Quantum “Cheshire Cat” created in the lab

BBC : Last year theorists proposed a way to perform a measurement on a particle at a location that is different from that of the particle itself. The separated property resembles the disembodied grin of the Cheshire Cat in Lewis Carroll’s Alice’s Adventures in Wonderland—hence the technique’s whimsical name. Now a team led by Yuji Hasegawa of the Vienna University of Technology has applied the technique to neutrons and their spin. The experiment, which was conducted at the the Institut Laue–Langevin in Grenoble, entails sending beams of spin-polarized neutrons through an interferometer. The researchers could perturb the neutrons’ state just enough to imprint a telltale phase change without destroying the state. Thanks to that so-called weak measurement and to careful selection of initial and final states, the researchers could determine the neutrons’ spin from one of the interferometer’s two arms while knowing that the neutrons themselves went through the other arm. A paper describing the experiment appeared yesterday in Nature Communications.