From: spOOk 19/02/2001 21:27:33 Subject: Quantum entanglement post id: 234112 This is from a book called "Hacker Attack" - Richard Mansfield. Published by Sybex. Now, Sybex seem to produce good copy but I have some issues with one particular chapter on quantum computing. Photons produce an electric field that vibrates. The actual direction of the vibration, called the photon's polarity, can be detected. The polarity can be in various degrees- 0,45,90 or 135. You can use the polarity property to transmit messages. You can send a pair of "entangled" photons scurrying off in opposite directions (at high speeds- they are photons after all). When entangled, two particles behave as if they share the same identity. Experiements have shown that if two entabgled particles are separated, if you change the polarity of one of them, its entangled mate changes its polarity instantaneously (faster than the speed of light! So, how are they communicating?). They appear to speak to each other in some spooky way, across space, faster than light. This opens up some really appealing improvements over the ways that the Internet sends information today, and the ways that computers compute. But speed isn't the only attraction of quantum computing. OKay - that's the end of the quote.. is it just me, or do you get the feeling that Richard is somewhat out of his depth? If the above explanation is accurate, then I think I've been missing something. Any comments? From: spOOk 20/02/2001 0:08:39 Subject: re: Quantum entanglement post id: 234255 This is more level headed.... http://www.sciam.com/1998/0698issue/0698gershenfeld.html From: James R (Avatar) 20/02/2001 11:12:07 Subject: re: Quantum entanglement post id: 234364 I think you're right, spOOk. Richard seems a little out of his depth. However, most of what he has said is actually correct. Photons produce an electric field that vibrates. The actual direction of the vibration, called the photon's polarity, can be detected. The polarity can be in various degrees- 0,45,90 or 135. It's not quite right to say photons produce and electric field. Photons are a vibrating electric and magnetic field, if you look at them classically. Polarity is the wrong term; the correct term is polarisation. The polarisation of a photon can be in any direction perpendicular to the direction of travel of the photon. This includes the angles mentioned, but also includes angles like 73 degrees and 157 degrees. You can use the polarity property to transmit messages. You can send a pair of "entangled" photons scurrying off in opposite directions (at high speeds- they are photons after all). When entangled, two particles behave as if they share the same identity. The part about entanglement is right, but you cannot use entanglement alone to transmit messages. You can, however, use entanglement in combination with a "classical" communication route (such as a phone line) to teleport the polarisation of a photon from one place to another. Experiements have shown that if two entabgled particles are separated, if you change the polarity of one of them, its entangled mate changes its polarity instantaneously (faster than the speed of light! Not exactly. If you measure the polarisation of one photon, the entangled photon will thereafter be found to always have a polarisation correlated with that of the measured photon. After the initial measurement, the entanglement is broken, and the photons no longer behave as if they are linked. So, how are they communicating?). They appear to speak to each other in some spooky way, across space, faster than light. That's one view. Another is that entangled photons are really a single quantum system with two widely-separated parts. Change one part of the system and the rest of the system will also look different. JR From: tritium ® 20/02/2001 11:37:47 Subject: re: Quantum entanglement post id: 234380 Can I just make a point about the polarisation of light... A beam of light can be polarised in any angle, however the polarisation can be described by calculating the amount of light that goes through polarising filters at 0, 45, 90 and 135 degrees and how much right hand and left hand circular polarisation there is Stoke's Parameters P1 = ( I(0) - I(90) )/( I(0) + I(90) ) P2 = ( I(45) - I(135) )/( I(45) + I(135) ) P3 = ( IRHC - ILHC )/( IRHC + ILHC ) where I is the detected signal measuring how many photons get through From: spOOk 20/02/2001 11:56:51 Subject: re: Quantum entanglement post id: 234395 Thanks James. You have confirmed all of my suspicions. The only thing that I was not sure about that Richard is right about was the angles of polarisation. I am still puzzled at the values and the lack of symmetry. ~:) From: tritium ® 20/02/2001 11:59:47 Subject: re: Quantum entanglement post id: 234397 lack of symmetry? polarisation at 0 degrees is equivliant to polarisation at 180 degrees... the photon vibrates back and forth... so if we include the opposite angles it is symmetric 0 and 180, 45 and 315, 90 and 270, 135 and 225 degrees Do you know how circular polarisation works? From: tritium ® 20/02/2001 14:15:18 Subject: re: Quantum entanglement post id: 234487 I haven't explained how circular polarization works, just how polarization in general works and how it is measured Circular polarisation works by constructing an electromagnetic field whose oscillations in the two directions, at right angles to each other and the direction of the wave, are out of phase with each other. This causes the apearance of the direction of polarisation slowly spinning about the direction in which the wave is going. Hence right hand and left hand circular polarisation depending on which direction it is spinning. From: Dave 20/02/2001 14:18:56 Subject: re: Quantum entanglement post id: 234488 This is a bit of a hijack and apologies if it should be in a new thread. Could polarisation be used to minimise glare from oncoming headlights? polarise light out and your windshield but not at full 90 degrees to each other. Remebering we want to reduce glare not turn the oncoming car invisible. From: tritium ® 20/02/2001 14:27:43 Subject: re: Quantum entanglement post id: 234494 the reason why we get glare is because the window is dirty... the best solution is to clean the window... BUT if could polarise the light coming out of headlights (this would cut the brightness in half, so this would really have to be useful) and set the windscreen to pick up the same polarisation (this would also reduce the amount of light that gets through our windscreen, another bad point), we'd see the headlight's perfectly well, but half of any light that get's scattered off dirt and loses polarisation will be absorbed reducing glare. This however would be expensive, wouldn't help against sun glare and would make oncoming headlights seem twice as bright as they do now (everything else will be twice as dark and presumable they'll double the power of headlights to compensate for the fact that half of it get's absorbed) Your suggesting of polarising to absorb headlights light would have the opposite effect, even if on a slight angle. Headlights would be much harder to see, but the glare would still be the same brightness compared to the rest of the light that gets in through the windscreen (it will lose it's polarisation after scattering) does this help? This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.