How are atoms entangled, and can it be done remotely?

[piningforthefjords]

I'm a bit confused about quantum entanglement. First of all, how exactly do you entangle two atoms? I've heard about how they entangled atoms in quantum teleportation experiments, but I don't get how they did it. I've heard that the atoms must "interact", but what exactly does that mean? Do they have to touch? Do they have to be within a certain distance - like a foot or an inch - from each other?

Also, can atoms be entangled if they're really far away? Like if one was in New York and the other was in LA, could you entangle them? Or would you have to, for example, entangle them both in LA and then take one to New York?

[Swift]

Thread moved from Science & Technology to Q&A

[HighGain]

pining,

They don't necessarily have to be atoms, they can be say a pair of photons too. But whatever pair of atomic/subatomic particles one cares to study, there are well described circumstances which "ties" the particles to one another in a sense. "Spin" is a common property studied, but one might choose to study polarization of photons as well.

For starters, don't even worry about spin or polarization per se. I am not trying to dumb it down for you, you may know more physics than me, but for the sake of understanding entanglement as a phenomenon itself, do not fuss so much about the details of the feature you are measuring. What you want to keep in mind is that as particles "separate" from one another, leave the local space-time of one another, they move away in space and time carrying this feature, this spin, this polarization, this feature which LINKS them across space time in a non local way.

So our photons travel a million light years from one another pining and you decide to now look at the spin of one, or the polarization of one. When you do, your looking will determine the spin or the polarization of the other, even though it is now a million light years away. So in a very real sense, information has traveled all that way, a determination has been made, and it has done so faster than light can travel.

Particles are tied to one another in a "non local way". That is in a way where they influence one another, or you influence one of them by measuring something and that influence is passed to the other tied particle/entity no matter how close or how far away it is. The tied particles could be at the opposite ends of the universe, or right next to one another. A measurement on one will determine a feature of the other, INSTANTANEOUSLY at the time a measurement is made.

Pretty cool. There is a simple, non dumbed down book entitled Einstein's Moon/Bell's Theorem by David PEat. Give it a read. You can buy a used copy for next to nothing. Fabulous book for a general orientation to the issue.

[HighGain]

Pining

To the best of my knowledge, particles must be together, must be local, in your terms "touch" to become entangled. The imparting of the feature's orientation which entangles the particles is local. One cannot have 2 particles that have never "seen" one another, one in NYC and one in Bejing, with random spin, random polarization, random whatever, not previously tied to one another and some way bring about their entanglement. I have never heard of that. Could not imagine how that could come about. They must be tied locally.

[piningforthefjords]

pining,

They don't necessarily have to be atoms, they can be say a pair of photons too. But whatever pair of atomic/subatomic particles one cares to study, there are well described circumstances which "ties" the particles to one another in a sense. "Spin" is a common property studied, but one might choose to study polarization of photons as well.

For starters, don't even worry about spin or polarization per se. I am not trying to dumb it down for you, you may know more physics than me, but for the sake of understanding entanglement as a phenomenon itself, do not fuss so much about the details of the feature you are measuring. What you want to keep in mind is that as particles "separate" from one another, leave the local space-time of one another, they move away in space and time carrying this feature, this spin, this polarization, this feature which LINKS them across space time in a non local way.

So our photons travel a million light years from one another pining and you decide to now look at the spin of one, or the polarization of one. When you do, your looking will determine the spin or the polarization of the other, even though it is now a million light years away. So in a very real sense, information has traveled all that way, a determination has been made, and it has done so faster than light can travel.

Particles are tied to one another in a "non local way". That is in a way where they influence one another, or you influence one of them by measuring something and that influence is passed to the other tied particle/entity no matter how close or how far away it is. The tied particles could be at the opposite ends of the universe, or right next to one another. A measurement on one will determine a feature of the other, INSTANTANEOUSLY at the time a measurement is made.

Pretty cool. There is a simple, non dumbed down book entitled Einstein's Moon/Bell's Theorem by David PEat. Give it a read. You can buy a used copy for next to nothing. Fabulous book for a general orientation to the issue.

Can you entangle more than one property? Also, I've heard that you can entangle atoms' properties by having them exchange photons. Can that be done remotely? How hard would that be? Would there have to be some special channel that the photons would have to go through? Would we have to worry about interference in the air or in space?

[Strange]

When you do, your looking will determine the spin or the polarization of the other, even though it is now a million light years away. So in a very real sense, information has traveled all that way, a determination has been made, and it has done so faster than light can travel.

It may be more accurate to say that in a very real sense, no information has been transferred.

You haven't "changed" the spin of B by measuring A. All that has happened is that by measuring particle A, you know what result will be obtained when particle B is (or was) measured.

