Can a particle be in one place and its property somewhere else?
In the quantum Cheshire Cat experiment, weak measurements of a neutron’s presence and spin point to different paths. This does not prove that spin detaches from the particle and travels independently.
In the classical world, you cannot pick up an apple and leave its redness on the table. Wherever an object goes, you expect its properties to go with it. So far, this is familiar physics. But at the quantum scale, an experiment challenges that expectation.
In 2014, Tobias Denkmayr and colleagues sent neutrons through a two-path interferometer. They specially prepared the neutrons’ initial quantum state and, at the output, selected only events corresponding to a particular measurement outcome. These steps are called preselection and postselection.
They used weak measurements to investigate the paths in between. Here, “weak” means that the measuring apparatus interacts only slightly with the system. Rather than continuously following a single neutron’s journey, the method extracts information from selected outcomes across many repetitions.
The surprising part is this: under these conditions, weak measurements of the neutron’s presence in a particular arm pointed to one path, while magnetic measurements associated with a spin component pointed to the other. The system behaved as if the neutron took one path and its magnetic property took another.
This is why the effect is called the quantum Cheshire Cat. The name comes from the cat in Alice’s Adventures in Wonderland whose grin remains visible after its body disappears. Here, the neutron plays the cat, and the magnetic property associated with spin plays the grin.
But “as if” matters. The experiment did not remove spin from a neutron and put it somewhere else. It demonstrated that, under particular preparation and selection conditions, measurements sensitive to different physical properties can respond in different arms.
The interesting point is that, in the quantum world, an answer to “Where is the particle?” does not always suffice to construct a classical picture placing all its properties at the same point. What is remarkable is that the limits of this picture can be investigated experimentally.
The experiment did not directly measure a neutron’s mass travelling through one arm; it examined observables associated with location and a particular spin component. Magnetic moment is related to spin, but they are different physical quantities. Interpreting the findings as a property separating from its particle remains disputed; explanations based on quantum interference and contextuality also exist. Spin has not become an independent object that abandons its owner.