Optical computer

An optical computer is a computer that uses light instead of electricity (i.e. photons rather than electrons) to manipulate, store and transmit data. Photons have fundamentally different physical properties than electrons, and researchers have attempted to make use of these properties, mostly using the basic principles of photosynthesis, to produce computers with performance and/or capabilities greater than those of electronic computers. Optical computer technology is still in the early stages: functional optical computers have been built in the laboratory, but none have progressed past the prototype stage.

Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. This approach appears to offer the best short-term prospects for commercial optical computing, since optical components could be integrated into traditional computers to produce an optical/electronic hybrid. Other research projects take a non-traditional approach, attempting to develop entirely new methods of computing that are not physically possible with electronics.

Optical components for binary digital computer
The fundamental building block of modern electronic computers is the transistor. To replace electronic components with optical ones, an equivalent "optical transistor" is required. This is achieved using materials with a non-linear refractive index. In particular, materials exist where the intensity of incoming light affects the intensity of the light transmitted through the material in a similar manner to the voltage response of an electronic transistor. This "optical transistor" effect is used to create logic gates, which in turn are assembled into the higher level components of the computer's CPU.

The problem with light-based transistors is that they need very large amounts of energy to operate and hence tend to be rather large in size. However, recently, a team at Queen's University in Belfast came up with a solution to this particular problem. Their technique provided a possible way to get two individual light beams to interact with each other in a way similar to how electrons interact in an electronic transistor. Now when a signal light beam is focused on a metal (in this case a gold film 220nm thick), free electrons are created which are called 'plasmons'. The plasmon changes depending on the intensity of the light. Using a second 'control light beam' at 45 degrees to the gold film, we can alter the state of the plasmon which in turn will affect the effect of the signal beam. The plasmon will then release energy in the form of light which acts at the resultant of the two initial light beams. This way we are able to get two separate sources of light to interact and produce a resultant light beam.

Misconceptions, challenges and prospects
Another claimed advantage of optics is that it can reduce power consumption, but an optical communication system will typically use more power over short distances than an electronic one. This is because the shot noise of an optical communication channel is greater than the thermal noise of an electrical channel which, from information theory, means that we require more signal power to achieve the same data capacity. However, over longer distances and at greater data rates the loss in electrical lines is sufficiently large that optical communications will comparatively use a lower amount of power. **As communication data rates rise, this distance becomes shorter and so the prospect of using optics in computing systems becomes more practical.**

A significant challenge to optical computing is that computation is a nonlinear process in which multiple signals must interact to compute the answer. Light, which is an electromagnetic wave, can only interact with another electromagnetic wave in the presence of electrons in a material and the strength of this interaction is much weaker for electromagnetic wave light than for the electronic signals in a conventional computer. This results in the processing elements for an optical computer requiring high powers and larger dimensions than for a conventional electronic computer using transistors.