Well, something I don’t understand just got a bit harder to understand. In this weeks Science and Technology section of the Economist, there is an article about work done by Drs Jonathan Friedman and James Lukens at SUNY Stony Brook. What they seem to have done, among other things, is to have worked out a way to observe a quantum mechanical state without disturbing that state. In the words of the article : ”What is remarkable about Dr Friedman’s and Dr Lukens’s experiment is that they have managed to measure the energy of superposed states without destorying them. This is like looking at the cat in a box with cloudy glass walls. You can observe the state of the dead-and-alive cat, but you cannot stroke it”
Their work relates mostly to quantum computing, but it seems that further applications of their techniques could have dramatic implications to our understanding (well, other people’s understanding) of QM. If different spins are associated with different energy levels (not directly implied by the article, btw) what would the energies of two electrons in an EPR state be? Would they differ? Would they differ after one is conventionally measured? If so, could one separate the particles, carry out a conventional measurement on one and detect the change in the other particle unconventionally? If there are non-local effects, this would not be a breech of law in and of itself, but only when that change was a signal for the Martians to attack. Well, of course not, the law is not that easy to break. But what other implications would there be if indeed we can measure the state of a superposed particle without disturbing that state.
Rhythmdvl
And what was it that they measured? They couldn’t have measured a superposed state, as that defies the very definition of superposition. What information does the measurement contain about the particle?
What they actually did (or at least, what was done in a similar experiment a while back; don’t know if it’s the same one), is measure a state in such a way that there’s a nonzero probability of leaving it unchanged. By varying the apparatus, it’s apparently possible to make that probability arbitrarily close to, but never equal to, one.
If I recall correctly, the method involved sending a polarized photon through a beam splitter, and putting a polaroid filter on one path at an angle, so there was only a certain probability of the photon getting through. This path shined on the atom to be measured, and the two photon paths were then recombined to form an interference pattern. The photon can travel along either path, so true to the nature of QM, it travels both, so you can make the measurement. However, since there’s only a small probability for the path with the atom, it probably doesn’t disturb it.
There was an article about it in Scientific American a couple of years ago; I’ll have to see if I can dig it up.
The Scientific American article is the cover story in the April 2000 issue. Scientific American has a website (www.sciam.com) but unfortunately the “Quantum Teleportation” article is not featured in the April 2000 online issue.
The authors point out that although the state of the photon can probably be transmitted FTL, the receiver has no way to be certain the state was correctly transmitted or not. The sender knows, however, and can verify the transmission by sending the receiver a message. But the verification has to be sent at, you guessed it, 186,000 miles per second.
I remember that article; it was several years ago. They used the example of forming an image of a ‘Medusa’s Head’, which could not be allowed to have even one photon strike it. (a more common example is a bomb that will detonate if struck by one photon)
I think you’re thinking of something else, pluto… As I understand it, the OP is asking about the “Medusa’s head” scenario that Willie described-- You don’t care how quickly you get the information; the important part is that you get it witout your subject noticing.
Anyone have access to SciAm indices or archives, and can tell us when the article was?