Observation of the quantum paradox of separation of a single photon from one of its properties

TL;DR

This paper reports an experimental demonstration of the quantum Cheshire cat phenomenon, where a single photon appears to be separated from one of its properties using weak measurements in a modified interferometer.

Contribution

The study provides the first experimental realization of separating a photon from one of its properties via weak measurements, confirming theoretical predictions.

Findings

Weak measurements reveal photon presence and polarization in different arms.

Absorber affects only the photon presence in one arm.

Polarization rotation affects only the polarization signal in the other arm.

Abstract

We report an experimental realization of the quantum paradox of the separation of a single photon from one of its properties (the so-called "quantum Cheshire cat"). We use a modified Sagnac interferometer with displaced paths to produce appropriately pre- and postselected states of heralded single photons. Weak measurements of photon presence and circular polarization are performed in each arm of the interferometer by introducing weak absorbers and small polarization rotations and analyzing changes in the postselected signal. The absorber is found to have an appreciable effect only in one arm of the interferometer, while the polarization rotation significantly affects the signal only when performed in the other arm. We carry out both sequential and simultaneous weak measurements and find good agreement between measured and predicted weak values. In the language of Aharonov et al. and in the sense of the ensemble averages described by weak values, the experiment establishes the separation of a particle from one its properties during the passage through the interferometer.