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Quantum Computing With Holograms

The US Air Force is developing simple but reliable quantum computers that can be built with off-the-shelf components

Light is one of the most promising carriers of quantum information. It is robust against decoherence because it does not interact with stray electric and magnetic fields and passes unscathed through transparent matter.

But this prized robustness is also a serious limitation. Photons do not easily interact with each other so processing the information they carry is tricky.

In recent years, however, physicists have worked out how to make photons interact using interferometers and to carry out quantum computations using the output of one interferometer as the input for another.

The trouble is that interferometers are notoriously fickle. Sneeze and they need re-calibrating. So cascades of them tend to be hard to handle.

Today, Jonathan McDonald at the Air Force Research Laboratory in Rome New York, and a few pals reveal a way round this problem.

Their idea is to make holograms of interferometers so that their properties become ‘frozen’ in glass. This makes them much more stable.

The researchers then plan to stack the interferometers to perform simple quantum computations. “The approach here will “lock” these interferometers within a tempered piece of glass that is resistant to environmental factors,” they say.

MacDonald and co suggest using a commercial holographic material called OptiGrate to store these holograms and show how these devices could carry out simple tasks such as quantum teleportation and CNOT logic.

There are two serious limitations to this approach, however. First, these devices are not scalable. The reason is that a hologram requires a certain volume of space to carry out each computation with high fidelity. And since computations scale exponentially in quantum computers, so must the volume.

Second, these devices are not reprogrammable, at least not with today’s technology. The reason is that OptiGrate is a write-once material. Re-recordable holographic media are available but not currently with the fidelity that allows this kind of work though clearly that could change in future.

Given these limitations it’s easy to dismiss this idea as just another of a growing number of exotic forms of quantum computation that are gathering dust on (metaphorical) library shelves.

But there are a number of emerging applications for the kind of reliable but low-dimensional quantum computations that these devices could perform. These include quantum memory buses, quantum error correction circuits and quantum key distribution relays.

For the moment, no technology does these jobs reliably well, although there are many pretenders for this crown. The difference with McDonald and co’s idea is that it ought to be possible to build these devices now with off-the-shelf technology.

In fact, it wouldn’t be surprising if this paper was a forerunner for practical work being done to develop prototypes. We’ll be watching!

Prototypes will be important for ironing out a number of practical question marks about this approach. For example, these holograms will have to be stacked to carry out even simple quantum computations. But nobody is quite sure whether this output from one hologram can be accurately channelled into the input for another.

There’s only one way to find out.

Ref: arxiv.org/abs/1112.3489: Quantum Computing In A Piece Of Glass

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