Creation of optical transistor from a single molecule moves toward optical IC

Scientists at ETH Zurich (Switzerland) say they have achieved a decisive breakthrough by successfully creating an optical transistor with a single molecule. The achievement, they note, has brought us one step closer to development of an optical integrated circuit (IC) / optical computer.

According to Jaesuk Hwang, first author of a study describing the work, amplification in a conventional laser is achieved by an enormous number of molecules. By focusing a laser beam on a single molecule, the researchers were able to generate stimulated emission using just one molecule. The scientists, working with Vahid Sandoghdar, Professor at ETH's Laboratory of Physical Chemistry, were helped in this by the fact that, at low temperatures, molecules seem to increase their apparent surface area for interaction with light. They therefore needed to cool the molecule down to minus 272 degrees Celsius (minus 457.6 degrees Fahrenheit), i.e. one degree above absolute zero. 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, the 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.

Sandoghdar says, "Many more years of research will still be needed before photons replace electrons in transistors. In the meantime, scientists will learn to manipulate and control quantum systems in a targeted way, moving them closer to the dream of a quantum computer."

Conventional central processing units (CPUs) limit the performance of today's computers because they produce an enormous amount of heat. The millions of transistors that switch and amplify the electronic signals in the CPUs are responsible for this. One square centimeter of CPU can emit up to 125 watts of heat, which is more than ten times as much as a square centimeter of an electric hotplate.

Integrated circuits that operate on the basis of photons instead of electrons would not only generate much less heat but 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. Sandoghdar explains that, "Comparing the current state of this technology with that of electronics, we are somewhat closer to the vacuum tube amplifiers that were around in the fifties than we are to today's integrated circuits."

His group's work made use of the fact that a molecule's energy is quantized: 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. This so-called stimulated emission, which Albert Einstein described over 90 years ago, also forms the basis for the principle of the laser.

For more information see the paper, A single-molecule optical transistor, published by Nature.