The Quantum Cheshire Cat: Can neutrons be located at a different place than their own spin?

Can neutrons be located at a different place than their own spin? A quantum experiment, carried out by a team of researchers from the Vienna University of Technology, demonstrates a new kind of quantum paradox.

The Cheshire Cat featured in Lewis Caroll's novel "Alice in Wonderland" is a remarkable creature: it disappears, leaving its grin behind. Can an object be separated from its properties? It is possible in the quantum world. In an experiment, neutrons travel along a different path than one of their properties -- their magnetic moment. This "Quantum Cheshire Cat" could be used to make high precision measurements less sensitive to external perturbations.

According to the law of quantum physics, particles can be in different physical states at the same time. If, for example, a beam of neutrons is divided into two beams using a silicon crystal, it can be shown that the individual neutrons do not have to decide which of the two possible paths they choose. Instead, they can travel along both paths at the same time in a quantum superposition.

"This experimental technique is called neutron interferometry," says Professor Yuji Hasegawa from the Vienna University of Technology. "It was invented here at our institute in the 1970s, and it has turned out to be the perfect tool to investigate fundamental quantum mechanics."