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| Illustration of molecules of radio communication |
A remarkable achievement of radio communication technology has been successfully performed. In the future, radio communications will probably be at the elementary level. Scientists at the ETH Zurich and the Max Planck Institute for the Science of Light in Erlangen have used two molecules as antennas (two molecules of dibenzanthanthrene (DBATT)) and transmitted signals in the form of single photons, i.e. light particles, from one to the other. Since a single photon usually has very little interaction with a molecule, the physicists had to use a few experimental tricks for the receiver molecule to register the light signal. A radio connection established via individual photons would be ideal for various applications in quantum communication – in quantum cryptography or in a quantum computer, for example.
In the future, quantum bits of communication will be replaced by individual particles of light. In quantum cryptography, single photons are already being used as information carriers which cannot be intercepted without this being noticed - in banking data exchange, for example.
“The difficulty with this experiment is that normally a single photon hardly interacts at all with a molecule,” explains Vahid Sandoghdar (Director of the Nano-optics Department at the Max Planck Institute for the Science of Light and holds a Humboldt professorship at the University of Erlangen).
Vahid using the method of direct interaction between photons and molecules in this experiment. First, the researchers embedded DBATT dye molecules into layers of other organic molecules. They then positioned two such layers doped with dye molecules a few metres apart and linked them with a fibre-optic cable. The next step was to select one molecule suitable for radio communication from each of the two layers. “This means that the transmitting molecule has to emit photons of exactly the same colour as the receiving molecule can absorb,” explains Professor Stephan Götzinger, who teaches at the University of Erlangen and also works in Vahid Sandoghdar’s group.
Thus not every molecule is suitable, because the dye molecules are stuck between other molecules just like raisins in a slice of fruit loaf. When the particles collide the colour of the light which the molecules transmit or receive changes, just like the dough changes the consistency of the raisins. The researchers therefore examined the molecular fruit cake for molecules with the same environment. They also reduced the collisions by cooling the samples down to minus 272 degrees Celsius, i.e. almost to absolute zero.
They then converted one of the two molecules into a source of single photons by irradiating it with a laser. The molecular antenna now transmitted a stream of single photons. The scientists focussed this stream of photons with a very good lens and guided it through the glass fibre. The weak flashes of light subsequently passed through a lens again at the other end. This enabled the researchers to focus the photons as much as possible. “However, it is not possible to limit a photon in the visible spectral range to less than a few hundred nanometres,” says Stephan Götzinger. A photon therefore easily overlooks a molecule, which normally measures only one nanometre or so, and it simply races past it.
“A couple of years ago, we noticed that we can nevertheless bring about a very strong interaction if the frequency of the photon agrees very accurately with the resonance frequency of the molecule. The molecule then appears to be much larger,” says Vahid Sandoghdar. This can be explained using the vibration of a tuning fork: musicians make a tuning fork emit a note by striking it against a hard object. The blow excites all the frequencies. To illustrate the situation between molecule and photon, a tuning fork would have to be made to vibrate by placing it onto a vibrating base. The prongs only vibrate as well if the base vibrates at precisely the natural frequency of the tuning fork, i.e. at 440 Hz of the concert pitch “A”.
“One single photon must therefore be strongly focussed onto one molecule,” says Vahid Sandoghdar. “This may sound easy, but in the laboratory at minus 272 degrees Celsius this is a challenge which we mastered only a short while ago.”
This article has edited by authors of threelas
Source: Max-Planck-Gesellschaft
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