Nothing has travelled between A and B and you can't use it to transfer information.

[HighGain]

No Strange, A has spin, BUT THAT SPIN'S ORIENTATION IS NOT DETERMINED UNTIL YOU MEASURE IT. Your measuring the spin "gives" A its spin orientation and thereby determines the orientation of B's spin. Both particles possess spin as they separate, but that spin is indeterminate. Your position as stated is essentially that of Albert Einstein. Again, I am not a professional physicist, but my fundamentals are sound all the way around. I am certain with respect to this point. No pun intended.

[Grey]

You haven't "changed" the spin of B by measuring A. All that has happened is that by measuring particle A, you know what result will be obtained when particle B is (or was) measured.

Sort of. It's worthwhile noting that nothing all that interesting results if you measure the spin of A and B on the same axis. All you find out is that yes, they always have the same spin (or maybe opposite spins, it doesn't matter). That could easily be the case if each particle "really" has spin up or spin down relative to a specific axis, and we just don't know which it is until we measure them after they've separated. You could imagine the particles to be carrying sealed envelopes with the possible results of various measurements, and the particles just have both the same results written down, set when they interacted in a way that entangled them. However, if you measure spins at different angles, you can find cases where the particles spins are correlated better than they "should" be if particle A doesn't know anything about which angle you choose to measure particle B's spin at, and vice versa. If you assume that the measurement results are set when the particles become entangled, and that once the particles separate nothing that happens to one has any effect on the other, you'd predict a lower correlation between the results of certain measurements than what is actually observed.

[HighGain]

When one measure's A's spin, something happens to B instantaneously. Its spin has become determined in the moment of the measurement on A and so information has passed to B. B now "knows" A's spin and its own. And for sure, nothing has traveled because all this has occurred instantly, and in traveling, one cannot go faster than the speed of light.

[HighGain]

For Grey, the whole concept of entanglement though does have to do with the fact making a measurement does "effect or impact the entangled particle that did not have a measurement taken.

[piningforthefjords]

But my question about how the properties of atoms are entangled still hasn't been answered. Do they HAVE to be in spatial locality? And what does that mean, anyway? Do they have to be within 1 meter of each other? 1 cm? 1 mm?

And I'm still wondering if they can be entangled remotely.

[HighGain]

Pining,

I believe one can entangle more than one property.

The remotely stuff I do not know about. Guessing somewhat, if you mean can you send photons from one atom to another and create entanglement, I believe the answer would be yes. Here the atoms are communicating and communicating locally/directly by way of the photons. So it would seem they could become entangled using your photons pining.

My non professional physicist opinion. If you put particles "in a box" and tie them that way, or if you connect them by way of light over long distances, it is the same thing. You are connecting them by way of imparting a tie to a feature that can later be measured and in so taking a measurement on the one, determine the feature's orientation on the other.

[HighGain]

Locality refers to one particle's influencing the other particle in such a way that the influence's speed of connection does not, as it cannot, exceed that of light. These are "classical" quantum mechanical interactions. By that one means, c, the speed of light, is limiting.

With entanglement, influence, effect, occurs outside the confines of such "classical events".

With entanglement, effects are realized instantaneously, across the breadth of the universe at the moment a measurement is made.

[loglo]

But my question about how the properties of atoms are entangled still hasn't been answered. Do they HAVE to be in spatial locality? And what does that mean, anyway? Do they have to be within 1 meter of each other? 1 cm? 1 mm?

And I'm still wondering if they can be entangled remotely.

The answer is yes, though usually it is done locally. Check the wiki on Entanglement Swapping.

[piningforthefjords]

But the point is that it can be done remotely by having atoms exchange photons, right? If so, if I wanted to perform quantum teleportation between two atoms, one on the moon and one on Earth, could I entangle them as is, or would I have to bring them together to entangle them and then take them apart?

[loglo]

Oh and here is a good post from Ken G about entanglement not requiring instantaneous action at a distance.

[loglo]

But the point is that it can be done remotely by having atoms exchange photons, right? If so, if I wanted to perform quantum teleportation between two atoms, one on the moon and one on Earth, could I entangle them as is, or would I have to bring them together to entangle them and then take them apart?

You need to have them interact in the right way, either directly or through an "agent" as in remote entanglement.

[piningforthefjords]

You need to have them interact in the right way, either directly or through an "agent" as in remote entanglement.

But that doesn't answer my question about remotely entangling an atom on the moon and one on Earth.

[loglo]

But that doesn't answer my question about remotely entangling an atom on the moon and one on Earth.

You either bring them together or you entangle one with a photon and send that to the other one and entangle those together.

[piningforthefjords]

You either bring them together or you entangle one with a photon and send that to the other one and entangle those together.

