|up, down> + |down, up> is not an eigenstate of the spin flip operator. Correct.
However, that doesn't mean you can't apply the spin-flip operator to it. I don't know how to explain it in more detail, other than to try and give an experimental realization, which I don't think is what you're looking for.
Perhaps this will make more sense... let's work in "vector notation"...
My notation was |spin of first electrion, spin of second electron>.
Initially, I was in a superposition of |up, down> and |down, up>.
Then, I performed an operation that flips the spin of only the first electron, leaving me in state |down, down> + |up, up>
I have not flipped two spins. And what does angular momentum conservation have to do with it? In this case, the expectation value of total angular momentum is conserved, but there's no need for that to be true in general of a unitary operation.
Teleportation only works in this scheme 25% of the time. It would not be sufficient to say "I'm done on this end", you would need to specify which teleportation attempts were successful. Thus, you need one classical bit for each teleported quantum bit.
This is a quantum teleportation experiement. Reading from the link: "Also, quantum teleportation does not allow for faster-than-light communication. Although the teleported particle attains the polarization value instantly, the people at the sending station must convey the fact that teleportation was successful by making a phone call or using some other light speed or sub-light-speed means of communication."
This statement, from the link you quote, contradicts your implication that quantum entanglement can be used to transmit classical information faster than the speed of light. Perhaps you did not mean to imply that in your post. At the time, given the other posts to this story in that vein, that's how I read your comment.
Aside from that, I don't see how this teleporation scheme can be used, in your words, to "shine a laser beam at a chemical vapor a mile away, and by reading the polarization states of trapped entagled photons, remotely measure the vapor's composition". Which photon is the one that passes through the vapor, photon M? If so, then a detector is needed at the site of the vapor, along with a beamsplitter and a second detector as described in the link, along with communication equipment to "phone home" when teleportations are successful.
Closer to "not yet set". But even that is misleading.
We can take electrons and put them in a known "indeterminate" state, such as |up> + |down>. While you might say the spin is not "set", in fact the QUANTUM state of the spin is precisely set.
And if we know what the quantum state of the particle is, we can put it into a determinate state even without measuring it.
Thought experiment.
In principle, according to quantum mechanics, one can construct a device that will perform the following transformation on an electron spin:
|up> -> |up> + |down>
|down> -> |up> - |down>
This can be done WITHOUT measuring the electron's spin, because what I have described, in math jargon, is a "unitary transformation". This basically means "reversible" or "information preserving".
Now, imagine that I have a bunch of electrons in the state |up> + |down>, and I put them through my device. What state do they end up in? We use the superposition principle to find out...
(as usual, I am ignoring all of the normilization constants)
Long story short, the indeterminate state of my input electroncs ends up in a determinite |up> state. But the device does not perform a measurment to do this.
The indeterminate state is transformed into a determinate state without measuring (ie "setting") the spin. Thus, it is a little problematic to think that indeterminate states are just states that haven't been "set" yet, because they can be transformed to determinate states without setting (measuring) them.
Further weirdness:
If I measure the spins of the particles only after they come out of my device, then I get all ups.
If I measure the spins before the device AND after the device, I get a mixture of ups and downs on both sides of the device. (why? because measuring before the device forces the electron into state |up> or state |down>, and both of these states are then transformed by my device back into indeterminate states).
Perhaps you don't realize that you've made an extremely controversial statement. What you claim this army physicist did is contrary to accepted quantum mechanics since circa 1930. Do you have any evidence to back up this claim?
According to quantum mechanics, the interaction of the "remote photon" can not produce a measurable change on the "local photon" in the way you have described.
What you said it mostly correct. However, it is theoretically possible (and has been done in practice as well, in fact) to flip a spin without performing a measurment. Flipping a spin is not always a measurment.
If you start in the state (normalization constants ignored):
2 * |up, down> + |down, up>
where the first electron is more likely to be measured in the up state, and you apply a "coherent spin flip" (ie a flip performed WITHOUT measuring the spin) to the first electon, you end up in the state
2 * |down, down> + |up, up>
This is still an entangled state, and the spin of each electron is still indeterminate. Contrary to the desires of the parent to your comment, however, you still have not managed to flip the spin of the second electron.
