A solid case of entanglement

Jan 11, 2010
This is an SEM image of a typical Cooper pair splitter. The bar is 1 micrometer. A central superconducting electrode (blue) is connected to two quantum dots engineered in the same single wall carbon nanotube (in purple). Entangled electrons inside the superconductor can be coaxed to move in opposite directions in the nanotube, ending up at separate quantum dots, while remaining entangled. Credit: L.G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk

Physicists have finally managed to demonstrate quantum entanglement of spatially separated electrons in solid state circuitry.

For the first time, physicists have convincingly demonstrated that physically separated particles in solid-state devices can be quantum-mechanically entangled. The achievement is analogous to the of light, except that it involves particles in circuitry instead of photons in optical systems. Both optical and solid-state entanglement offer potential routes to and secure communications, but solid-state versions may ultimately be easier to incorporate into electronic devices.

The experiment is reported in an upcoming issue of Physical Review Letters and highlighted with a Viewpoint in the January 11 issue of Physics.

In optical entanglement experiments, a pair of entangled photons may be separated via a beam splitter. Despite their physical separation, the entangled photons continue to act as a single quantum object. A team of physicists from France, Germany and Spain has now performed a solid-state entanglement experiment that uses electrons in a superconductor in place of photons in an optical system.

As conventional are cooled, the electrons they conduct entangle to form what are known as Cooper pairs. In the new experiment, Cooper pairs flow through a superconducting bridge until they reach a that acts as the electronic equivalent of a beam splitter. Occasionally, the electrons part ways and are directed to separate -- but remain entangled. Although the quantum dots are only a micron or so apart, the distance is large enough to demonstrate entanglement comparable to that seen in optical systems.

In addition to the possibility of using entangled electrons in solid-state devices for computing and secure communications, the breakthrough opens a whole new vista on the study of quantum mechanically entangled systems in solid materials.

Explore further: 'Cavity protection effect' helps to conserve quantum information

More information: Carbon Nanotubes as Cooper-Pair Beam Splitters, L. G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk, Phys. Rev. Lett. 104, 026801 (2010) - Published January 11, 2010, Download PDF

Provided by American Physical Society

4.4 /5 (28 votes)

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Question
2.6 / 5 (5) Jan 11, 2010
Quote from article:
"In optical entanglement experiments, a pair of entangled photons may be separated via a beam splitter. Despite their physical separation, the entangled photons continue to act as a single quantum object."

Any proof of the above statement? If one reflects one of the entangled photons does the other get reflected at the same time? If not, how can they be considered entangled? These two "entangled" photons or pulses of light are like two sides of a coin. Split the coin in half, head from the tales, and as soon as you see that one half is heads, you know the other half is tales. Does one half of this coin have any influence on the other half once they are separated?
fuzz54
3.7 / 5 (3) Jan 11, 2010
@Question: Distance has no effect on entanglement as far as the universe is concerned. One could reflect one of the photons as many times as desired and the entangled system would still exist. By your logic if one photon does not experience the same acceleration as the other then the system is not entangled. This does not make sense since local variations in gravity are always causing different accelerations for any photons that don't have the exact same velocity and position.
Question
3 / 5 (4) Jan 11, 2010
fuzz54: Then exactly what is entangled about the photons? Couldn't one do exactly the same thing with a coin cut in 1/2? I do not see a single thing going on here that classical physics cannot explain.

fuzz54
3.7 / 5 (3) Jan 11, 2010
Information is contained in the entaglement and operations can be done on the entangled information to perform calculations. At the end of the calculation an observer would "measure" the state of a photon and the system would collapse down to the classical heads or tails answer that you are thinking of. The difference here is that a quantum system can be 0 or 1 or both at once (superposition). Through superposition as you add more entangled photons to the system (qubits) you get exponentially more information that can be stored and manipulated in a single operation. With enough qubits in your system you could for example add fifteen numbers together in one operation instead of fifteen separate operations. It's all kind of mind blowing and does not correspond to classical physics much, so you have to stop thinking in a classical yes or no sense. The exact physics for performing calculations on groups of entangled objects is well beyond the scope of this forum or my brain.
Question
2.3 / 5 (3) Jan 11, 2010
fuzz54: Oh, so what you are telling me is that the photons are in two states at the same time. It is like saying my split coin halves are both heads and tails until I examine one and then the other always becomes the opposite. Well no, these coin halves always were one or the other, never both. What proof does anyone have that these entangled photons are ever in both states, once they exit the splitter?
Auxon
not rated yet Jan 11, 2010
@Question: They entangled the spin state of the electrons in the superconductor, according to the paper. When a chosen quantum state is entangled, distance and time do not matter, changes to one particle's state effect the other instantly. Experiments have been done which show that one entangled particle would have to go back in time and tell the other particle, somehow, what it's spin state is going to be when it is observed. Obviously this seems very counterintuitive, if not impossible. Hence, the many-worlds interpretation, or the fish-in-a-bowl-viewed-with-different-cameras hypothesis (I have no idea what it is called) - holographic interpretation? - or something does travel back in time, there is only one particle it just travels back and forth in time, creating duplicates of itself and causing all other particles to form from interactions etc..., etc.... Lots of crazy ideas.
Auxon
not rated yet Jan 11, 2010
fuzz54: Oh, so what you are telling me is that the photons are in two states at the same time. It is like saying my split coin halves are both heads and tails until I examine one and then the other always becomes the opposite. Well no, these coin halves always were one or the other, never both. What proof does anyone have that these entangled photons are ever in both states, once they exit the splitter?


