optical computing

According to Albert Einstein, nothing travels faster than light and, unlike the electrons in an electronic circuit, the passage of photons doesn't generate heat. Light is faster and cooler than the electronic equivalent, so surely an all optical computer would provide the best of both worlds?

It will come as no surprise that research into optical computers has a long heritage, but so far there's been little to show for that work – until recently, as Professor Vahid Sandoghdar of the Max Planck Institute for the Science of Light explained.

Small problems

We started by asking why progress has been so slow. "One of the main challenges in making competitive optical computers is miniaturisation," he told us. "While an electronic wire can be as narrow as a few tenths of a nanometer, an optical wire – a waveguide – is much thicker. In addition, no one has managed to transmit light around sharp bends. Another major difficulty is the miniaturisation of elements like transistors."

Referring to work he started while at the ETH Research Centre in Zurich, Professor Sandoghdar suggested that all this could be about to change. "Our work has shown that a single quantum emitter like a single molecule can function as a transistor because its interaction with light is intrinsically non-linear.

"However, packaging is still an issue because you can easily perturb the optical fields around the emitter if you bring anything else too close to it. Furthermore, our system has the practical problem that it works at liquid helium temperature. In all these efforts, our goal is to optimise the efficiency of the interaction between photons and emitters. This means that losses become less important."

Atoms as memory

Optical silicon

But there's another key element, as Sandoghdar reminded us. "Optical memories, at least for transitory purposes, are of course also important, but I could imagine that you could use micro-resonators for times as long as microsecond. But again, these things are huge compared to their equivalent electronic components. Here it's thinkable to use single atoms, ions, or colour centres as memory in a similar fashion as we have demonstrated transistor and phase shifting functionalities."

So does this mean that the all-optical computer is on the way? "I certainly believe that it should be manageable to build an optical computer one day," Professor Sandoghdar said, "but I really can't say anything about its size and cost. To that end, it's hard to believe my laptop would be replaced by such a device."

On the other hand, a recent IBM development suggests a halfway solution might be closer. Its so-called silicon nanophotonic chip maintains silicon for switching, but connects everything together using high-speed optical links.

Where common sense ends

D-Wave

Although particles like electrons and photons behave in what we think of as a common sense fashion en mass, individually they obey the laws of quantum physics, which are counter-intuitive.

Qubits explained

For example, electrons have a property called spin, which can be up or down, but a single electron can also have an up and a down spin at the same time. This is called a state of superposition, and it forms the basis of quantum computing.

If you use electrons to represent a binary data, they can represent either a 0 or a 1, or they could be in superposition. Because of this latter possibility, the term 'qubit' (quantum bit) is used instead of the more familiar 'bit'. Now let's imagine a 64-qubit computer.

Because each qubit can represent both a 0 and a 1 simultaneously, each 64-bit register is capable of holding 18,446,744,073,709,551,616 different values at the same time. If you were to carry out some computation, it would therefore be carried out on all those values at once.

This might be the ultimate in terms of parallelism, but the technique isn't without its problems. First you have to work out some pretty clever algorithms, because as soon as you try to read the result of any computation the state of superposition collapses and you end up seeing just one result. Much the same problem makes it rather tricky to even build a quantum computer.

Absolutely any interaction with the outside world destroys the superposition and preventing this from happening gets more difficult each time you add another qubit to the register width. For this reason, most claims from academia have involved modest numbers of qubits, demonstrations often involving fewer than eight.

Despite this, a company called D-Wave Systems launched the world's first commercially available quantum computer earlier this year. Called the D-Wave One, it boasts 128 qubits, but many in the research community are sceptical.