What do you mean by "occurs in a correlated manner until the end state is reached."?
You could argue that flipping the spin of one does "affect" the other. It changes the quantum state of the pair, and the "other" is a member of the pair. But it does NOT change the state in a way that:
a) Can be determined with measurments performed only on "the other". or b) Can be used to transmit information.
Proviso: When I said that modifying the properties of one member of the pair ruins the entanglement, that was not completely correct. If you managed to come up with a scheme to flip the spin of one without measuring the spin, then entanglement would be maintained. However, this would still not flip the spin of the other electron -- the entanglement would not have a different character.
Example: You start with the electrons having opposite (but indeterminate) spins, in the entangled state
|down, up> + |up, down>
(normalization constant ignored)
Now you flip the spin of the first electron. This puts you in the entangled state
|up, up> + |down, down>
Entanglement is preserved, however, you have not "flipped the spin" of the second electron. You have changed the sense of the correleation though. But you still haven't transmitted any information. The spin of each individual electron was indeterminate before you meddled, and was after you meddled.
When I said the measuring the relevant property of one of the pairs ruins the entanglement, well, that was still correct. And try as you might, there is no way to transmit classical information without performing a measurment.
By "Pair off two electrons", I presume you mean put them in an entangled state where the spins of the two electrons are correlated? (For example, in the state |up, down> + |down, up>).
In that case, your system won't work. Putting one of the electroncs in this spin-flipping device would destroy the fragile entanglement. In other words, flipping the spin of one would do nothing to the other.
This is how it always is with entanglement -- entangled particles only remain entangled as long as you leave their entangled properties alone. Once you measure or modify the properties of one, the entanglement is ruined.
No, the original poster is correct. Open up an optics textbook. There are solutions to the wave equation that do no fall off as the square of distance. In fact, Gaussian Beams, a special solution to the wave equation, do not fall of at all. They are, of course, highly directional.
Regarding the point about molecules on the order of a wavelength resonating:
The radio waves refered to here have wavelengnths on the order of millimeter to meters. In fact, with AM, the wavelength is around.5 meters (300,000,000 meters per second divided by 500,000,000 cycles per second). There aren't any molecules this long which could resonate as described.
That's a major problem, which I think will take several years to solve (not with nanobots, but with better hijacking of biological systems). But it is dwarfed by the problem of identifying the right cocktail of proteins necessary to get the DNA to work in a living cell.
Venter is known in the public genomics community as an ambitions, ruthless, and unprincipled egotist. Example: At the end of Celera's (Venter's company) much publicised private human genome project it was revealed that the genome they had sequenced came in secret from none other than - you guessed it - Craig Venter. If that isn't vanity...
Good question. The only way I can see this system being secure against emulating the client is if the chips have an onboard private key, and the public key is made available in a public database linked against some sort of chip serial number. There goes anonymity.
By the way, I was one of the UBC students (Aviv). The one in the orange / yellow shirt. The guy with the camera is a friend of mine, so I'll bug him to put the picture somewhere soon. Who were you? I don't remember anybody's name unfortunately, so I'll need a visual cue.
I was another atendee. Just posting this so I can remember your username easily.
Arbiters... Something doesn't make sense.
on
Clockless Computing
·
· Score: 2
The story spends a lot of words discussing Arbiters and the Buridan's Ass "paradox". A quote from the story: "Although Arbiter circuits never grant more than one request at a time, there is no way to build an Arbiter that will always reach a decision within a fixed time limit."
I don't understand this at all - is it just Scientific American oversimplification? Why can't an Arbiter simply decide that if two pieces of data need to pass through the same component, it will let the left one through first this time, and next time there is a conflict it will let the right one through first (in order to avoid systematic "discrimination" against one part of the chip). This decision making process will always take the same amount of time.