Actually, imagine that you have one coin and it's spinning in space and it's not going to stop until you slap it on the table. Now you take two coins and you spin them both at the same time, but in a special way, that actually causes them to somehow become entangled. (In the real world there are various ways to do it.) Now, imagine that you slap one down and it's heads, but the other one is still spinning. Now you slap the second one down and it is tails. Repeat. Every single time they are opposites no matter how you try to make them different they are entangled.
Auxon
not rated yet Jan 11, 2010
fuzz54: Well no, these coin halves always were one or the other, never both. What proof does anyone have that these entangled photons are ever in both states, once they exit the splitter?


There is lots of proof of that ... the interference pattern caused by light interacting in the two-slit experiment was the first evidence of that. Given the fact that the two-slit experiment has shown that a single photon will interfere with itself as well, this cannot be explained by classical physics. You'd probably enjoy this video on YouTube about the two-slit experiment: http://www.youtub...eprQ7oGc
Question
3 / 5 (4) Jan 11, 2010
Auxom: As far as the two coin experiment, all I can say is show me how two coins can be entangled to always come up opposites. I do not see how you or anyone else can. And if it cannot be done for these coins, how can it be done for photons or pulses of light?

And the two slit experiment can also be considered proof that light is made up of waves of particles and not a single particle called the photon.
Auxon
2.3 / 5 (3) Jan 11, 2010
Auxom: As far as the two coin experiment, all I can say is show me how two coins can be entangled to always come up opposites. I do not see how you or anyone else can. And if it cannot be done for these coins, how can it be done for photons or pulses of light?

And the two slit experiment can also be considered proof that light is made up of waves of particles and not a single particle called the photon.


I provided an introductory video describing just how it happens and how it's proven. It's up to you if you want to remain ignorant or learn.
Objectivist
5 / 5 (1) Jan 12, 2010
And the two slit experiment can also be considered proof that light is made up of waves of particles and not a single particle called the photon.


This is because of the particle-wave duality. Why do you favor waves? You've clearly seen light behave both ways. Surely even you can understand how in everyday physics particles could be the foundation of waves and vice versa. You seem to have made up your mind to favor waves without any proper reason to do so.

The quantum world is telling us that all data is both particle and wave, and that depending on which specific piece of data we observe we'll find it to be more like one or the other.
bluehigh
2.8 / 5 (5) Jan 12, 2010
No, that is step too far. Particle wave duality is a term that describes how light can ACT LIKE a wave or ACT LIKE a particle. It does NOT suggest suggest that EM radiation, photons or anything else is BOTH particle and wave.
simpletim
1 / 5 (1) Jan 12, 2010
This question will be answered in the next 2-3 years. Either quantum computers will work and thus provide proof on entanglement, or they will not work as advertised and we will need a new paradigm to explain wave / particle duality.
Mr_Man
5 / 5 (1) Jan 12, 2010
Auxom: As far as the two coin experiment, all I can say is show me how two coins can be entangled to always come up opposites. I do not see how you or anyone else can. And if it cannot be done for these coins, how can it be done for photons or pulses of light?


I think you are not understanding that quantum and classical physics work differently - that is what this all about.
Entangled photon pairs will have the same "spin", even if you split them and change the spin of one of the 2 photons again after the initial split the two will still always remain entangled. That may not have been the best way to explain this, but space is limited for this text.

The mystery comes from how the two photons "communicate" their change instantaneously even if they are light years apart. This is the main difference between quantum and classical physics. Someone correct me if I am wrong - I am not an expert in the field, I just love learning about it.

