Macroscopic Quantum Entanglement

meckardt writes: "We laugh at the science fiction of such programs as Star Trek, but it can almost be stated as a truism that what is fiction today may be science tomorrow and engineering next week. Researchers at the University of Aarhus in Denmark report in the science journal Nature that they have been able to cause particles to interact over a distance using lasers. The effect, called quantum entanglement, has been observed before, but never with such large amounts of matter. Don't expect transporters next week, but it is interesting that this report hits the streets the same day that Enterprise debuts."

Comments

Clarification...?

[melquiades]

As I've understood these experiments in the past, entanglement involes splitting a particle, or taking two existing particles, and "entangling" their states -- so that, for example, if you change the spin of one electron, its partner electron's spin also changes, even at a great distance (or something to this effect).

The application to faster-than-light information transmission is obvious. But teleportation? The article doesn't give enough specifics. Can anybody shed light on this? How would this experiment lead to a teleporter??

Re:Clarification...?

[renard]

Can anybody shed light on this? How would this experiment lead to a teleporter??

Well, this will get us into some of the most dangerous neighborhoods of quantum mechanics, but I'll see what I can do.

The quantum entanglement of two particles means that (just as you say) the behavior of one particle becomes perfectly correlated with the behavior of another. In the classic example case, two photons are generated with opposite polarizations. If you can transmit them a distance apart without any interference, then the photons will remain entangled, and a measurement of the polarization of one photon will have immediate implications for the polarization of the other.

Although this is very useful for quantum cryptography, please note that it will NOT allow you to transmit information any faster than the speed of light. To take the cryptographic example, it allows you to generate a safe one-time pad, known to both sides and to no one else, but you still have to transmit your actual message separately.

How can quantum entanglement be used for something like teleportation? Well, let us agree first that if I can produce a perfect quantum replica of a distant system, that is equivalent to teleporting the system. Any given electron (for example) is indistinguishable (in a very deep sense) from every other electron in the universe. So for teleportation all we need to do is reproduce a quantum state. You might say it's more akin to a quantum xerox machine than to most people's classical idea of teleportation.

Okay, so here's how it works: take your two quantum-entangled photons, and instead of simply measuring the polarization of the one nearby, get it to interact with a "target photon" that you want to teleport. If you set things up properly, and observe the outcome very carefully, then the interaction of the two photons on your end will cause the entangled photon - an arbitrary distance away - to enter a new state which is perfectly correlated with the state that your target photon had. Then, once you tell your distant collaborator about the exact outcome of the photon interaction on your side, your collaborator will be able to apply that knowledge to her entangled photon, and produce an exact quantum replica of your original target photon. Voila! Teleportation.

Note again that no faster-than-light communication is enabled by this. You still have to communicate a regular light-speed message between collaborators to get this to work. The actual experiment was carried out several years ago and is old hat by now. The current experiment improves upon previous efforts by entangling so many more (trillions!) particles.

The quantum entanglement of so many particles makes the actual teleportation of a similar number feasible, but one final note - even trillions of particles is many orders of magnitude less than the 10^27 or so particles in your average Starship Captain.

-Renard

Not true teleportation

[Wind_Walker]

This is actually NOT teleportation; this is akin to an episode of The Outer Limits I saw once where they create an exact copy of a person on the other end of a "teleportation" machine, and then destroy the copy that currently resides on the transmitting end. It's a great show, but I digress...

I'm amazed that this worked with "trillions" of atoms; this kind of phenomenon is usually restricted to very small, very energetic particles. But it's NOT teleporation. Teleportation involves taking an object from point A and moving it to point Z without crossing the in-between space, C through Y. This is like taking an object from point A, running it through the world's biggest and best Fax machine, then putting the result at point Z, without crossing C through Y.

Still, it's an interesting and ground-breaking result, one that (I hope) will make it past the peer review process, which kills more scientific papers than anything else.

Re:Not true teleportation

[dragons_flight]

Nature is a peer reviewed journal, and one of the more prestigious ones to boot. This means that there is nothing wrong (unless very subtle) with the setup or analysis of their experiment provided the data they report is accurate. Of course something might still be wrong with their results, but that will found out when other scientists try to replicate the experiement.

From the book about Milliways...

[Chris Brewer]

I teleported home one night,
With Ron and Sid and Meg,
Ron stole Meg's heart away,
And I got Sidney's leg.

Scientists observer quantum entanglement and

[scotch]

Scott Bakula returns to television tonight. Coincidence? Don't believe it.

Ansible

[mmmmbeer]

I don't see how this would allow for teleportation. As many others have already mentioned, how do you draw a link between this and the ability to transport (or even duplicate) matter?

However, I do see a possibly very significant use of this technology. If you can maintain an entangled state between macroscopic objects, wouldn't this allow a change to one object to be seen immediately in the other? If so, couldn't this be used to create computer networking devices which would work over any distance without any delay, and without any necessary wires or similar infrastructure? This sounds like it could potentially create the "ansible" predicted by Ursula K. Le Guin and Orson Scott Card.

