Photon Pair Coupled in Glass Fiber

Trachman writes: Austrian scientists have discovered a way to couple photon pairs. When two identical photons are coupled and the phase of one is changed, then thanks to the magic of quantum mechanics, the phase of the other photon also changes (abstract). Scientists predict this can advance quantum optics and quantum computations, taking us a step closer to having data transmissions secure from the nosy agencies of the world.

If any of you have expertise in this area, could you share your thoughts on the essence of this discovery and its associated potential practical applications?

Comments

Re:News for nerds!?!

[Anonymous Coward]

http://arxiv.org/pdf/1403.1860.pdf

The Summary is Wrong: Photon AND Gate?

[blavallee]

According to the article, "When both hit the resonator at the same time, both of them together experience a phase shift by 180 degrees."
It's not advancement in quantum communications, it's an advancement in quantum computation.

The potential practical application... it resembles an AND logic gate function, with photons!

Media and the Copenhagen interpretation

[amaurea]

Only 42% of quantum physicisists would agree with the statement in the summary that "When two identical photons are coupled and the phase of one is changed, then thanks to the magic of quantum mechanics, the phase of the other photon also changes", and 40% of them would actively disagree. While the mathematics and measurement predictions of quantum mechanics is quite uncontroversial, the interpretation beyond that is a topic of much debate (much of which belongs in philosopy rather than physics).

The summary is using one such metaphysical interpretation, called the Copenhagen interpretation, which has more "magic" than most (spooky, faster-than-light action at a distance; wavefunctions that collapse when I, the Observer, looks at them, but not when anyone else does), and might be the most confusing one to the public (though admittedly, all the interpretations are confusing to some extent).

Re:Media and the Copenhagen interpretation

[amaurea]

I think it is important to distinguish between three things here. The theory (the equations and predictions of measurements), our interpretations of the theory (what picture of the world we associate with the theory), and the real world itself.

For example, let's say that you need a theory for describing how a hypothetical time-ray works. The observed effect is that physical processes of whatever it is shot at occur at twice the rate as before compared to the rest of the world. This is straightforward to measure and can be modeled exactly using equations. But how should we interpret what happens? One interpretation is that the time-ray speeds up the passing of time for the object it hits. But another, equivalent interpretation is that the ray slows down the passing of time for the entire rest of the world, and protects only the target object from the effect. These interpretations both lead to the same observations, since all we can observe are the *relative* rate of events, but they make different claims about what actually happens. In this case one interpretation is clearly more appealing because it is simpler, but no experiment could distinguish between them. So in some sense the distinction is meaningless.

Similarly, the theory of general relativity, which is our modern description of gravity, can be interpreted as spacetime being curved by the presence of energy, and the curvature affecting the paths of objects. But it is also possible to interpret it as spacetime being flat, but filled with a field of self-interacting, massless, spin-2 particles (gravitons). Both these pictures lead to the same predictions, so in that sense they are the same theory. But they are clearly very different descriptions of reality.

The point of making the distinction between theory and interpretation is that the former can be tested, while the latter can't. The theory of general relativity has been put through a huge number of tests, and it has held up under all of them. Like most theories of fundamental physics it has been tested to exquisite precision, and if it is wrong, it has to be wrong in a very subtle way. But the interpretation of general relativity can't be tested at all. Which one to use is a bit like choosing whether to use a cartesian or polar coordinate system in maths. One might be easier to use or prettier in some situations, but they give exactly the same results.

The same applies to quantum physics to perhaps an even greater extent. Quantum Electrodynamics, one of the building blocks of the standard model of particle physics, may be the most precistly tested theory in science. The archetypical example is the anomalous dipole moment which is correctly predicted to 14 decimal places (all the ones we could measure so far). So the theory part of quantum physics is trustworthy. It may not be 100% correct, but it is pretty damned close. But there is a plethora of interpretations of quantum physics, and these are completely uncertain - we can't tell them apart because they are mathematically equivalent and hence all make the same predictions. Each one corresponds to a different real world, but we can't tell which one it is.

Electrons bound to atoms are relatively simple quantum systems, and I don't think our ability to measure them is the limiting factor. It sounds like you are arguing for a Hidden Variables description of the electron, where a point-like electron moves around the nucleus in an well-defined particle orbit like a planet, and it only looks like it's this complex non-local wavefunction (electron cloud) because it moves to quicky for us to resolve its actual orbit. The good news is that it it is possible to interpret standard quantum physics that way

Re:Naive optimism in headline

[Rei]

Here's how collapse of the waveform works. If you take a measurement, you will get a value.

1. The value you get is completely random to you.
2. You cannot in any way choose the value.
3. You cannot know, by reading it, if anyone else has already collapsed the waveform, or if the value you're getting is new.
4. If someone does collapse the waveform, however, when the other side tries to measure it, they'll get the same value - instantaneously.

The problem with trying to use this as some sort of instantaneous information teleportation system is that while it is instantaneous, it's not sending information. Your reading it does not give any information to the other side. You don't choose the value and they can't tell that you've read it. All they get is random noise.

It is, however, potentially valuable for cryptography, in that you can simultaneously generate the same one-time pad in two locations without any snoopable channel, which you can then use to encode or decode data. The data still has to be sent by conventional means - as mentioned above, you're not sending any information by measuring quantum states, the other side has no clue what you've done or not done - but the pad itself is perfectly random and unsnoopable.