Quantum Cheshire cat

In quantum mechanics, the quantum Cheshire cat is a quantum phenomena that suggests that a particle's physical properties can take a different trajectory from that of the particle itself. The name makes reference to the Cheshire Cat from Lewis Carroll's Alice's Adventures in Wonderland, a feline character which could disappear leaving only its grin behind. The effect was originally proposed by Yakir Aharonov, Daniel Rohrlich, Sandu Popescu and Paul Skrzypczyk in 2012.^[1]

In classical physics, physical properties cannot be detached from the object associated to it. If a magnet follows a given trajectory in space and time, its magnetic moment follows it through the same trajectory. However in quantum mechanics, particles can be in a quantum superposition of more than one trajectory previous to measurement. The quantum Cheshire experiments suggests that previous to a measurement, a particle may take two paths, but the property of the particle, like the spin of a massive particle or the polarization of a light beam, travels only through one of the paths, while the particle takes the opposite path. The conclusion is only obtained from an analysis of weak measurements, which consist in interpreting the particle history previous to measurement by studying quantum systems in the presence of small disturbances.

Experimental demonstration of the quantum Cheshire cat have already been claimed in different systems, including photons^[2] and neutrons.^[3] The effect has been suggested as a probe to study properties of massive particles by detaching it from its magnetic moment in order to shield them from electromagnetic disturbances.^[4][5] A dynamical quantum Cheshire cat has also been proposed as a counterfactual quantum communication protocol.^[6]