Researchers from ETH Zurich have recently managed to create an optical transistor from a single molecule in what is yet another important achievement on the road to quantum computing.

Quantum photonics is a particularly attractive field to scientists and engineers alike in that it could, once some core issues have been resolved, allow for the production of integrated circuits that operate on the basis of photons instead of electrons, which would in turn enable considerably higher data transfer rates as well as dramatically reduced heat dissipation.

Fiber optics are a typical example for the outstanding data transfer rates of light particles when compared to electrons; however, we still need to generate the necessary encoding of the information using electronically controlled switches, which acts as a bottleneck and slows down the process considerably.

But the research group from ETH Zurich has now achieved a decisive breakthrough by successfully creating an optical transistor with a single molecule, which could harness the full potential of quantum optics.

ETH's Martin Pototschnig told us more about the molecule used for the experiments. "It is a small hydrocarbon molecule called dibenzanthanthrene (DBATT). The molecules are doped in n-tetradecane, an organic solvent. So the sample is a pink liquid at room temperature. Then we cryogenically cool the small portion of the sample then the n-tetradecane freezes and forms a molecular crystal."

The molecule itself is about 2 nanometers in size, over ten times smaller than standard transistors, which means that a lot more could be integrated in a single chip.

The reason behind the cooling at the cryogenic temperature of 1.4 K (or -272 °C) is that, under these conditions, quantum mechanical phenomena can be observed much more easily than at room temperature, and particularly to increase a parameter known as coherence time, the time during which the phase between the "0" and the "1" states remain well-defined.

We asked Dr. Hwang, who was part of the research team and lead author of the paper that appeared on the journal Nature detailing the experiment, to give us a sense for how much heat dissipation is affected using these kinds of quantum technologies.

"Our single-molecule optical transistor generates almost negligible amount of heat. When a single molecule absorbs one photon, there is some probability (quantum yield) that the molecule emits a photon out. The rest of the energy absorbed turns into heat in the matrix. For the case of the specific hydrocarbon molecule that we use, the quantum yield is near 100%. So almost no heat is generated."

Determining how a quantum transistor stacks up against a traditional electronic one in terms of performance is however considerably harder because of how the two paradigms differ as its "quantum superposition" can't be directly translated into a precise number of 01 or 10 transactions in standard electronics. Some measure of the performance, Dr. Hwang told us, can still be obtained by analyzing the coherence time, which is about 10 nanoseconds.

By using a laser beam to impose the quantum state of a molecular transistor, the research team demonstrated control of a second laser beam, which reflects the way in which a conventional transistor works.

"The next step is to 'connect' two or more [single-molecule optical transistors]," Pototschnig told us with regard to future areas the team will be focusing on. "In other words, we have to connect two molecules in a way that the quantum mechanical superposition state of each molecule is exchanged in a coherent manner. Only that way the strength of the quantum computing principles can be fully taken advantage of. We are in the middle of coming up with actual ways to implement the connection idea."

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