Question
1 / 5 (2) Jan 12, 2010
Mr_Man: Yes that is where the difference comes into play. It is really no mystery. The two photons do not communicate, they are what they are from the moment they were split or entangled. There is not different than the coin split in an earlier posting. Toss the two halves into the air and then go pick up one half and you will intantly know what the other half is. It will always be the opposite with no "communication" between these two coin halves or the two split photons a light-year apart. They are what they are from the moment of creation.
fuzz54
5 / 5 (2) Jan 12, 2010
This question will be answered in the next 2-3 years. Either quantum computers will work and thus provide proof on entanglement, or they will not work as advertised and we will need a new paradigm to explain wave / particle duality.
Simpe quantum computer calculations are already being done using entanglement. Wave / particle duality does not need to explained. It has been observed and theories have been proposed and tested that match the observations with a high degree of accuracy. That is all you can ask for with the scientific process.
fuzz54
5 / 5 (1) Jan 12, 2010
Auxom: As far as the two coin experiment, all I can say is show me how two coins can be entangled to always come up opposites. I do not see how you or anyone else can. And if it cannot be done for these coins, how can it be done for photons or pulses of light?

And the two slit experiment can also be considered proof that light is made up of waves of particles and not a single particle called the photon.

You can't be shown how two "quantum" coins can be made to come up opposites. That's just how it works. It's the possibility that the coins are heads, tails, or both that allows for quantum computing and entanglement. This possibility vanishes once you observe what one coin is, but until you make the observation, the crazy idea that both coins are tails or heads contains useful information. In this way entanglement can store information on all of the possible states of an unknown system at once and do operations on each state simultaneously.
flaredone
5 / 5 (1) Jan 12, 2010
Explanation of entanglement is closely related to wave function collapse. Try to imagine, you're a sailor, who is staying at night on the end of floating wharf, to which some boat is attached. Because night sea is stormy, both sailor, both wharf and boat are wobbling up and down, but in different phases. From the perspective of sailor this boat sways randomly.

The observation of quantum particle is analogous to situation, when sailor touches the boat for a moment, thus exchanging some kinetic energy with it. What will happen, after then? The wharf and boat will begin to oscillate at phase. It means, the sailor will keep his relative position with respect to boat, so he cannot detect any boat wobbling anymore, because he moves by the same way. We can say, the wave motion/function of boat has collapsed from local perspective of that sailor.
Sciencebee
1 / 5 (1) Jan 12, 2010
Does entanglement have the potential to enable instant communication across space? (I may not be understanding correctly) Going to check out that video above when I get home.
flaredone
not rated yet Jan 12, 2010
The nanotube experiment discussed could be modeled by undulating oil droplets, splitted by flowing through T-shaped pipe into two halves like droplets in lava lamp (or even better by droplets under diamagnetic levitation).

The resulting parts will remain undulate at phase with respect to the center of their common mass. The smaller droplets will therefore "remember" the state of original droplet - so they can serve as a model of quabit memory. Such pair will create their own "inertial reference frame" and/or "local universe" by many worlds interpretation. The original state of droplet could be restored only by combining of both halves back again (no other droplet vibrating at same phase can be used) - on this fact the quantum cryptography is based. The main difference is, the droplets system exhibit an extremelly high quantum number with compare to electron in nanotube.
flaredone
5 / 5 (2) Jan 12, 2010
Does entanglement have the potential to enable instant communication across space?

Entanglement is mediated by gravitational waves - you can imagine like communication by underwater sound waves at water surface, which ignores the surface wave spreading. So it could violate causality, but violation of causality is not sufficient as a proof of superluminal communication: you can be never sure by your partner in such communication.
flaredone
5 / 5 (2) Jan 12, 2010
Wave / particle duality does not need to explained
This is just a Holly Church stance, which prohibits people to ask deeper questions. Such stance is quite common in modern physic (Feynman: "shut up & calculate!") and it demonstrates clearly, how mainstream physics transformed into modern theology.

We should never replace understanding of reality by its numeric regression. The fact, we can extrapolate path free fall by parabola still doesn't mean, we understand, why is it so. The same mistake was done by proponents of Ptolemaic epicycles: they could describe path of planets well and they can even lead to testable predictions of eclipses and conjuctions - the only "subtle" problem is, their underlying physical model is completelly wrong. Just because it replaced understanding by its formal model fitted onto observations.
Rynox77
not rated yet Jan 12, 2010
Thank you for the video, Auxon; I rather enjoyed it. I felt like I was in 2nd grade again, though. Maybe they could have just explained it like we're big boys and girls. :)
fuzz54
not rated yet Jan 12, 2010
Wave / particle duality does not need to explained
This is just a Holly Church stance, which prohibits people to ask deeper questions.

