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Crystal memory allows efficient storage of quantum information in light

The views expressed are those of the author and are not necessarily those of Scientific American.


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Rare-earth doped crystal for efficient quantum memoryLight makes for a terrific carrier of information—witness the prevalence of fiber optics in telecommunications—and the realm of quantum communication is no different. Photons are key quantum objects that can carry information over large distances and that can be entangled in relatively large numbers.

But photons are a hyperactive lot, zipping around at light speed, making the stationary storage of the information they carry a challenge. One cannot simply encode the photonic information in a standard electronic memory—any measurement of a quantum object destroys some of the information contained therein, thereby casting aside the benefits of quantum communication and computation.

Researchers have devised several methods to park the information stored in photons, by encoding the quantum state of the photons into the quantum state of atoms, which can be more easily held in place. But those systems tend to be woefully spotty in their ability to store and recall information. Now a study in the June 24 issue of Nature presents a quantum memory that can store photonic information in a crystal with efficiencies of up to 69 percent. (Scientific American is part of Nature Publishing Group.)

The study’s authors used a yttrium orthosilicate (Y2SiO5) crystal doped with the rare earth element praseodymium, carefully tailored to have very specific absorption properties, as a quantum memory. Cooled to just three degrees Kelvin and held in an electric field, the crystal memory absorbs a pulse of laser light. But when the external electric field is switched, the memory produces an "echo" of the original photons, their quantum state intact.

"Light entering the crystal is slowed all the way to a stop, where it remains until we let it go again," lead study author Morgan Hedges, a student at the Australian National University’s Laser Physics Center, said in a prepared statement. "When we do let it go, we get out essentially everything that went in as a three-dimensional hologram, accurate right down to the last photon."

The storage times demonstrated in the new research are rather short—measured in millionths of a second—but the researchers predict that even more efficient memories should be feasible, with storage times measured in seconds.

Photo credit: ANU





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  1. 1. ildenizen 4:01 pm 06/29/2010

    I am confused. On one hand we are given that there is up to 69% efficienty, then at the end we are told that "we get out essentially everything that went in as a three-dimensional hologram, accurate right down to the last photon".
    So where is the loss off efficiency?

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  2. 2. jtdwyer 7:23 pm 06/29/2010

    As I understand, physicists’ references to quantum information should not be confused with intellectual information understandable by human earthlings. As I understand, no real world information, such as any of the words used in this comment, have been encoded and decoded using quantum information technologies, which refer only to the state of the characteristic properties of fundamental particles, perhaps encoded onto atoms. Of course, use of quantum information encoding of useful data is the eventual objective of this research which may or may not someday be achieved. Anyone please correct me if I’ve misstated anything.

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  3. 3. MEscribe 9:37 pm 06/29/2010

    I think you might be right.
    Nowhere in this article does it mention the ability to "write" to the quantum states of a group of photons. One of the links discusses the use of photons as "quantum keys" for unbreakable encryption, but that is based on the photons’ states being random. I suppose this storage method could be used for those?

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  4. 4. jtdwyer 5:21 am 06/30/2010

    MEscribe – Very good. I had missed the issue of quantum ‘information’ being random. Some mechanism would be required to control or at least select necessary states for useful data encoding.

    Reading a little further, it seem that ildenizen’s confusion is produced by the terminology used. The term ‘efficiency’ seems to be used here to to indicate reliability. The primary issue seems to be the ability to reliably retrieve the same information that was stored – an absolutely crucial requirement for any data storage use.

    I think you’re correct – random key generation may seem be the only application quantum information technology is suited to. However, while the remote entanglement (or more simply ‘mirroring’) of information might seem to allow private generation of secure access keys, if they cannot be reliably stored and retrieved they are worthless. Who wants a bank account that’s so secret nobody can access its contents?

    Principal benefits envisioned for quantum computing include increased information density and processing speeds. If a supercooled gaseous containment device is required for the physical storage of individual information bits, these benefits may not be practically attainable.

    This could take a while – I wouldn’t get overly excited about this research yet, but that’s just my opinion. Hopefully this issue of storage reliability isn’t a fundamental property of this nasty quantum uncertainty principle. That would be a real problem!

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  5. 5. sparcboy 1:31 pm 06/30/2010

    "Cooled to just three degrees Kelvin "…I guess when there is a device in a PC that can maintain a crystal at these temps, this will be practical.

    I remember reading about 20 years ago that scientist were trying to store information within the lattices of lithium niobate crystals.

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  6. 6. jack.123 6:52 pm 06/30/2010

    I had an idea some years ago of storing photons on a coil of fiber optic with a amplifier repeater connected between each end of the coil thus the information would be going around in a loop.Could someone please tell me why this simple storage device wouldn’t work?Could someone please,as well tell me how much information could be stored on a 100 mile coil?

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  7. 7. morgatron 10:11 pm 06/30/2010

    ~70% efficiency can be interpreted in a few ways. Just thinking of a ‘number of photons’, 70% of those that went in come out again- but the ones that do are exactly the same as the ones that went in. Compare that to a video camera- a few % of the photons that go in are actually detected, and a good deal of the information that describes those photons (in particular, the ‘quantum information’) is lost. You don’t notice though- when you replay the video, the device makes up for it- efor every photon it detected, your TV puts out 100, so the brightness is the same, even though the very fine detail is lost.

    The term ‘quantum information’ is a funny one though, and unfortuneatly depends a bit on one’s interpretation of quantum mechanics. It’s certainly possible to prepare things (eg. photons) in a definite quantum state, such that you can write down everything about it. But, if you have only a single unknown quantum state, you can’t access all that information. The best you can do is make a choice about which measurement you’ll make, and thus which information you’ll extract- in the process destroying other information. So the quantum state of an object is real, it interacts with the quantum state of other objects, and knowing what it is let’s you predict the outcomes of experiments. But generally for it to be of use for information processing, the only measurement must take place at the end.

    I hope that makes sense.

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  8. 8. jtdwyer 10:55 am 07/1/2010

    morgatron – If your employer lost 30% of the digits on you paycheck amount and the automatic deposit account number the impact would be highly significant to you.

    To give you some idea of the functional performance capabilities of this new device, the referenced ‘prepared statement’ stated:

    "Dr Sellars’ team has previously performed an experiment that `stopped’ light in a crystal for over a second, more than 1,000 times longer than was previously possible."

    I don’t think they’ve gotten around to the fundamental issues involved with quantum memory refresh yet.

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  9. 9. Quinn the Eskimo 11:35 pm 07/2/2010

    Random storage of random data. Encrypted.

    So, what is it for?

    And who would know?

    Or did I miss something, again.

    Link to this

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