"It's the most curious thing I ever saw in my life!" Alice thought to herself when she saw a Cheshire cat disappear and leave only its grin behind. It is not only in Wonderland, however, that properties of objects can exist independently of the objects themselves. That is the conclusion of a group of physicists from Israel and the UK, which has shown how the strange laws of quantum mechanics permit a photon to be in one place and its circular polarization in another.
This counterintuitive result was achieved thanks to the quantum-mechanical concept of post-selection. In classical physics, the initial conditions of a set of particles and the rules governing the behaviour of those particles are in principle enough to determine the properties of the particles at any arbitrary point in the future. That is not the case in quantum mechanics, in which a particle's evolution is inherently probabilistic. So while the results of a measurement carried out on a set of particles will have a known probability distribution, individual results cannot be predicted.
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