A Quantum Memory Storage Prototype

eldavojohn writes "An Australian National University project has completed a proof-of-concept storage unit that relies on bringing light to a standstill inside a crystal and then releasing it later for a read-once storage device. There are a few complexities to work out, such as the -270 degrees Celsius requirement to stop the light. And there is an interesting side effect noted by the team lead: 'We could entangle the quantum state of two memories, that is, two crystals. According to quantum mechanics, reading out one memory will instantly alter what is stored in the other, no matter how large the distance between them. According to relativity, the way time passes for one memory is affected by how it moves. With a good quantum memory, an experiment to measure how these fundamental effects interact could be as simple as putting one crystal in the back of my car and going for a drive.' Hopefully this will lead to a better understanding and simple testing of quantum entanglement."

Comments

So, no storage, but instant transmission?

[stanlyb]

From you explanation, this device is more like a transmission device, not a memory device? Even better, if you ask me. I am beginning to dream about some fancy, quantum cell phone, untraceable, and extremely secure.

Re:So, no storage, but instant transmission?

[Locke2005]

Also extremely heavy due to the liquid nitrogen cooling requirement, and point-to-point use only. Makes it more of a big walkie talkie than a cell phone.

Quantum communication?

[Locke2005]

Spooky action at a distance still seems fundamentally wrong to me. At what speed does information propagate between the entangled particles?

Re:Quantum communication?

[TexVex]

According to the theory here if I modify Side A, Side B must also change right?

No. That's what the GP's post said multiple times.

Quantum entanglement's strangeness is all about how observations of entangled particles correlate in a way that defies explanation without resorting to time travel, faster-than-light exchange of information, reverse causality, or a deterministic universe.

First, we would have to be making continuous measurements of Side B and placing them into a buffer.

You cannot observe a quantum thing without changing it. This means all subsequent measurements are invalid. Example: you observe the polarization of a photon by passing it through a polarizer and seeing if it hits a detector on the other side. This happens 50% of the time. The process involves the destruction of the photon either way, because it's either absorbed by the polarizer, or by the detector. Boom, no more photon.

Now, interestingly enough, if you put a second polarizer between the first polarizer and the detector, and you orient the second polarizer at exactly the same angle as the first, ythe chances of each individual photon reaching the detector are still 50%. Why? Because after a photon makes it through the first polarizer, it will subsequently pass through any polarizer oriented the same way. The photon has been altered in such a way that it is now polarized to that exact angle. This effect is classic physics, observable to the naked eye with two pieces of polarized glass.

At the quantum level, the results are the same even if you suspend an individual atom and observe its spin with magnetic fields. The first time you detect the atom's spin, its spin becomes aligned with the angle you tested for, and subsequent measurements at that same angle will always give the same result. Your observation changed the object you measured.

It is not possible to observe the precise angle of a photon's polarization. You can only try to pass it through a polarizer and see if it passes through or not. It's the same for spins -- you can test for "up" or "down" along any axis you choose, but you cannot discover any particular axis along which the particle can be said to be spinning. This is the nature of quantum mechanics: it's the place where the math of the universe becomes integral; there are no real numbers there, only whole ones. The Planck length is 1 unit of distance that cannot be subdivided. Photon wavelengths are always in even multiples of the Planck length. Spin is either up or down. Polarization is either "yes" or "no" for any particular angle (or "clockwise" or "counter-clockwise" for circular polarization filters).

According to you, and many interpretations I have read on Slashdot

The problem is, science reporting tends to sensationalize (because dry science is boring to the layman) and journalists tend to misunderstand (because they're writers, not rocket scientists, god dammit Jim!). In the jargon of quantum mechanics, it is understood that to observe something is to change it, and that to determine something is to learn it (not to cause it). So, by simple misunderstanding of the language, it gets reported that by changing one particle (by observing it) you can change its entangled mate (by determining one of its properties by inference). That is quite the sensational misunderstanding! And the fallacy gets repeated until so many believe it that it becomes self-sustaining.

Re:FTL communications

[TexVex]

Because you cannot both entangle the two photons and store information in them at the same time. Entangled quantum particles are by definition in a "superposition of states", which basically means that their values are essentially random when observed.

Storing information in a quantum particle requires observing it, to wrangle it into a desired non-random state. Observation destroys entanglement, because an observed particle is no longer in a superposition of states. Entangling quantum particles requires re-superposing their states. Creation of entanglement destroys information.

So, a pair of these quantum memory cells can store only one of the three following:
1> The same information
2> Unrelated information
3> Entanglement (which is unknown randomness that is correlated between the two cells)

The "spooky-action-at-a-distance" thing is in how the observations of separated but entangled quantum systems correlate. It's weirder than it seems on the surface -- read up on what a Bell Inequality is. That's where the strangeness is; because separate observation of entangled pairs of particles correlates more than is possible by the rules of classical physics and the rules of math and logic.

Quantum Entanglement does not "transfer" anything!

[CyberBill]

I am so sick of news reports claiming that if you alter one entangled particle, that the other entangled particle is affected too - like if you push one, the other one moves. IT DOESN'T!

What happens is if you measure the state of one particle, and then you measure the state of the other particle, they are always equal (or opposites, depending on the entanglement type).

Think of it this way... You have a CD burner that burns two CDs at the same time and puts random data on both, but the random data is identical. Obviously, no matter how far away the CDs are, if you read them, they contain the same information. There is absolutely zero information transfer going on here!

Serious question: Faster than light communication?

[RichiH]

One thing I always wondered:

Let's say I have a total of 1024 entangled pairs; well contained and stable. Now, I take the one half of those pairs and transport them somewhere else. Then, I proceed to measure the state of them _or not_. When checking the other half, shouldn't I get a total of 1024 "altered" and "unaltered" read-outs, resulting in the transmission of 128 bytes?

Granted, it's still hard to do all this, but afaik, what I just described is FTL transmission of actually useful data.

As I am sure there is some pitfall with which the quantum theories foil FTL plans (they seem to do that pretty reliably), I am eager to learn what trick those pesky laws of physics will pull out of their, admittedly tiny, hat, this time.