Quantum Computing

[cailloux]

One real posiblity for quantum entanglement would be in the area of quantum computing and distributed processing. The theory in a quantum computer is that every possible state of every computation can exist simultaneously. Only after you decide you want to know the answer to a specific problem will you find it - in effect any complex calculation is speeded up my magnitudes of order. In a distributed environment, quantum entanglement would allow for 2 (or more) quantum computers to join together and each work on a distributed/parallel process program and instantly share data, as well as solutions. For example, in gene research the refinement of proteins into useful medications could take place at a much faster rate because each quantum computer could "see" what the other got for evolutionary results and apply those changes along separate lines of reasoning while still being aware of what worked and what did not.

In a non-quantum computing environment, data networking could happen much faster (blowing the doors of gigabit ethernet) by being able to instantly transfer the entire contents of a hard drive from one place to the next along fiber; no longer are you sending electrons at high speed (c), but now you are transferring the entire data packet straight from one network card to the next.

-cailloux

A Clarification...

[HEbGb]

Clarification:

Quantum entanglement involves creating a system in which the state (polarization, spin, etc.) of two or more particles are 'dependent on' each other. Measuring the state of one particle defines the state of the other, 'magically', over some distance.

HOWEVER make no mistake, nothing in quantum mechanics or entanglement theory allows anything resembling faster-than-light information traveling, nor teleportation as we understand it. This is pure fantasy that many physicists subtly or not-so-subtly use to solicit grants, or at least popular press. (There's plenty of this nonsense in sci-tech magazines.) It certainly worked here.

Here's another example of macroscopic 'quantum entanglement'. I have a bag with two billiard balls, one black, one white. I close my eyes, pull one out and put it in a second bag. Then, I hand you the first bag, and walk across the room with the second bag, and open it. Once I look at the color of the billiard ball in my bag, the color of the ball in your bag 'magically' changes color and assumes a defined state. These billiard balls are entangled, very much like subatomic particles are.

Can you ever transport information faster than light using this method? NO. Can matter be teleported? NO. I really wish these pop-sci articles would put an end to these misconceptions once and for all...

Re:A Clarification...

[jaoswald]

It doesn't necessarily "change the state" of the second particle. (It can't, since the particles cannot causally interact; the particle's state evolves according to the local environment). However, the results of measurements on the second particle are inter-dependent with the results of the measurements of the first particle, even though the acts of measurement themselves cannot be connected causally (in the sense of special relativity).

The really funky thing is that the *choice* made to determine what kind of measurement to make on the first particle affects the inter-dependence. The idea being that "somehow" the measurement apparatus is communicating its setup to the distant particle, even though it really can't. This is really disturbing, but probably doesn't have any better explanation than "that's just how it is."

Re:A Clarification...

[renard]

Your billiard ball example is equivalent to Einstein's "hidden variables" attempt to explain away quantum entanglement. Bell's theorem demonstrated that the predictions of quantum mechanics were actually inconsistent with such a theory - and subsequent experiments proved him right. The universe is far more mysterious than you or Einstein give it credit for.

In fact the reproduction of a quantum state - in all its particulars - is as perfect a teleportation as we might ever expect to achieve - see my accompanying comment. So I don't think your criticisms are entirely justified.

I say "not entirely" because extrapolating 13 orders of magnitude, and to real systems rather than super-cooled ones - as required for useful teleportation - still requires a bit of hutzpah. But the scientists cannot take all the blame. After all, the Trekkies were there long before...

-Renard

Re:A Clarification...

[caffeinated_bunsen]

The faster-than-light seeming aspect of it appears disturbing at first. But after a while, you realize that it occurs anywhere in quantum mechanics when a wave function collapses.

Think about it. Consider 2 polarized photons, 2 electron spins, 2 billiard balls, anything entangled such that a particular measurement performed on each always returns opposite results. When the system is set up, each object's probability of being, say, spin up, is 50%. The two spins are described by coupled wave functions, so that the 50% that corresponds to A being spin up also corresponds to B being spin down and vice versa. When one is measured, its wave function collapses into a single eigenstate, and its partner's wave function collapses into the other eigenstate. Thus, the final eigenstate of B is decided by the same measurement that measures the state of A.

This seems disturbing, the instantaneous change of B's wave function an arbitrary distance from A, when only A is being measured. But the simultaneous collapse of 2 coupled wave functions is mathematically no different from the collapse of a single wave function. When you have a particle with a large uncertainty in position, mesuring its position causes it to collapse to a single position eigenstate. If you have 2 detectors some distance apart, and use each to measure the presence or absence of the particle some very short time apart, you know that if you observe it at one, you won't observe it at the other. Say the detectors are 10m apart, and they take their measurements 1ns apart. If you detect the particle at the first one, you KNOW that the second won't detect it. But the 'information' about the wave function's collapse at the first detector would take 33ns to reach the second, if it travelled at the speed of light. So a single wavefunction's instantaneous collapse from all of space to a single point is just as much 'communication' as an entangled particle pair's simultaneous collapse.

So you have a choice: Either the entangled particles' behavior isn't that disturbing, any measurement of a quantum system is really disturbing.

Old news

[dickDragon]

This was presented at the

International Conference on Quantum Information
June 10-13, 2001 at the University of Rochester campus in Rochester, New York.