Can anybody explain to me what I have misunderstood - I'm sure there must be something I'm not getting, otherwise Sun wouldn't be researching this one piece so deeply.
It is not faster than light. Observable information never moves faster than light, even in Quantum Mechanics. It's true that entangled particles can "communicate" in a sense faster than light with eachother, but classical (measurable) information will never move faster than light.
In this experiment, a radio signal was sent between the teleportation sender and the teleportation receiver. At each end, there were some particles entangled with the particles at the other end, however, the classical information - that is the laser beam - could not be reconstructed without sending a radio signal (at the speed of light) from one end to the other.
The reason his writing has authority, and the reason he is famous, is precisely because so many people have done the evaluation you just described and have come up with a positive result. I haven't done so myself, granted.
I don't agree that he is a false authority either. Usually, Noam Chomsky is a linguistics professor arguing about propaganda, which is right up the linguist's alley.
Anyway, what's wrong with "Argument by Authority" as a first-order approximation? (this is only/. after all, can't get TOO deep) I'm sure that, on average, renowned humanities professors are more informed about politics then the mean.
This is not just some whacko, this is one of the most respected Intellectuals in the US. He's not making it up, though he might be emphasizing certain aspects to make a point. His homepage at MIT (where he is a professor of linguistics) is here. Don't just dismiss him, his writing carry a lot more weight and respect then, for example, yours.
Now that I think about it, that talk does not particularly well adress this topic. A better work of Noam Chomsky, which gives much better background information, is the documentary about him entitled "Manufacturing Consent". Give it a try.
I suggest a little bit of reading. Noam Chomsky's writings are a good place to start, and here is an excellent piece by him. I agree that the US has done many good things with its military, but it has also done many bad things, which are mostly swept under the carpet.
Slashdot Mirror
|up, down> + |down, up> is not an eigenstate of the spin flip operator. Correct.
However, that doesn't mean you can't apply the spin-flip operator to it. I don't know how to explain it in more detail, other than to try and give an experimental realization, which I don't think is what you're looking for.
Perhaps this will make more sense... let's work in "vector notation"...
define |up, up> = (1, 0, 0, 0)
|up, down> = (0, 1, 0, 0)
|down, up> = (0, 0, 1, 0)
|down, down> = (0, 0, 0, 1)
The spin flip operator for the first electron can now be written in matrix form as
0 0 1 0
0 0 0 1
1 0 0 0
0 1 0 0
Since this matrix is unitary (see http://mathworld.wolfram.com/UnitaryMatrix.html), then this is a valid time-evolution operator in quantum mechanics.
I guess I really haven't said anything new here, so I doubt that actually clarifies anything. But I'm not sure what it is you don't understand.
My notation was |spin of first electrion, spin of second electron>.
Initially, I was in a superposition of |up, down> and |down, up>.
Then, I performed an operation that flips the spin of only the first electron, leaving me in state |down, down> + |up, up>
I have not flipped two spins. And what does angular momentum conservation have to do with it? In this case, the expectation value of total angular momentum is conserved, but there's no need for that to be true in general of a unitary operation.
Teleportation only works in this scheme 25% of the time. It would not be sufficient to say "I'm done on this end", you would need to specify which teleportation attempts were successful. Thus, you need one classical bit for each teleported quantum bit.
This is a quantum teleportation experiement. Reading from the link:
"Also, quantum teleportation does not allow for faster-than-light communication. Although the teleported particle attains the polarization value instantly, the people at the sending station must convey the fact that teleportation was successful by making a phone call or using some other light speed or sub-light-speed means of communication."
This statement, from the link you quote, contradicts your implication that quantum entanglement can be used to transmit classical information faster than the speed of light. Perhaps you did not mean to imply that in your post. At the time, given the other posts to this story in that vein, that's how I read your comment.
Aside from that, I don't see how this teleporation scheme can be used, in your words, to "shine a laser beam at a chemical vapor a mile away, and by reading the polarization states of trapped entagled photons, remotely measure the vapor's composition". Which photon is the one that passes through the vapor, photon M? If so, then a detector is needed at the site of the vapor, along with a beamsplitter and a second detector as described in the link, along with communication equipment to "phone home" when teleportations are successful.
