Using quantum entanglement to stack light particles: Physicists play Lego with photons

While many of us enjoyed constructing little houses out of toy bricks when we were kids, this task is much more difficult if bricks are elementary particles. It is even harder if these are particles of light - photons, which can only exist while flying at an incredible speed and vanish if they touch anything.

A team at the University of Calgary has accomplished exactly that: by manipulating a mysterious quantum property of light known as entanglement, they are able to mount up to two photons on top of one another to construct a variety of quantum states of light - that is, build two-story quantum toy houses of any style and architecture.

The results of their research, written in the paper 'Quantum-optical state engineering up to the two-photon level', will be published on Nature Photonics's website on Feb. 14.

"This ability to prepare or control complex quantum objects is considered the holy grail of quantum science" says Andrew MacRae, a co-author of the paper and PhD physics student at the U of C. "It brings us closer to the onset of the new era of technology."

This new generation of technology is expected to endow us with qualitatively new capabilities. This includes measurement instruments of extraordinary sensitivity, dramatically faster computers, secure communication systems and enhanced control over chemical reactions.

"Light is a particularly interesting quantum object," says paper author Alexander Lvovsky, a professor in the Department of Physics and Astronomy, "because it's an excellent communication tool. No matter what future quantum computers will be made of, they'll talk to each other using photons."

U of C researchers used mirrors and lenses to focus a blue laser beam into a specialized crystal. This crystal takes high energy blue photons and converts them into a of lower energy red photons, which emerge in two directions, or 'channels'. By measuring one of the channels using ultra-sensitive single detectors, the physicists prepare the desired in the other.

Such an operation is possible because the photons in the two channels are entangled: a measurement made in one channel would result in an immediate change in the other, regardless of whether the particles were an arm's length apart or light years away. Albert Einstein called this quantum weirdness "spooky action at a distance."

"Quantum is like an ocean," says Lvovsky, "and it's full of mysteries and treasures. Our task is to conquer it. But so far, physicists were able to control only a tiny island in this ocean. What we have done is to make this island bigger."

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Feb 14, 2010
Am I daft or (false dichotomy) is this just ordinary creation of entangled photons? What stacking is involved - even metaphorically? What is novel here? I'm no physicist and my reading comprehension skills are not perfect but I've been over this twice and I can't see what I'm missing. Maybe it will jump out at me when I have posted this. That's usually what happens.

Feb 14, 2010
What they're talking about is similar to the 'communications device in Stargate', if you're familiar with it. When you break apart entangled particles, they are mirror images of each other despite not being entangled anymore. So you could potentially excite one half of this previously entangled particle and the other half (wherever it is) would share the same energy level. Theoretically, you could have arrays of these particles and use them with a binary protocol for computers or communications or what not. Instant transmission of as much data as you want.

Feb 15, 2010
Sean_W, this is just another example of quantum physics news gone bad. First off, the article comes from a university that has a "Physics and Astronomy" department. They might as well throw in chemistry and biology, while they're at it.

I'd wager the interviewing and writing was done by an over-eager journalism major of the same uni, or something. The interviewees must've thought the article would end up in some teen magazine, otherwise I can't imagine why they'd talk the way they did.

As usual, the only salient bit of information is contained in one paragraph, the rest being clichee QM explanations&metaphors. As far as I can tell, the real novelty is that "specialised" crystal. Of which nothing is said.

This particular piece does raise the bar by quoting Einstein out of context, though.

Feb 15, 2010
Bloodoflamb has it right: You can't alter one part of an entangled pair and expect the other to alter also.

What you can do is _measure_ the property of one and the be confident that the other will have that property, too. But since you don't know what state the property is in before you measure it (obviously) you can't use this for information transmission.

Feb 15, 2010
Sean_W, this is just another example of quantum physics news gone bad.

I thought that might be the case but I was not prepared to be the one to call it. Thanks.

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