Can a Measurement Be Undone? Recovering the State of a Measured Microscopic System with a Reversible Measuring Apparatus

Abstract

We propose a direct, model-independent search for irreversible coherence loss during a reversible measurement. A mesoscopic apparatus measures a microscopic two-state system by storing which-state information and is then returned to its pre-measurement state; unitary quantum mechanics predicts recovered coherence, while collapse leaves residual endpoint loss. In one representative device, a coherently controlled molecular force source displaces a charged nanoparticle that serves as the apparatus; during one $4.19\mathrm{ms}$ measurement-and-reversal cycle, its two states become almost fully distinguishable. Such a device would give a first direct bound in a measurement-and-reversal setting. Repeating this cycle $5\times 10^5$ times would bound the corresponding irreversible coherence-loss rate beyond ordinary decoherence at $5.51{\mathrm{s}}^{-1}$. Continuous Spontaneous Localization is included as a secondary benchmark on the same apparatus history.