In some interpretations of quantum mechanics, quantum effects may be transmitted at speeds greater than c (indeed, action at a distance has long been perceived as a problem with quantum mechanics: see EPR paradox). For example, the quantum states of two particles can be entangled, so the state of one particle fixes the state of the other particle (say, one must have spin +½ and the other must have spin −½). Until the particles are observed, they exist in a superposition of two quantum states, (+½, −½) and (−½, +½). If the particles are separated and one of them is observed to determine its quantum state then the quantum state of the second particle is determined automatically. If, as in some interpretations of quantum mechanics, one presumes that the information about the quantum state is local to one particle, then one must conclude that second particle takes up its quantum state instantaneously, as soon as the first observation is carried out.
Even though it is impossible to make perfect copies of an unknown quantum state, it is possible to produce imperfect copies. This can be done by coupling a larger auxiliary system to the system that is to be cloned, and applying a unitary transformation to the combined system. If the unitary transformation is chosen correctly, several components of the combined system will evolve into approximate copies of the original system. Imperfect cloning can be used as an eavesdropping attack on quantum cryptography protocols, among other uses in quantum information science.
Classical error correction employs redundancy: The simplest way is to store the information multiple times, and—if these copies are later found to disagree—just take a majority vote; i.e. if, say, one copy says, the bit is a 0, and two others claim it to be a 1, then probably all three were a 1 and the first bit got corrupted.
However, this is not possible with quantum information, as it cannot be copied: see no-cloning theorem.
Therefore it was a relief when Peter Shor realized that, even if the information cannot be copied, the information of one qubit (now called a logical qubit) can be spread onto several (physical) qubits by using a quantum error correcting code. Now, if noise or decoherence corrupts one qubit, the information is not lost.
tada! now i just need to understand half the words i posted... Posted by signine (Member # 7671) on :
The first time I heard about the bizzare property of quantum spin being instantaneously replicated (literally instantaneously) I had to ask my physics professor (at the time I was a physics major) if this could be used to transfer data. His first answer is that which you'd expect from any physicist "I don't know." He told me I should ask him in a week. During that week I continued to research (this was pre qubit research) and discovered that measuring the spin of one electron would change the spin of the other electron, and the spin of the electron would change as soon as one measured it Heisenberg Uncertainty kind of sucks in that way.
Assume two electrons have spin X and spin Y. As soon as you measure the spin of electron X you change both it, and the spin of electron Y. If the spin you measure is actually a spin you INDUCE by measuring the spin, then transferring data via the link is impossible as per the following example:
Measuring X changes the spin of X to -3/4 Y, as a result of the change of X, becomes +3/4 You measure Y to see if there has been a change in spin, but by measuring Y you change its spin, thus making it impossible to determine whether or not you had a bit.
On the other hand, say it DOES change the spin to measure it, but the measurement you get is the original spin. Well this makes it nearly impossible too without some tricky computer work. Every time you measure the spin it changes, so your method of sending data is inconsistent as you cannot be sure you will receive the same result twice. You need to know your initial values before you can determine whether or not they have changed, and to determine whether or not they've changed, *sigh* you have to change them.
Heisenberg Uncertainty REALLY sucks.
On the other hand, if it's actually possible to measure the spin without changing the spin, then all bets are off and we can transfer data freely.
What I'm more concerned about is why we can push a photon faster than light but we can't use said photon to transfer binary data. I mean, if we can detect an emitted photon BEFORE it's actually emitted, why can't we send data using it.
Meh.
Posted by TomDavidson (Member # 124) on :
"On the other hand, if it's actually possible to measure the spin without changing the spin, then all bets are off and we can transfer data freely."
Sadly, it is the nature of the universe that this is impossible. Posted by Noemon (Member # 1115) on :
And here I thought we had another fan of Dave Langford's SF newsletter.
Posted by kaioshin00 (Member # 3740) on :
I dont see any ansible ;/
Posted by wiggin105 (Member # 7897) on :
"Therefore it was a relief when Peter Shor realized that, even if the information cannot be copied, the information of one qubit (now called a logical qubit) can be spread onto several (physical) qubits by using a quantum error correcting code. Now, if noise or decoherence corrupts one qubit, the information is not lost."
if this can be done, we could send info in multiple packets and use error correction (simple averaging of the code) to get information transfered. although it wouldn't be a perfect copy, neither is the stuff i download from the internet (all legal). information is slightly corrupted by the time it reaches my computer. even so, my computer can still open the slightly corrupted pdf files i download from Penn State just fine.
it would take some nifty computer work to interpret the corrupted files sent along quantum connections, but Peter Shor believes that it is possible (or so i read).
Posted by signine (Member # 7671) on :
He believe's it's possible to store data on the qubits, but then you're using them as a storage device, not a communications method. When you don't have any way of knowing what data is coming across the stream you lose a lot of error-checking capability. Data transfer over Ethernet at Layer 1 of the OSI model uses voltage changes to store the data. The funny part is that 100Mb ethernet actually operates at 125Mhz. A 1 is indicated by a change in voltage, and a zero is indicated by voltage not changing during the clock stroke.
We keep track of the timing by using the last bit out of each set of five bits as a 1 if all other bits are zero, and as a 0 if the other bits contain one. This is how we keep the clock synced when we're transmitting zeros. Neat huh?