Closer to "not yet set". But even that is misleading.
We can take electrons and put them in a known "indeterminate" state, such as |up> + |down>. While you might say the spin is not "set", in fact the QUANTUM state of the spin is precisely set.
And if we know what the quantum state of the particle is, we can put it into a determinate state even without measuring it.
Thought experiment.
In principle, according to quantum mechanics, one can construct a device that will perform the following transformation on an electron spin:
|up> -> |up> + |down>
|down> -> |up> - |down>
This can be done WITHOUT measuring the electron's spin, because what I have described, in math jargon, is a "unitary transformation". This basically means "reversible" or "information preserving".
Now, imagine that I have a bunch of electrons in the state |up> + |down>, and I put them through my device. What state do they end up in? We use the superposition principle to find out...
|up> + |down> -> ( |up> + |down> ) + ( |up> - |down> ) = |up>
(as usual, I am ignoring all of the normilization constants)
Long story short, the indeterminate state of my input electroncs ends up in a determinite |up> state. But the device does not perform a measurment to do this.
The indeterminate state is transformed into a determinate state without measuring (ie "setting") the spin. Thus, it is a little problematic to think that indeterminate states are just states that haven't been "set" yet, because they can be transformed to determinate states without setting (measuring) them.
Further weirdness:
If I measure the spins of the particles only after they come out of my device, then I get all ups.
If I measure the spins before the device AND after the device, I get a mixture of ups and downs on both sides of the device. (why? because measuring before the device forces the electron into state |up> or state |down>, and both of these states are then transformed by my device back into indeterminate states).
Perhaps you don't realize that you've made an extremely controversial statement. What you claim this army physicist did is contrary to accepted quantum mechanics since circa 1930. Do you have any evidence to back up this claim?
According to quantum mechanics, the interaction of the "remote photon" can not produce a measurable change on the "local photon" in the way you have described.
Yes.
But flipping the spin of the first particle still does not flip the spin of the other particle.
To "set the spin" (ie put it in a known state) is equivalent to performing a measurment, and spoils the entanglement.
What you said it mostly correct. However, it is theoretically possible (and has been done in practice as well, in fact) to flip a spin without performing a measurment. Flipping a spin is not always a measurment.
If you start in the state (normalization constants ignored):
2 * |up, down> + |down, up>
where the first electron is more likely to be measured in the up state, and you apply a "coherent spin flip" (ie a flip performed WITHOUT measuring the spin) to the first electon, you end up in the state
2 * |down, down> + |up, up>
This is still an entangled state, and the spin of each electron is still indeterminate. Contrary to the desires of the parent to your comment, however, you still have not managed to flip the spin of the second electron.
What do you mean by "occurs in a correlated manner until the end state is reached."?
You could argue that flipping the spin of one does "affect" the other. It changes the quantum state of the pair, and the "other" is a member of the pair. But it does NOT change the state in a way that:
a) Can be determined with measurments performed only on "the other". or
b) Can be used to transmit information.
Proviso: When I said that modifying the properties of one member of the pair ruins the entanglement, that was not completely correct. If you managed to come up with a scheme to flip the spin of one without measuring the spin, then entanglement would be maintained. However, this would still not flip the spin of the other electron -- the entanglement would not have a different character.
Example: You start with the electrons having opposite (but indeterminate) spins, in the entangled state
|down, up> + |up, down>
(normalization constant ignored)
Now you flip the spin of the first electron. This puts you in the entangled state
|up, up> + |down, down>
Entanglement is preserved, however, you have not "flipped the spin" of the second electron. You have changed the sense of the correleation though. But you still haven't transmitted any information. The spin of each individual electron was indeterminate before you meddled, and was after you meddled.
When I said the measuring the relevant property of one of the pairs ruins the entanglement, well, that was still correct. And try as you might, there is no way to transmit classical information without performing a measurment.
