Mattskii
26th January 2011, 05:18 AM
It is a new twist on the double slit experiment, where if you shine a beam of light on a card with 2 slits cut in it, the 2 rays of light which come out of the other side interfere with each other so when detected on a screen or digital device an interference pattern of light and dark stripes is observed. This is because waves add up or subtract from each other.
So some bright spark things, well what would happen if we fired a single photon at a double slit experiment? Would it go through one or other slot, like a particle, or would it go through both like a wave. turns out if you send enough single particles of light - photons - at a double slit experiment you still build up the interference patern, which kind of proved that light travels as a wave. But hold on, we are talking about a single photon, which as we can't possibly know which slot it passed through, quantum physics tells us it went through both, hence the interference pattern.
So some bright spark said OK, lets measure the photon as it goes through one slot or the other so that we do know which slot it traveled through. And you know what - the interference pattern stopped appearing on the detector, and just a clump of hits were seen. This proves that light is a particle. As soon as it was measured it stopped being in a quantum stated and just went through one slot or the other.
Yet another even brighter spark then said OK, what if we then errase the which path information we gained, and do this by splitting the photon into an entangled pair, and make the measurements after the fact? So they did. They created an entangled pair of photons from each photon which passed through either one or other of the two slots. They then measured the path of roughly 50% of them, and "errased" the path information for the other 50%. They arranged the experiment so that the 1 of the entangled photons, or twins, was directed to a detector, whilst it's twin was sent on a much longer path to a beam splitter which would allow roughly 50% of photons to have their path recorded, and 50% of them not to. And this which path information was not recorded until about 8ns after it's twin hit the detector. And guess what?
Those photons who's twins were going to have their path information recorded in the future (and as they were entangled it was therefore possble know their path too) acted as particles and did not show any interference pattern. Those particles whose twin in the future was going to have that path information errased DID show an interference pattern. So the quantum state was affected by observations which were going to be made in the future. Mind boggling.
Here is a link to the full artcle: Delayed Choice Quantum Erraser (http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm)
So some bright spark things, well what would happen if we fired a single photon at a double slit experiment? Would it go through one or other slot, like a particle, or would it go through both like a wave. turns out if you send enough single particles of light - photons - at a double slit experiment you still build up the interference patern, which kind of proved that light travels as a wave. But hold on, we are talking about a single photon, which as we can't possibly know which slot it passed through, quantum physics tells us it went through both, hence the interference pattern.
So some bright spark said OK, lets measure the photon as it goes through one slot or the other so that we do know which slot it traveled through. And you know what - the interference pattern stopped appearing on the detector, and just a clump of hits were seen. This proves that light is a particle. As soon as it was measured it stopped being in a quantum stated and just went through one slot or the other.
Yet another even brighter spark then said OK, what if we then errase the which path information we gained, and do this by splitting the photon into an entangled pair, and make the measurements after the fact? So they did. They created an entangled pair of photons from each photon which passed through either one or other of the two slots. They then measured the path of roughly 50% of them, and "errased" the path information for the other 50%. They arranged the experiment so that the 1 of the entangled photons, or twins, was directed to a detector, whilst it's twin was sent on a much longer path to a beam splitter which would allow roughly 50% of photons to have their path recorded, and 50% of them not to. And this which path information was not recorded until about 8ns after it's twin hit the detector. And guess what?
Those photons who's twins were going to have their path information recorded in the future (and as they were entangled it was therefore possble know their path too) acted as particles and did not show any interference pattern. Those particles whose twin in the future was going to have that path information errased DID show an interference pattern. So the quantum state was affected by observations which were going to be made in the future. Mind boggling.
Here is a link to the full artcle: Delayed Choice Quantum Erraser (http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm)