Everything is quantum: what I measure, my detector, myself!
Since the first formulation of quantum mechanics, Nature has been divided into two categories: the classical world (that of tennis balls) and the quantum world of atoms and photons. The transition between the two is made by the wave function collapse during the measurement process. From the multitude of possible measurement results included in the wave function, only one value comes out, the one actually measured, corresponding to a particular state of the wave function. This is an irreversible process, quite different from the (reversible) evolution of the system just before the measurement according to the Schrödinger equation. Two worlds and two types of dynamics… too complicated! Another approach is possible by considering that everything is of a quantum nature: the system studied, the measuring apparatus, the observer and even the watch that marks the time of the measurement. A researcher of the team Clusters and surfaces under intense excitation (ASUR acronym in French) of the INSP has shown that, with this approach, the probability function of the measurement results is simple and unique. Moreover, it is valid for tennis balls as well as for photons.
Artistic illustration of the case of Wigner’s friend. Wigner’s friend makes a measurement on a system which can give as a result “up” or “down”. Wigner measures the system as well and, at the same time, the result of the friend’s measurement, but with a different basis: “left” or “right”. Therefore, Wigner and his friend have a different perception of the same system studied; hence the apparent paradox.
« Conditional probabilities of measurements, quantum time, and the Wigner’s-friend case », M. Trassinelli, Phys. Rev. A 105, 032213 (2022) hal-03179772
« Conditional probability and interferences in generalized measurements with or without definite causal order », M. Trassinelli, Phys. Rev. A 102, 052224 (2020) hal-02933221
« Relational Quantum Mechanics and Probability », M. Trassinelli, Found. Phys. 48, 1092 (2018) hal-01723999