But wouldn't that photon just collide with a particle in the air or with radiation in space? How hard would it be to ensure that the photon gets from the atom on Earth to the atom on the moon?

[Grey]

Oh and here is a good post from Ken G about entanglement not requiring instantaneous action at a distance.

Ken's statements there are absolutely correct. But note what he's pointing out: that to avoid instantaneous action at a distance, you have to dispense yourself of the notion that the properties of a particle can be considered local to the particle in the first place. That is, the outcome of a measurement may depend on events that are arbitrarily distant from the measurement event itself. Ken calls that "holistic" because for some reason he doesn't like to say "nonlocal", but most other physicists use the term "nonlocal" to mean what Ken seems to mean when he says "holistic".

[EigenState]

Greetings,

Ken's statements there are absolutely correct. But note what he's pointing out: that to avoid instantaneous action at a distance, you have to dispense yourself of the notion that the properties of a particle can be considered local to the particle in the first place. That is, the outcome of a measurement may depend on events that are arbitrarily distant from the measurement event itself. Ken calls that "holistic" because for some reason he doesn't like to say "nonlocal", but most other physicists use the term "nonlocal" to mean what Ken seems to mean when he says "holistic".

I would disagree with the interpretation that KenG's use of the term "holism" necessarily corresponds to "nonlocality". I would instead offer the interpretation that it corresponds to the inherent, inseparable nature of the states that actually describe the system under investigation.

Consider a system of two particles. If those two particles are not entangled, then the system is completely characterized by separable states composed of the pure states of the individual particles. That is not the case if the system is entangled. In the latter case, the states are inseparable and the system cannot be described within the basis of the pure states of the individual components. To me, the term "holism" better describes the inherent necessity to characterize an entangled system with full consideration of both particles comprising that system. That is applicable even if the constituent particles of the entangled state are not physically separated to afford nonlocality.

To simplify, characterization of an entangled system requires full consideration of all of the particles that make up that system. Consideration of the individual constituents is not valid.

Best regards,
EigenState

[HighGain]

Pining,

Back in the earlier days of the Quantum debate, before the Einstein-Podolsky-Rosen thought experiment was fully digested, before David Bohm published QUANTUM THEORY in 1951 addressing the possible role of hidden variables, before John Bell read Bohm's work and "saw the impossible done", before Bell realized John von Neuman wasn't always right and that "quantum" should and would be questioned, before Bell's own struggle trying to preserve locality, before Bell's own startling publication in just the third issue of PHYSICS, before Clauser's and then Aspect's photon polarization correlation measurements confirmed the violation of Bell's inequality, the Dane Neils Bohr pointed out that perhaps the problem people were having with "quantum" was in essence Wittgensteinian.

Wittgenstein had pointed out that often times, at least in his mind, philosophical problems could be resolved simply by looking at how we use every day language. Of course we all know what "time" means, what time is. Metaphysical problems arise only when philosophers start using words in funny ways, words like "time", and demanding these words be understood outside of their every day context. What is time? Look at how you use the word in ALL OF ITS MANY WAYS and you you will find your answer. The meaning of a word, is often its use.

Bohr pointed out with "quantum" there was a new context for words such as "velocity", "momentum" and so forth. Physicists were playing the role of wrong headed philosophers, trying to force an old "meaning", an old "use", of these words. Here in the quantum world, there was a new context for "position", different from how the term had previously been used. Do not torture the word by compelling it to be what it is not by using it the old way in a new context. The use and so the meaning of position is different in the world of quantum.

Just as the word "time" has not one but many meanings depending on the context of its use, so too position, momentum and other words have different meanings as they are USED differently by physicists making quantum measurements.

Try this out with your word "remote" pining. What does remote mean in a holistic view, a non-local view? To see what it may mean, ask yourself how it is used, but do not force on the word a use from our "local lexicon".

[dgavin]

Also with entanglement, if you only entangle a pair, then you can also determine if one, or the other is entangled itself. However this only works wiht a pair, and soon as more then a pair are entangled the uncertainty priciple kicks in and you also can't tell what's entangled and what isn't.

Addtionaly you can break entanglements, which in the case of a pair, is deteactable are the other end instaneously.

That is at a quandry with information theory that states this can't happen, as it in effect it is a FTL(instant) state change that /is/ detectable without the uncertainty priciple kicking in. As far as I know there has be no resolution of that quandry yet, it may be years before it can be explained. I've read about expiriments exploring this odd feature of entaglement, but have not heard of any results from them. The best guess i've heard on this is that the action of observing 'if a particle is entangled', will break the entanglement if it is, thus preventing any usefull signal encoding via this oddity anyway. I think thats what the experiments are trying to determine.