By "Pair off two electrons", I presume you mean put them in an entangled state where the spins of the two electrons are correlated? (For example, in the state |up, down> + |down, up>).
In that case, your system won't work. Putting one of the electroncs in this spin-flipping device would destroy the fragile entanglement. In other words, flipping the spin of one would do nothing to the other.
This is how it always is with entanglement -- entangled particles only remain entangled as long as you leave their entangled properties alone. Once you measure or modify the properties of one, the entanglement is ruined.
No, the original poster is correct. Open up an optics textbook. There are solutions to the wave equation that do no fall off as the square of distance. In fact, Gaussian Beams, a special solution to the wave equation, do not fall of at all. They are, of course, highly directional.
Regarding the point about molecules on the order of a wavelength resonating:
.5 meters (300,000,000 meters per second divided by 500,000,000 cycles per second). There aren't any molecules this long which could resonate as described.
The radio waves refered to here have wavelengnths on the order of millimeter to meters. In fact, with AM, the wavelength is around
That's a major problem, which I think will take several years to solve (not with nanobots, but with better hijacking of biological systems). But it is dwarfed by the problem of identifying the right cocktail of proteins necessary to get the DNA to work in a living cell.
Venter is known in the public genomics community as an ambitions, ruthless, and unprincipled egotist. Example: At the end of Celera's (Venter's company) much publicised private human genome project it was revealed that the genome they had sequenced came in secret from none other than - you guessed it - Craig Venter. If that isn't vanity...
Good question. The only way I can see this system being secure against emulating the client is if the chips have an onboard private key, and the public key is made available in a public database linked against some sort of chip serial number. There goes anonymity.
By the way, I was one of the UBC students (Aviv). The one in the orange / yellow shirt. The guy with the camera is a friend of mine, so I'll bug him to put the picture somewhere soon. Who were you? I don't remember anybody's name unfortunately, so I'll need a visual cue.
I was another atendee. Just posting this so I can remember your username easily.
I don't understand this at all - is it just Scientific American oversimplification? Why can't an Arbiter simply decide that if two pieces of data need to pass through the same component, it will let the left one through first this time, and next time there is a conflict it will let the right one through first (in order to avoid systematic "discrimination" against one part of the chip). This decision making process will always take the same amount of time.
Can anybody explain to me what I have misunderstood - I'm sure there must be something I'm not getting, otherwise Sun wouldn't be researching this one piece so deeply.
The particles can "communicate" faster than light, but humans cannot extract information from this communication.
It is not faster than light. Observable information never moves faster than light, even in Quantum Mechanics. It's true that entangled particles can "communicate" in a sense faster than light with eachother, but classical (measurable) information will never move faster than light.
In this experiment, a radio signal was sent between the teleportation sender and the teleportation receiver. At each end, there were some particles entangled with the particles at the other end, however, the classical information - that is the laser beam - could not be reconstructed without sending a radio signal (at the speed of light) from one end to the other.
The reason his writing has authority, and the reason he is famous, is precisely because so many people have done the evaluation you just described and have come up with a positive result. I haven't done so myself, granted.
/. after all, can't get TOO deep) I'm sure that, on average, renowned humanities professors are more informed about politics then the mean.
I don't agree that he is a false authority either. Usually, Noam Chomsky is a linguistics professor arguing about propaganda, which is right up the linguist's alley.
Anyway, what's wrong with "Argument by Authority" as a first-order approximation? (this is only
This is not just some whacko, this is one of the most respected Intellectuals in the US. He's not making it up, though he might be emphasizing certain aspects to make a point. His homepage at MIT (where he is a professor of linguistics) is here. Don't just dismiss him, his writing carry a lot more weight and respect then, for example, yours.
Now that I think about it, that talk does not particularly well adress this topic. A better work of Noam Chomsky, which gives much better background information, is the documentary about him entitled "Manufacturing Consent". Give it a try.
I suggest a little bit of reading. Noam Chomsky's writings are a good place to start, and here is an excellent piece by him. I agree that the US has done many good things with its military, but it has also done many bad things, which are mostly swept under the carpet.