What is the difference between entangled and normal photons?

I think there is no test that can distinguish between the two, therefore I'd say there is no difference. In fact, any photon has to be entangled with the system it originated from.

But I may be wrong, professional physicists on this forum should explain you better.

 

Well, simplistically, with two photons that are entangled, you can examine one's polarity and know that the other's polarity is opposite.

 

Dave is correct, somewhat, I think. If you determine a photon's polarity to be one way, and another photon's polarity to be completely out of phase, this does not necessarily mean that the two photons were entangled to begin with. So is there any way, without knowing the photons' source and without immensely separated detectors, to determine the difference between entangeld and unentangled photons with only one run of an experiment?

 

No. There is no objective property of a photon (or another particle) that makes it entangled vs. not. Entanglement is only about the relationship between two or more particles - the shared history.

 

If my understanding is correct, I think, in general, entanglement may apply to two particles/photons created in a process where conservation laws require certain attributes eg spin/polarization to be correlated.
Is it not true that a measurement which 'disturbs' the attribute of one particle also disturbs that of the other - even if they are separated beyong speed of light contact?

 

Yes, if two entangled particles are measured in the same way (in any direction, but in the same), they will show for example the same polarization, or the opposite spin.

If you have a single pair of particles which are not entangled, they might show the same behavior only by coincidence. However, if you measure many non-entangled pairs, they will display a very different average behavior.

As I understand, even if you somehow make sure, for example, that the non-entangled particles have an opposite spin (before you measure them), they will behave quite randomly when you measure them in a direction of 90 degrees, whereas the entangled pair will still always have opposite spin. Yet again, if you have a single pair, they might behave as-if-entangled by coincidence. I don't know whether quantum teleportation could provide a single pair test.

 

Since entanglement is neither all or nothing nor limited to pairs of particles, doesn't it stand to reason that every particle in the universe is in some way entangled with every other? But for the overwhelming majority, however, the entanglement is so attenuated due to entropy that it is not detectable?

The only thing all physicists can agree on (as my other thread demonstrated) is that the probability of observing a particle with an attribute A is correlated with the probability of observing an entangled particle with a complementary attribute A'. For polarized photons, this probability is always related to the cosine of the difference between polarizer angles. There are two possibilities - the photons "knew" at the outset what the polarizer angles would be, and agreed on the outcome, or the photons did not "know" until one or the other struck a polarizer, in which case no one really knows how or why the correlation occurs. One thing that is certain is that it cannot be a "classical" force that causes the correlation because such a force, if it existed, would have to ignore space and time entirely.

Perhaps it is the same "force" that causes 2+2 to equal 4?

 

Which means that for an angle difference of zero, the probability is one, so that appears to be the simplest case to explain, and the most "striking" effect, even though this case can't be used to disprove the more complex local-hidden-variable theories.

But it does provide the case where the difference is most visible, between an entangled pair of initially opposite spin, and a non-entangled pair of initially opposite spin, when looking at how a non-entangled pair actually behaves. (Rather than comparing to how an pseudo-entangled pair might theoretically behave according to some contrived hidden-variable theory).

The entangled pair, when measured in 90 degrees to the original spin (as one can test only by making many random runs), will still have opposite spin, whereas the non-entangle pair will have 50/50 opposite and same spin, even though the initial spin of both particles was opposite, as well.

 

Consider many pairs of correlated photons. If the "normal" photon-pairs are correlated by identical linear or circular polarization, they will not yield the same test results as the entangled pairs.

The measured correlation of the "normal" photon-pairs will be less. So there is a difference; Yes?

Perhaps we should therefore open up the possibility that each entangled pair shares identical spherical polarizations?? Could that be the difference?

 

Something to add here is that the property of being entangled can be transferred with quantum teleportation, and it can be extended using "entanglement-swapping" to photons that have no common source. This means that there is a property in a photon, the state of being entangled, which can be passed-on to other photons.

Also, the entanglement is always in regard to specific properties, for example photons can also be entangled regarding their impuls (as in the double-double-slit experiment).

 

Also, it is also possible to entangle e.g. two electrical circuits (solid state qubits) meaning entanglement is not a "property" of a system, whether or not two systems can be considered entangled only depends upon the relationship between.

 

What do you mean with "relationship" ? Where does it exist if not in "both" photons?

(Actually many particles can be entangled, not just two.) The largest distance for entanglement, that has been experimentally achieved, AFAIK, is 144 km (90 miles), and there is no theoretical limit. Would be difficult to imagine that there is some kind of physical connection covering that distance, especially since the effect is instantaneous.

