Multiplying light signals could give optical computers a boost


Optical multiplication: a new technique for combining light signals could give optical computers a boost. (Courtesy: iStock/7io)

Researchers in Russia and the UK have proposed a new and simple way to produce binary output signals in the logic gates of optical computers. Developed by Nikita Stroev at the Skolkovo Institute of Science and Technology in Russia and Natalia Berloff at the UK’s University of Cambridge the technique involves multiplying the input signals to the gates, instead of adding them linearly. With further improvements, their approach could drastically reduce the number of light signals required for optical computers to operate, improving their potential for complex problem solving.

Optical computers are emerging solution to the limitations of conventional electronic devices. Not only could they enable information to travel much faster through their component circuits; they should also have a far lower energy consumption and allow information to be processed in new ways that would be much more efficient at solving certain problems.

Instead of electrical signals, optical computers use the continuous phase of photons to encode and distribute binary information. A big challenge in building optical computers is that photons normally do not interact with each other, making it difficult to create logic gates. One way of making photons interact is to use materials with nonlinear refractive indices. When two or more input optical signals are combined in such materials, they interact with each other via the material’s electrons. By carefully engineering these interactions, researchers could build devices in which the two signals add together to deliver the desired output — and least in principle

Unpredictable phase

In practice, however, nonlinear materials can have unpredictable effects on the phases of output signals. To deal with this problem an external resonant excitation is introduced to set the phases of output photons to clear binary states, which is not an ideal solution.

Stroev and Berloff explored a more robust approach in their study. Instead of adding the input signals linearly, they combined them by multiplying their wave functions. Under appropriate conditions, the researchers calculate that the phases of each coupled signal changed to reach a minimum energy configuration. This produced an output signal with a phase clearly associated with either a 0 or a 1, without any need for additional signals.

The duo’s model system used polaritons: quasiparticles that form through a strong coupling between light and matter, giving them hybrid physical properties. In their design, they multiplied the wavefunctions of coherent, superfast polariton pulses, guiding them towards the correct output phase by temporarily altering their coupling strengths.

The inherent noise in the signals means that further improvements will be needed before the system can be integrated into the large-scale production of optical computers. However, the early success of Stroev and Berloff’s approach reveals a promising new route towards superfast, real-world problem solving, in areas too complex for conventional computers to handle.

The research is described in Physical Review Letters.

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