We should never replace understanding of reality by its numeric regression. The fact, we can extrapolate path free fall by parabola still doesn't mean, we understand, why is it so. The same mistake was done by proponents of Ptolemaic epicycles: they could describe path of planets well and they can even lead to testable predictions of eclipses and conjuctions - the only "subtle" problem is, their underlying physical model is completelly wrong. Just because it replaced understanding by its formal model fitted onto observations.

We have no true understanding of reality. It's all just an approximation and all of our current theories could be in error. Nobody will pat us on the back someday and tell us that we finally got things right. Using logic based theories instead of emprical approaches is better in my opinion too.
flaredone
Jan 13, 2010
This comment has been removed by a moderator.
ShadowRam
not rated yet Jan 14, 2010

Actually, imagine that you have one coin and it's spinning in space and it's not going to stop until you slap it on the table. Now you take two coins and you spin them both at the same time, but in a special way, that actually causes them to somehow become entangled. (In the real world there are various ways to do it.) Now, imagine that you slap one down and it's heads, but the other one is still spinning. Now you slap the second one down and it is tails. Repeat. Every single time they are opposites no matter how you try to make them different they are entangled


This is still a classical situation.

The first coin was heads all along (you just didn't know it until you measured it), as was the 2nd coin was tails.

Unless you can 100% control the outcome of the first coin, there is no 'action at a distance'....
Jake_Love
not rated yet Jan 14, 2010

On 12JAN Flaredone said, "We can say, the wave motion/function of boat has collapsed from local perspective of that sailor."

That's preceded by a lotta mental imaging just to slip in the notion of 'wave function collapse' as if something in the real world were collapsing.

But like entanglement (see the excellent original comment by 'Question' on 11 Jan), what's bobbing up/down in the theorist's head is 'information' perceived probabilistically and that, my friends, is readily entangled.

We 'know' the whole coin has a 50-50 chance of landing heads while it's spinning in mid-air and that knowledge 'collapses' to 0 or 100% when the coin lands and is read. If there is a similar coin a mile away, no magical entanglement allows us knowledge of its state before it lands.

But if the quantum mechanic initially 'prepares the entangled state' by splitting the coin into head and tail...

QED
FenderFennec
1 / 5 (1) Jan 16, 2010
And if you were to turn over the "heads" coin so that it shows tails instead, the "tails" coin would become heads without user intervention.
antialias_physorg
5 / 5 (1) Jan 16, 2010
No. Once you have performed a measurement then it's over. Changing the spin (or other previously entangled property) of one of the two will then not change the state of the other.
DigiMc
not rated yet Jan 17, 2010
How do we know the electrons have collapsed into final state only at the moment of measurement, but not earlier? By measuring magnetic field (if it is small to nonexistent, then the electron's spin must be in both state simultaneously = entangled)?

If we do know whether an electron is entangled or not, then why that can't be used for FTL communication?
antialias_physorg
not rated yet Jan 17, 2010
How do we know the electrons have collapsed into final state only at the moment of measurement, but not earlier?

Double slit experiment (or its equivalent for spin). If it had collapsed earlier there would be no interference pattern. But we do observe interference patterns so he collapse must happen at the point of measurement.

If we do know whether an electron is entangled or not, then why that can't be used for FTL communication?

Because you either collapse one now and then send the other particle (which is regular - i.e. non-FTL - information transmission. Here entanglement is pointless since you can do it better in other ways.)

Or you transmit the particle and collapse it later. But since you _don't know_ which state it will collapse into (and since there is no way to force it to collapse into one state) you will not know _what_ you 'transmit'. So you'll be sending completely random noise which carries an informational value of zero (see Shannon information theory)
DigiMc
not rated yet Jan 18, 2010
Or you transmit the particle and collapse it later. But since you _don't know_ which state it will collapse into (and since there is no way to force it to collapse into one state) you will not know _what_ you 'transmit'. So you'll be sending completely random noise which carries an informational value of zero (see Shannon information theory)


I understand that. But - if we can detect for a particular particle whether it is entangled state or not - we can collapse particles at 'transmitter' side and simply observe time intervals at which particles become de-entangled at 'receiver' site. Say if interval between two de-entanglements is 1s, that means bit 0 was sent, if interval is 2s, that means bit 1 was sent. So we don't care into which states particles will collapse at either side, just whether they did collapse or not - which, according to the article, should be possible...?