# Entangled particle question

Could entangled particles be used to send information instantly? For example, a mars rover could be given a particle and earth could be given a particle. If we did something to our particle, it would also happen to the entangled particle. We could write a computer program to interpret these changes. A code could be developed based on changes in the particles. Right? Or am I making too many assumptions?

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jcsd
Gold Member
No they specifically cannot be used to send information.

Look at this article. It is very interesting and deals with this.

http://www.space.com/scienceastronomy/atom_teleportation_040617.html [Broken]

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jcsd
Gold Member
That's describing quantum teleporation that uses entanglement. In order to accomplish quantum teleportation you still need a classicla communications channel (which can only send information at c or under).

I guess I don't quite understand. It seems like if you have two entangled particles and you put one one place and one another you could do something to one that would affect the other. This could very easily be used to send information between great distances.

jcsd
Gold Member
The problem is though a measuremnt on one partcile instantly decide the state of the other particle it doesn't change the values we should expect to measure. Therefore someone measuring the particle whose state was decie instanteously by a measuremnt on that particle has no way of telling if the value was already determined or not.

iamgore46 said:
I guess I don't quite understand. It seems like if you have two entangled particles and you put one one place and one another you could do something to one that would affect the other. This could very easily be used to send information between great distances.
I may be wrong, but it sounds to me like you are thinking that entangled particles remain entangled for many measurements. This is not the case.

The entanglement is lost the instant that one of the particles interacts with something else. The particles don't remain entangled forever. The very idea of "putting" an entangled particle someplace were you can play with it is a completely impossible idea. The only thing you can do to an entangled particle is measure it where you find it. Anything else that you might do to it will cause it to lose any previous entanglement that it might have had with another particle.

You only get one measurement, and even that measurement must be made before the entangled particle interacts with anything else. If you want to use entangled particles to communicate you would need to use a whole stream of them. But since you can't do anything special to them (like polarize them or something) without destoying their entanglement, then you can't actually use them for communication. Unless you want to communicate a bunch of random noise. They are good for that!

Unfortunately communicating random noise isn't a very useful conversation.

The entanglement is lost the instant that one of the particles interacts with something else.
Does it also happens in the so-called Non-demolition measurements (measurements that don't change the state of a system)?

Quantum entanglement is an aspect of quantum information science that is seriously being considered as a way of transporting data.

Quantum Entanglement
Quantum entanglement is a subtle nonlocal correlation among the parts of a quantum system that has no classical analog. Thus entanglement is best characterized and quantified as a feature of the system that cannot be created through local operations that act on the different parts separately, or by means of classical communication among the parts.

In the case of a pure quantum state of a system divided into two parts, the entanglement can be completely characterized because it can be reversibly converted to a standard currency. If many identical copies of a given state are available, then it is possible with local operations and classical communication to "distill" the entanglement into a standard form — many copies of a two-qubit Bell pair. And the Bell pairs, with local operations and classical communication, can be transformed back into many copies of the original state, with negligible losses. Thus the number of distillable Bell pairs provides a universal measure of bipartite pure state entanglement.

The situation is far more subtle and interesting for the case of entangled bipartite mixed states, or for pure-state entanglement with more than two parts. For example, some bipartite mixed states exhibit bound entanglement -- though entanglement is necessary to create these states, none of this entanglement can be distilled into Bell pairs. Another significant surprise is that even bipartite states with no entanglement can exhibit a peculiar kind of quantum nonlocality. One can construct a quantum book with two pages, such that it is impossible to read the book one page at a time, even though the two pages are not entangled with one another.

Since entanglement cannot be created locally, an entangled state shared by two widely separated parties can be a valuable resource (Fig. 3). One application of shared entanglement is a novel quantum communication protocol called quantum teleportation. If one party (Alice) possesses a qubit in an unknown state, she cannot observe the state without disturbing it. But if she shares a Bell pair with another party (Bob), then Alice can convey a perfect replica of her state to Bob by sending him just two bits of classical information. In the process, the shared Bell pair is consumed, and Alice’s original is destroyed. An odd feature of quantum teleportation is that the unknown state can take values in a continuum; nevertheless, thanks to the pre-existing shared entanglement, Bob needs to receive only two classical bits to recover the perfect replica. This protocol has been convincingly demonstrated in the laboratory.

http://www.nsf.gov/pubs/2000/nsf00101/images/figure3.gif

Figure 3: Two related tasks that require quantum entanglement as a resource. In quantum teleportation, Alice receives a qubit in an unknown state, and destroys it by performing a Bell measurement on that qubit and a member of an entangled pair of qubits that she shares with Bob. She sends a two-bit classical message (her measurement outcome) to Bob, who then performs a unitary transformation on his member of the pair to reconstruct a perfect replica of the unknown state. In superdense coding, Alice receives a two-bit classical message, transmits the message by performing a unitary transformation on a member of an entangled pair that she shares with Bob, and then sends that qubit to Bob. Thus one qubit suffices to carry two classical bits of information.
SOURCE: http://www.nsf.gov/pubs/2000/nsf00101/nsf00101.htm#b2

chroot
Staff Emeritus
Gold Member
Information cannot be transmitted by quantum-entangelement alone, Imparcticle. You'll also need a classical channel.

- Warren

meteor said:
Does it also happens in the so-called Non-demolition measurements (measurements that don't change the state of a system)?
I'm not aware of any so-called Non-demolition measurements that can be made on quons without changing their state? Seems to me that would be in direct violation of the uncertainty principle. You'll have to provide a link to more information about non-demolition measurements on quons so I can read up on this. There must be a catch somewhere.

LURCH
Gore, as has been said more than once, the popular wisdom stats that information cannot be transmitted via quantum entanglement. Althoug I myself remain unconvinced of this statement, it is broadly accepted by the scientific community in general.

You're right, I was confused. A quantum non-demolition measurement does not perturb the measured observable but the canonically conjugate observable is perturbed. This document assures this:
http://plato.phy.ohiou.edu/~ulloa/611-612/612papers/Shaleen%20Shukla--Q%20Non-demolition.pdf [Broken]

Anyway, and I don't want to seem obstinate, I have heard some things about something called entanglement degradation. Does it happens by decoherence of the particles entangled with the medium?If it's this way, this is a measurement that don't destroys the entanglement, only degrades it. But is possible that entanglement degradation can also happen without any interaction of the entangled particles with anything. I'm not sure.

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A "non-demolition" measurement refers to a measurement in which the state of the system is left in the eigenstate corresponding to the eigenvalue observed. This is very easy to achieve with things like polarization and spin, but much harder for a general observable. For example, a measurement of photon number will usually absorb the photons you are trying to measure. A number of clever techniques have been developed in quantum optics in order to measure a general observable in a non-demolition way.