 

I believe there is a discernible difference between an entangled photon and one that is not entangled. Entangled photons do not self-interfere when they are sent through a double slit set-up. This has been pointed out by Zeilinger and others.

 

Actually, that depends on the experimental set up, entangled photons can interfere, for example in an experiment which Zeilinger calls the double-double-split experiment.

In this experiment, a source of light is used which creates two entangled photons a time, which are entangled in regard to their impuls, and fly away form the source in opposite directions. If one positions a double-slit on both sides, the photons will produce a detectable interference pattern, although it takes a 'trick' to discover it.

One of the interesting things in this experiment is that if one tries to measure the path of the photons on one side, not only these photons (as in the simple double-slit experiment), but also the entangled photons will stop producing interference patterns.

[Edit 12/22/07:] Actually it might be more precise to say: The entangled photons will either stop producing interference, or there won't be any means to discover these patterns anymore. (Which is rather complex to understand).

 

That's very interesting! But why is that so?!?

 

When you discard information about the other particle, you are left with a statistical mixture. (mathematically, this amounts to taking a partial trace) Given a large collection of particles with identical states, you can distinguish between that state being a pure state and a mixed state.

(Of course, this does not tell you "why" the particle is in a mixed state; you simply know that it's not in a pure state)

 

There is no such thing as a photon that isnt entangled.

 

Who knows, perhaps not, however they can certainly be entangled in different ways.

 

Solid state qubits are usually entangled using capacitive or inductive coupling. Mathematically the Hamiltonian will have one term for each qubit+ at least one coupling term which depends on the strength of the coupling (i.e. the value of the effective capacitance/inductance) and the bias points of the qubits. Often the coupling strength can not be changed (because C or L is determined during fabrication) which means that whether or not the qubits are entangled is only determined by bias.
Hence, obviously there is nothing "special" about each qubit, the reason why it can become entangled is due to the coupling circuitry and the fact that there are other qubits it can couple to.

I prefer to "man made" systems as examples when disussing entanglement and related phenomena. Partly because it is what I work with, but also because photons are "strange" and it is easy to make the misstake of thinking that the effects are due to "propeties" of photons, whereas in fact is just basic QM valid for types many systems.
Optical photons are nice to work with because they do not interact much with the environment meaning the coherence times are usually very long, but there is no fundamental difference between a system of photons and e.g. a electrical circuit with a bunch of qubits in this case.

 

If you are saying that entanglement is not a property specific to photons, that is certainly correct, since entanglement isn't limited to photons. What I meant is that the entanglement must be anchored somewhere (or somehow).

As you seem to say, already existing particles can be entangled by some action (the example I know about ("entanglement-swapping") is the use of Bell measurements to entangle two entangled pairs of photons, where the Bell measurement is performed on two particles, one of each pair, resulting in the other two being entangled as well, even though they have never met and are not from the same source.). And also, the entanglement can be lost (often too easily).

So where in reality (unless it has some non-local existence, whatever that would be) might the state of entanglement be located? I don't know, but I wouldn't think that it might be in the physical space between the particles.

 

Look at the fig.1 in this paper:
http://arxiv.org/abs/cond-mat/0312332

(it is an old paper, but illustrates my point). In this experiment qubit A was entangled with qubit B (the same experiment can be done with more than 2 qubits). Now, it should be quite obvious that the entangled state is not "located" anywhere, the qubits do not move and are obviously not "delocalized" particles.
Entanglement simply means that the two subsystems (qubits) are coupled in such as a way that there are off diagonal terms in the total system Hamiltonian; nothing more.

 

I'm not familiar with the specific physics here. Which are the variables here, based on which you calculate the 'total system Hamiltonian' ? As I understand, entanglement is lost with decoherence of each respective subsystem, so I would assume that entanglement is a function of the specific coherent status of each subsytem, potentially involving some kind of non-local connection or wormhole-like modification of space, or something in that direction, generally and naively speaking. This would seem completely independent of the qubits moving, or not. With photons the entanglement can persist when they are moved very far apart, through glass fibers, so it doesn't seem to have anything to do with their relative positioning. I see the following possibilities for what contributes to entanglement:

(1.) local state(s) of each subsystem.
(2.) local state(s) in the physical space between them, or the relative positioning.
(3.) Non-local state(s) of an unknown kind.

Of these, (2.) seems most unlikely to me. That's all I'm saying.