Quantum Entanglement:
The bell-state quantum eraser

This is an extract from my third quantum mechanics assignment for this semester, it talks about an experiment called "the bell-state quantum eraser" which demonstrates the phenomena of quantum entanglement, among other things. Quantum entanglement is fricken amazing, the entire reason I study physics is to learn about things like this and to try and understand the implications that have on reality. Reality is a tricky thing.
I'm not sure how much of it anyone will understand, but hopefully you'll get the general idea.


"...the concept of quantum entanglement. This refers to a phenomena whereby two or more quantum systems cannot be described without reference to each other, even though they may be spatially separated. This allows information about one system to be determined by making measurements on the others. A spectacular example of this is an experiment called the ‘bell-state quantum eraser’. In this experiment a beam of light is passed through a special type of crystal (BBO) that can split a photon into a pair of lower energy photons with orthogonal linear polarisations, say x and y. These photons are entangled because by measuring the polarisation of one photon we know with certainty what the polarisation of the other is. A diagram of the experimental setup is included left. The photons are emitted slowly so that we can be sure there are only the two entangled photons passing through the apparatus at any one time, and can thus identify the pairs as they reach the detectors. Consider first the setup without the polariser in front of detector 2 or the quarter wave plates in front of the slits. The entangled photons each travel along separate paths, we shall call them p1 and p2, towards the detectors D1 and D2. The p1 photons pass through the double slit and form an interference pattern as is expected. Next we place the quarter wave plates in front of each slit. These plates will alter the polarisation of the light so that photons going through one slit will be circularly polarised in one direction and those going through the other slit will have the opposite polarisation. Not only this, but if our initial photon was linearly polarised in the other direction (say y instead of x) then it would emerge from the quarter wave-plates with the opposite circular polarisations to the first situation. So in order to determine which slit the photon has travelled through, we need to know the initial linear polarisation AND the final circular polarisation. We can determine the final circular polarisation of a photon at detector 1 and by measuring the polarisation of its entangled partner at detector 2 we can determine its initial linear polarisation. Both pieces of information are required to determine which slit the photon has travelled through.
Next we add the polariser in front of detector 2, oriented to admit light that is a combination of the two possible polarisations. It is now no longer possible to determine the initial polarisation of the p2 photons, and so determine the initial polarisation of the p1 photons. We now no longer have the two pieces of information required to determine which slit a path 1 photon has travelled through, and so the interference pattern re-emerges. We have done this without doing anything at all to the path 2 photons, and so demonstrated the entangled nature of the photon pairs.
To make it even stranger, we can move detector 2 backwards so that we detect photons at detector 1 first, and we remove the polariser. Now the photons hit the screen and the interference pattern has been destroyed, before their pair photon has even had a chance to pass through a polariser or not and hit detector 2. Somehow the photons ‘know’ that the polariser isn't there before we have even had a chance to find out which slit they had passed though. The mere possibility that we could know seems to destroy the pattern.
Even more strangely we could have the polariser in place while the p1 photons are hitting the screen and then take it away after they've formed their pattern, but before their entangled partners reach the polariser and detector 2, and we would find that there was no interference pattern. This is where it gets too strange for me to consider further, but the point is that spatial and temporal separations of the detectors theoretically have no impact on the result of the experiment, which is the truly amazing aspect of quantum entanglement."

References:
http://encyclopedia.thefreedictionary.com/measurement+problem
http://encyclopedia.thefreedictionary.com/quantum%20decoherence
http://www.qubit.org/library/intros/entang/index.html
http://www.joot.com/dave/writings/articles/entanglement/spookiness.shtml
http://en.wikipedia.org/wiki/Quantum_entanglement
http://grad.physics.sunysb.edu/~amarch/ (diagram)
http://en.wikipedia.org/wiki/Polarisation