal boundary prescription without ad hoc regulators and isolates a narrow two-quantum sector at the Unruh boost frequencies. A \emph{local herald}—a click indicating detection of the targeted quanta in wedge I—marks a successful run; conditioned on this herald, the output state closely matches the ideal $Z$-flipped state, and the fidelity improves as the switching is made longer and smoother. Without conditioning, the detector simply experiences ordinary thermalizing noise.