The control of a single photon – the elementary quantum carrier of light and all forms of electromagnetic radiation – is seen as the Holy Grail of quantum computing.
If researchers could effectively make a photon do the work of a switch, or a transistor, it would be a revolutionary achievement with regard to telecommunication and computer speeds. In its most elementary form, such a photon would be what is called an optical switch – it would enable signals traveling in optical fibers, or integrated optical circuits, to be effortlessly and speedily switched from one circuit to another.
In the case of a photon doing the work of a transistor, the same principle applies, but a transistor not only starts and stops electronic signals (as a switch), it also controls the degree or amplification of signals as they pass through the system. We are all familiar with silicon-based transistors and semiconductors that regulate electrical conductivity in varying degrees, but these multi-layered components pale in comparison to the speed at which a singular photon could accomplish the same job.
Another drag on electronic systems deals with the issue of resistance, or the hindrance encountered during the flow of electrons from terminal to terminal, as in a circuit board on a computer. With the employment of optical fibers, resistance is greatly reduced, as compared to the performance of copper wires, for example. The signals within optical fibers can be re-routed by electronic switches or controlled by electronic transistors. Better still, these signals can be controlled by photonic switches that can switch wavelengths or signals within a given fiber. An optical switch that can be turned on by a single photon could very well result in radically new designs for future computers – especially quantum computers.
As such, optical computing – using light rather than electricity to perform calculations – could pay dividends for both conventional computers and quantum computers, which at this point are largely hypothetical devices that could perform computations exponentially faster than classical computers. For the United States Air Force, the ability to process data exponentially faster would be a boon for communications, target acquisition and cryptography, to name but a few.
There are hurdles to be overcome though, and the Air Force Office of Scientific Research (AFOSR) has been funding a research group that is attempting to make a photon do the opposite of what it does in nature. Purely optical computing requires that light particles – photons – be required to initiate and control the actions of other photons. But in nature, photons are not designed to do this–quite the opposite, in fact, as they ignore each other by simply passing through one another when traveling in a vacuum.
In an article published last month in the journal Science, AFOSR-supported researchers at MIT’s Research Laboratory of Electronics, along with team members at Harvard University and the Vienna University of Technology, have demonstrated an optical switch that is controlled by a single photon–light controlling light. And as described above, the result is an optical version of a transistor, the fundamental component of a computing circuit.
Without getting too technical, the optical switch is produced, and controlled, by two mirrors, which effectively act as an optical resonator producing an electromagnetic field in a cavity between the two mirrors. The cavity between the two mirrors is then filled with a gas of supercooled cesium atoms. By themselves, this gas cloud of atoms would not interfere with the light passing through the mirrors, but the key to making this cloud an optical switch occurs when a single photon is fired into the cesium atoms at a different angle than the light source. This “gate photon” proceeds to kick one electron of one cesium atom into a higher energy state, and thereby changing the physics of the cavity so that light can no longer pass through it, thus creating an on/off switch.
What optical computing offers is a significant advantage in power management – far less power and far less heat compared to electricity driven circuits. But the far more advantageous application is in the quantum computing realm is the photon’s innate ability to maintain its superposition ability.
While pure optical quantum computing is a long way off, the single photon switch is a significant achievement to build on.