Quantum computer nudges closer

ETH Zurich researchers have created an optical transistor from a single molecule, bringing them one step closer to an optical computer.

Internet connections and computers need to be ever faster and more powerful. However, conventional central processing units limit the performance of computers, for example, because they produce an enormous amount of heat.

The millions of transistors that switch and amplify the signals in the CPUs are responsible for this. One square centimetre of CPU can emit up to 125 W of heat, which is more than 10 times as much as a square centimetre of an electric hotplate.

Scientists have been trying for some time to find ways to produce integrated circuits that operate on the basis of photons instead of electrons. The reason is that photons do not only generate much less heat than electrons, 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.

Vahid Sandoghdar, professor at the Laboratory of Physical Chemistry of ETH Zurich, says 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 research group has now achieved something of a breakthrough by creating an optical transistor with a single molecule. For this, they have used 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.

This so-called stimulated emission, which Albert Einstein described over 90 years ago, is at the heart of a laser.

Jaesuk Hwang, first author of the study and a scientific member of Sandoghdar’s nano-optics group, says that amplification in a conventional laser is achieved by an enormous number of molecules. By focusing a laser beam on only a single molecule, the ETH Zurich scientists have generated 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 needed to cool the molecule down to -272°C, ie, 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, scientists could 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.

Component parts such as the single molecule transistor may also pave the way for a quantum computer.

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.”