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ScienceWise - Summer 2012

Total recall

Article Illustration
Above: HAL, the thinking computer in Stanley Kubrick’s science fiction classic 2001, employed a holographic memory. Below: A real working holographic quantum memory crystal

Holographic quantum memory becomes science-fact

Quantum mechanics is possibly one of the strangest areas of physics.  Central to the theory is the notion of wavefunctions; mathematical functions that describe the probability of a particle being in a particular position at a particular time. But it’s not like there being a 50:50 chance of your car being in the garage on a Sunday afternoon, because the car will either be there or it won’t. In the quantum world the car would exist at every possible place including the garage until someone looked for it. Then in effect, the universe would make up its mind and decide, yes it’s in the garage or no it isn’t.

Of course you could try to cheat by looking if the car keys were on their peg. Then you could infer that it must be in the garage without actually making any measurement on the car itself. The strange thing is that in the quantum world, the act of measuring the keys would also instantly collapse the car’s wavefunction into a definite value no matter how far away it was. Physicists call this entanglement. A situation where the wavefunctions of two things are inherently linked so that a measurement made on one instantly affects the other.

Entanglement is an essential part of the concept of quantum computing. In an electronic computer such as the one I’m writing this article on, each of my letters is stored as electrons in transistors in the RAM. As long as when requested, the RAM gives back electrons from that location, it really doesn’t matter if they’re the same ones that went in or what their spin is. Just like money in the bank. You don’t have to get the exact same dollar coins back that you deposited so long as you get back the right number of coins.

With a quantum computer that isn’t the case. The extraordinary computational power that’s potentially available hinges on the preservation of every detail of the quantum particles as they’re stored and processed. An electron or photon submitted to the memory must be given back in an identical form, including any entanglement that it has with any other system.

Whilst in principle a quantum computer can be built using a variety of architectures based around electrons, atoms or photons of light, some very promising results have come from light based systems. Back in 2009, an ANU team lead by Dr Matt Sellars, were the first to demonstrate a working two qubit logic gate based on photons of light interacting with a crystal lattice. Since then the group have been developing a number of crucial quantum computer components, the most recent of which is a holographic memory again based on a perfect single crystal.

In effect when a photon of light enters the crystal it interacts with deliberately added impurity centres in the form of praseodymium ions. The light is absorbed and its information transferred to the spin states of the particular ion that did the absorption. But because any of the ions within the crystal lattice could have done the absorption and you don’t know which one, in effect all the ions in the crystal are now entangled.

But how do you then play back the photon? The key comes from the fact that the laser used to generate the “write” beam has a very, very narrow range of wavelengths, even by monochromatic laser standards. In fact the spectral range is so narrow that the chance of hitting the equally narrow absorption of a given ion is virtually zero. To get round this the scientists apply an electric field to the crystal. Atomic transitions shift in energy slightly when an electric field is applied, a phenomena known as the Stark effect. Because there are trillions of individual ions in the sugar cube sized crystal and because the applied field varies across it, the ion absorption energies form a gradient. And somewhere within that lattice will be some ions with the perfect absorption to match the incoming photon.

If the field is now reversed, the absorption process is reversed and the trapped photon is effectively released with all its entanglements intact. The really clever thing about this system is that because the crystal is very cold, close to absolute zero, the excited ion states are very stable. This means that they can potentially hold photons for hours. That might not sound like a particularly long time, but it’s thousands of times better than can be achieved by any other method and plenty long enough for computational calculations.

“In effect what we have is a working holographic quantum memory” Dr Sellars explains, “It’s really just a matter of the technological aspects being perfected.”

Don’t expect to be seeing a quantum processor in your next laptop, but the technology required to build these remarkable machines is, step by step, becoming reality.

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