Molecular transistor

Researchers in Switzerland have successfully created an optical transistor from a single molecule.

The development from ETH Zurich, the Swiss Federal Institute of Technology, has brought researchers one step closer to an optical computer.

Such a technology has been the goal of scientists who have been trying for years to find ways to produce integrated circuits that operate on the basis of photons instead of electrons. This is because photons do not only generate much less heat than electrons, but they also enable considerably higher data transfer rates.

Although a large part of telecommunications engineering nowadays is based on optical signal transmission, the necessary encoding of the information is generated using electronically controlled switches. A compact optical transistor is still a long way off.

The ETH research group was able to create an optical transistor with a single molecule by making use of the fact that a molecule’s energy is quantised. When laser light strikes a molecule that is in its ground state, the light is absorbed. As a result, the laser beam is quenched. Conversely, it is possible to release the absorbed energy again in a targeted way with a second light beam. This occurs because the beam changes the molecule’s quantum state, with the result that the light beam is amplified.

Jaesuk Hwang, first author of the study, explained amplification in a conventional laser is achieved by an enormous number of molecules. By focusing a laser beam on only a single tiny molecule, the ETH Zurich scientists have now been able to generate stimulated emission using just one molecule. They were helped in this by the fact that, at low temperatures, molecules seem to increase their apparent surface area for interaction with light. The researchers therefore needed to cool the molecule down to -272ºC. In this case, the enlarged surface area corresponded approximately to the diameter of the focused laser beam.

By using one laser beam to prepare the quantum state of a single molecule in a controlled fashion, scientists could significantly attenuate or amplify a second laser beam. This mode of operation is identical to that of a conventional transistor, in which electrical potential can be used to modulate a second signal.

The research team hopes that component parts such as the new single-molecule transistor may pave the way for a quantum computer.