By Hamish Johnston
Three months ago we ran a news article about a “quantum Cheshire cat” experiment that was proposed by Yakir Aharonov of Tel Aviv University and colleagues. Now, an international team of physicists has created a quantum Cheshire cat using polarized neutrons at the Institut Laue-Langevin (ILL) in Grenoble, France.
The work was done by Yuji Hasegawa and colleagues at the Vienna University of Technology, ILL, the University of Cergy-Pontoise and Chapman University.
In Lewis Carroll’s story Alice’s Adventures in Wonderland, Alice encounters a smiling Cheshire cat that fades away leaving behind just its grin. A quantum Cheshire cat is a physical system that can somehow be separated from its quantum state, only to be reunited at a later time.
In Hasegawa’s experiment the cat is a neutron and the quantum state is the polarization of the neutron. The experiment involves sending a beam of polarized neutrons through an interferometer. “Inside the interferometer the cat goes though the upper beam path, while its grin travels along the lower beam path,” he says.
The team says that it was able to confirm that the neutron and its polarization took different paths by making “weak measurements” on the neutrons as they pass through the experiment – something that was suggested by Aharonov and colleagues. Unlike a conventional measurement in quantum mechanics, which would have a profound effect on the outcome of an interferometry experiment, a weak measurement provides a small amount of information about an intermediate state without having a significant effect on the final outcome.
Hasegawa and colleagues describe their experiment in a preprint on arXiv entitled “Observation of a quantum Cheshire cat in a matter wave interferometer experiment“.
You can find out much more about weak measurement in the Physics World feature article “In praise of weakness” by Aephraim Steinberg, Amir Feizpour, Lee Rozema, Dylan Mahler and Alex Hayat.