For the particular settings of these boxes, Alice and Bob’s observations, which perfectly agree with the prediction of QM, are the following: Shortly after, however, they realise there are strong correlations between the behaviour of the two boxes. There seems to be no rhyme or reason to which one or why. Whichever Alice presses on her box, either its red light or its green light flashes with apparently 50% probability. Immediately, and before looking at the other’s data, the first thing they agree is that there is no obvious pattern to which light flashes in response to which button is pressed.
Later, Alice and Bob meet up and compare notes. They each record which lights flash in response to each measurement. Their choice of which button to press each minute is independent of all previous choices, and completely up to them. The measurements have two possible outcomes, which show up as either a red light on the box, or a green light.Īlice and Bob go off to distant locations and every minute press either one of the buttons. These boxes each have two buttons (labelled 1 and 2), which when pressed perform a measurement on the next particle in line inside the box. We similarly package the other particles and give them to Bob. We take one particle from each pair, line them up in order and put them all in a box, which we give to an experimentalist called Alice. Imagine some quantum mechanical process (in my lab we use Spontaneous parametric down-conversion ) produces a bunch of particle pairs. No need to go into the maths because fortunately, the basic concept is quite intuitive. Now, to properly understand quantum entanglement, it’s crucial to talk about Bell's theorem. Anyone who confuses entanglement with classical correlation deserves to be glove-slapped.