We still, however, have the problem of Heisenberg Uncertainty. Then again, Heisenberg Uncertainty only really says we cannot know both the velocity and position of a particle without changing either it's velocity or position. The interesting thing about both of these qubit and electron spin discourses and something I still haven't found an answer on is this:
1. To know whether or not the spin has changed, you must know it's initial value, and thus, measure it. 2. Thus, it is possible to measure change in spin without knowing whether or not something else has altered the change.
In fact, all the theoretical papers I've read indicate that the particles have no spin property until they are measured, but that just reeks of Schrodinger relativism. Sometimes I wish I'd kept being a physicist.
Posted by joebb3 (Member # 7164) on :
Now I'm no rocket scientist but...
It seems to me that the problem could be solved with a Reference Particle. A particle that you can replicate over and over again with the exact same properties (or spin). Detecting the changes in THAT particle should be able to tell you the properties of the particle you measured with it.
Yes? Or should I go back to greeting people as they enter Wal-Mart?
Posted by DaiTenshi (Member # 7759) on :
I realize that I'm perhaps years our of my league, but what if it were possible to undue the changes of the electron spin? Like, maybe measuring it again, only negating the first effects, so that the end result is the original measurement?
Or maybe pigs can suddenly evolve wings and dominate the stratus . . .
Posted by TomDavidson (Member # 124) on :
"It seems to me that the problem could be solved with a Reference Particle. A particle that you can replicate over and over again with the exact same properties (or spin)."
Can't exist. Or, rather, if you could create such a particle, FTL communication would be the least of your practical applications.
"I realize that I'm perhaps years our of my league, but what if it were possible to undue the changes of the electron spin? Like, maybe measuring it again, only negating the first effects, so that the end result is the original measurement?"
Nope. That's not how it works. It's actually because of misconceptions like this that I'm not entirely satisfied with the use of "spin" to describe situations like these.
Let's use another metaphor. When you view a particle to determine its location and state, you collapse a possibility waveform that includes all its potential locations and states. When two particles are part of a single "spin pair," what happens when you determine the spin of one particle is that you suddenly know the spin of the other particle; in one sense, the other particle has been spinning that way all the time.
But we also know -- and this is the funky bit -- that neither particle was spinning in any given way until you looked. We've actually confirmed this in experiments; the particles really do exist in multiple states simultaneously until they are observed, at which point all the other possibilities go away and they're locked into a single state. So the second particle, no matter how far away it is from the first particle, will "begin" spinning the appropriate way instantly, considerably faster than light.
This is really cool, since it suggests some form of communication between the two particles below the basic fabric of the universe -- a kind of layer three quantum switching. *grin* But it's also useless, because all it tells you is what direction that particle's going, and this by itself doesn't make communication possible.
Keep in mind that the state the particle assumes when you look at it is NOT something that we can reliably control. If it were, quantum mechanics would be an applied science.
It would be possible, once you lock the particles down, to allow them to return to a state of uncertainty -- to get a "second chance," as it were. But unfortunately, this would in the first place break the spin pair, and in the second place still result in an uncontrollable observation.
It would be theoretically possible to store data in this way for future retrieval. It would not be possible, sadly, to transmit data using this mechanism. And since the only part of this mechanism which is not subject to relativity is exactly the part which we cannot use for communications purposes, we're still stuck without a form of FTL transmission.
[ May 02, 2005, 01:14 PM: Message edited by: TomDavidson ]
Posted by Morbo (Member # 5309) on :
quote: So the second particle, no matter how far away it is from the first particle, will "begin" spinning the appropriate way instantly, considerably faster than light.
This is really cool, since it suggests some form of communication between the two particles below the basic fabric of the universe -- a kind of layer three quantum switching. *grin* But it's also useless, because all it tells you is what direction that particle's going, and this by itself doesn't make communication possible.
It's a useless oddity of the theory, now, but future generations could hack into this FTL quantum communication system, similar to the ansible system. At a minimum manipulating such a system could produce FTL communications.
The maximum potential is much, much greater--almost unlimited, really. It would be the Ultimate Hack. A sophisticated manipulation of quantum events could produce or destroy or transport or transmute both matter and energy.
It's certainly made for some great sci-fi, especially Greg Bear's Moving Mars and Anvil of Stars (Forge of God sequel).
Even if such a system is possible, it probably won't happen for centuries. I'll settle for a good film based on Moving Mars.
Posted by Dread Pendragon (Member # 7239) on :
Um . . . didn't you guys know? There are functioning ansibles right now. On Nextel you can choose between the 2-way radio thing and the ansible function. I think you can get it on the newer blackberries too I think.
Posted by Morbo (Member # 5309) on :
That's even worse than no FTL, Pendragon: FTL that's out of my budget. With my luck, all the cool people will be surfing the Galactic Net and creating vacation planets around distant nebulae while I muddle along with my cruddy old comp and basic cable. Posted by Dread Pendragon (Member # 7239) on :
Morbo, don't tell anyone I'm telling you this, but I know a way to use FTL-based technology just by using your basic cable, some tin-foil, some of that wire stuff Bean used in the battle room, and . . . oh crap, somebody else is reading this . . . I'll tell you how later!
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