[Grey]

Addtionaly you can break entanglements, which in the case of a pair, is deteactable are the other end instaneously.

Can you provide a citation for this? To my knowledge, there are no measurements that you can perform on one member of an entangled pair that can tell you anything about whether something has been done to the other particle. It's only when you correlate the measurements of the two particles that you can see that something unusual is going on.

[piningforthefjords]

Pining,

Back in the earlier days of the Quantum debate, before the Einstein-Podolsky-Rosen thought experiment was fully digested, before David Bohm published QUANTUM THEORY in 1951 addressing the possible role of hidden variables, before John Bell read Bohm's work and "saw the impossible done", before Bell realized John von Neuman wasn't always right and that "quantum" should and would be questioned, before Bell's own struggle trying to preserve locality, before Bell's own startling publication in just the third issue of PHYSICS, before Clauser's and then Aspect's photon polarization correlation measurements confirmed the violation of Bell's inequality, the Dane Neils Bohr pointed out that perhaps the problem people were having with "quantum" was in essence Wittgensteinian.

Wittgenstein had pointed out that often times, at least in his mind, philosophical problems could be resolved simply by looking at how we use every day language. Of course we all know what "time" means, what time is. Metaphysical problems arise only when philosophers start using words in funny ways, words like "time", and demanding these words be understood outside of their every day context. What is time? Look at how you use the word in ALL OF ITS MANY WAYS and you you will find your answer. The meaning of a word, is often its use.

Bohr pointed out with "quantum" there was a new context for words such as "velocity", "momentum" and so forth. Physicists were playing the role of wrong headed philosophers, trying to force an old "meaning", an old "use", of these words. Here in the quantum world, there was a new context for "position", different from how the term had previously been used. Do not torture the word by compelling it to be what it is not by using it the old way in a new context. The use and so the meaning of position is different in the world of quantum.

Just as the word "time" has not one but many meanings depending on the context of its use, so too position, momentum and other words have different meanings as they are USED differently by physicists making quantum measurements.

Try this out with your word "remote" pining. What does remote mean in a holistic view, a non-local view? To see what it may mean, ask yourself how it is used, but do not force on the word a use from our "local lexicon".

But it's a fairly straightforward question. There's an atom on the moon and an atom on Earth. Initially, they're not correlated or entangled at all. Can you entangle them by having them exchange photons? Would you have to worry about the photons getting messed up or not hitting their targets?

[EigenState]

Greetings,

But it's a fairly straightforward question. There's an atom on the moon and an atom on Earth. Initially, they're not correlated or entangled at all. Can you entangle them by having them exchange photons? Would you have to worry about the photons getting messed up or not hitting their targets?

No, you cannot entangle them as described. I am quite certain that the only known method of creating an entangled state from states that have not interacted directly (locally) is by entanglement swapping. However that process requires initial preparation of two sets of pre-entangled systems.

As an example: Location 1 has two particles one of which is entangled with one particle at Location 2 and the other is entangled with another distinct particle at Location 3. Manipulation of the two particles at Location 1 can yield a pair of entangled particles at Locations 2 and 3 despite those particles having never interacted directly with one another.

Best regards,
EigenState

[dgavin]

Can you provide a citation for this? To my knowledge, there are no measurements that you can perform on one member of an entangled pair that can tell you anything about whether something has been done to the other particle. It's only when you correlate the measurements of the two particles that you can see that something unusual is going on.

I'll see if i can dig it up. We touched on it some years ago on Baut, and the references was easy to find then, looking at search today though it's going to take a bit to find it now. It does exist, just need to re-locate it!

[dgavin]

Grey I did not find the original source that talked about entangelment detection and breaking. However I did find a followup that talks about detecting broken entanglements, and from what i'm reading of it, once entangled, you can't fully disentangle them. So it looks like the quandry of being able to test entanglement state itself is resolved experimentally. I suppose this is the uncertainty priciple kickign in again as i sounds like with more and more entangled items, it becomes less and less possible to break the entanglements.

Source http://iopscience.iop.org/1751-8121/43/27/275306

[HighGain]

For your post at # 26 pining, just my way of encouraging you to perhaps focus on basics. Sometimes that helps me when I am working on something and sort of getting frustrated. I ask myself, OK, what's going here, what are the basics, what do I know and not know for sure". In the case of quantum, a lot of times confusion over basics has to do with the use of words. Maybe slow down a little pining. Try to define remotely for yourself and then ask if given that definition correlation could occur.

An aside. Sort of sad that Bell died so young. It think he was 61 or something like that. Died in 1990 I believe. Not that a Nobel prize is the be all and end all. But if anyone deserved the very highest honors, he sure did. What a great idea he had. So eloquent he was with his mathematics and his physics.