From micro- to macrorealism: addressing experimental clumsiness with semi-weak measurements

Abstract

We propose a protocol that allows to assess the precise correspondence between thought and practical experiments, a critical point for addressing experimental clumsiness in a test of macro- or micro-realism. Two-time generalized von Neumann measurements of properties $\mathcal{A}$ and $\mathcal{B}$ are shown to obey the so-called no-signaling in time condition for initial states defined as an incoherent sum of eigenstates of $\mathcal{A}$ . An experiment for witnessing the use of this type of measurements in the laboratory is then devised by proving the existence of five conditions that have to be fulfilled by any generalized von Neumann measurement. Ensuring the use of this type of measurements and then testing the no-signaling in time condition for a range of system-meter coupling strengths allows to test realism in a highly reproducible manner and to critically narrow the so-called clumsiness loophole. The resulting protocol is applicable to general (not only dichotomic) variables, and it is employed to show, both analytically for general systems and numerically for a collection of harmonic oscillators, that quantum systems made of a large number of uncorrelated particles are genuinely macrorealist, i.e., realistic with respect to all intensive properties at any time.

Keywords

• Leggett–Garg inequalities
• no-signaling in time
• macrorealism
• generalized von Neumann measurements
• quantum-to-classical transition
• weak measurement