Alone as a croud: the latest example of the weirdness of quantum mechanics:
Quantum mechanics is a realm of weirdness: electrons being linked to each other even though the vastness of the universe might separate them, things being in two places at once, and, of course, knowledge precluding knowledge. This last is the standard bearer of quantum oddity: measuring the momentum of an object precludes precise knowledge of where that object is. But I think I have found something that is stranger than them all.
Researchers have suggested that it might be possible to make measurements that trick a photon into thinking it is, in fact, a crowd of photons.
Let’s imagine that we want to introduce a phase shift to one single photon through a control photon. A phase shift is basically a time delay. In traditional optics this delay is applied through high-intensity light beams: a high intensity pulse can modify the refractive index of the medium through which it propagates. Our signal photon traveling through that medium will see that different refractive index and either be delayed or sped up.
The problem is that we want to do this all with single photons, and just one photon does not fit the definition of high intensity.
It seems a bit hopeless, right? However, in quantum mechanics, things are not all that they seem. One type of measurement in particular—called a weak measurement—can give very strange results. For instance, if you measure the spin of an electron using a weak measurement, you can be reasonably certain that you haven’t disturbed the spin state of the electron, but, you might get a strange value. Electrons only take on spin values of +1/2 or -1/2, but a weak measurement could return something like 100. So, under the right circumstances, that single electron can behave as if it had the spin effect of 200 electrons.