Experiments testing macroscopic quantum superpositions must be slow

TL;DR

This paper argues that testing macroscopic quantum superpositions requires a minimum measurement time proportional to the system's mass or charge, preventing superluminal communication and implying fundamental limits on such experiments.

Contribution

It introduces a fundamental principle that local experiments distinguishing superpositions from mixtures must take time proportional to the system's mass or charge, resolving paradoxes in macroscopic quantum tests.

Findings

Measurement time scales with mass or charge, preventing superluminal signaling.

Experiments with charged particles involve photon entanglement or vacuum fluctuations.

Results suggest gravitational vacuum fluctuations are relevant for massive particles.

Abstract

We consider a thought experiment where the preparation of a macroscopically massive or charged particle in a quantum superposition and the associated dynamics of a distant test particle apparently allow for superluminal communication. We give a solution to the paradox which is based on the following fundamental principle: any local experiment, discriminating a coherent superposition from an incoherent statistical mixture, necessarily requires a minimum time proportional to the mass (or charge) of the system. For a charged particle, we consider two examples of such experiments, and show that they are both consistent with the previous limitation. In the first, the measurement requires to accelerate the charge, that can entangle with the emitted photons. In the second, the limitation can be ascribed to the quantum vacuum fluctuations of the electromagnetic field. On the other hand, when applied to massive particles our result provides an indirect evidence for the existence of gravitational vacuum fluctuations and for the possibility of entangling a particle with quantum gravitational radiation.