How do photonic integrated circuits work

Researchers at ETH Zurich have succeeded in producing an optical transistor from a single molecule. This has brought them a step closer to the optical computer.

Internet connections and computers should be faster and more powerful today. In the case of computers, however, conventional main processors (CPUs) limit the performance, for example by producing enormous amounts of heat. This is due to the millions of transistors in the CPUs that switch and amplify electronic signals. One square centimeter of CPU can emit up to 125 watts of heat, which is more than ten times as much as one square centimeter of an electric hotplate.

Photons instead of electrons

Scientists have therefore been trying for some time to find ways to produce integrated circuits that function on the basis of photons instead of electrons. Because photons not only generate much less heat than electrons, they also enable significantly higher transmission rates.

Today, a large part of communications technology is based on optical signal transmission, but the necessary coding of the information is generated with the help of electronically controlled switches. A compact optical transistor is still a long way off. "If you compare the current state of this technology with that of electronics, you are more likely to find yourself with the tube amplifiers that were common in the 1950s than with today's integrated circuits," explains Vahid Sandoghdar, professor at the Laboratory for Physical Chemistry at ETH Zurich.

However, his research group has now made a decisive breakthrough by creating an optical transistor with a single molecule. To do this, they use the fact that the energy of a molecule is quantized: If laser light hits a molecule that is in its ground state, the light is absorbed. This results in the laser beam being extinguished. On the other hand, it is possible to specifically release the absorbed energy with a second light beam. This is done by the beam changing the quantum state of the molecule, which then amplifies the light beam. This so-called stimulated emission, which Albert Einstein described more than 90 years ago, is also the basis of the laser principle.

Focusing in the nano range

"In an ordinary laser, the amplification is achieved by a huge number of molecules," explains Jaesuk Hwang, first author of the study and research associate in the nano-optics group at Sandoghdar. By focusing a laser beam on just a single tiny molecule, the ETH scientists have succeeded in generating stimulated emissions with exactly one molecule. They were helped by the fact that molecules seem to enlarge their surface in a certain resonance oscillation at low temperatures. The researchers therefore had to cool the molecule down to minus 272 degrees Celsius, one degree above absolute zero. In this case the enlarged surface corresponded approximately to the diameter of the focused laser beam.

Switching light with light

As a result of the controlled preparation of the quantum state of the molecule with a laser beam, a single molecule was able to cause a significant weakening or amplification of a second laser beam. This mode of operation is analogous to a conventional transistor in which a second signal can be modulated using an electrical voltage.

There is still a long way to go to the new type of computer

Components such as the new single-molecule transistor can also pave the way for a quantum computer. “It will take many years to research before photons replace electrons in transistors. In the meantime, scientists will learn to manipulate and control quantum systems in a targeted manner and thus come closer to the dream of a quantum computer. " says Sandoghdar.

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J. Hwang, M. Pototschnig, R. Lettow, G. Zumofen, A. Renn, S. Götzinger, V. Sandoghda: A single-molecule optical transistor, Nature (2009) 460, 76-80, doi: 10.1038 / nature08134