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.
Individual particles of light are the means of choice to transmit quantum bits. In the future, the smallest units of quantum information could replace the conventional bits if the computer advances into new dimensions of computing speed with the aid of the special properties of quantum physics. 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.
In experiments conducted at the ETH Zurich, physicists working with Vahid Sandoghdar, who recently became 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, have transmitted single photons between the smallest antennas in the world, i.e. between two molecules of dibenzanthanthrene (DBATT). "The difficulty with this experiment is that normally a single photon hardly interacts at all with a molecule," explains the Max Planck Director.
The transmitter molecule must emit photons of a suitable colour
A single photon is almost as invisible to a molecule as it is to the human eye. So until now, physicists trapped atoms or molecules between two tiny mirrors between which the single light particles were reflected innumerable times. This method significantly increases the probability that the atom notices the photon. In order to do without mirrors and thus bring about a direct interaction between a photon and a molecule, Vahid Sandoghdar and his team of physicists had to use a few experimental tricks.
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.
To a photon of the right frequency the molecule appears to be larger
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 objective is the exchange of photons in quantum radio communication
"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."
Thanks to the experimental ingenuity developed by Vahid Sandoghdar's team, the receiver molecule scatters a sizeable three percent of the photons transmitted in the quantum radio communication. The scattering process, i.e. the absorption and emission of the light particles, delays the wave train of the light. This delay can be used as information. "This could be very useful for processing quantum information," says Vahid Sandoghdar.
He and his team now want to further optimise the quantum radio tested at the ETH Zurich back in Erlangen for applications in quantum physics. "We are working on increasing the efficiency of our receiver antenna further," says the researcher. His team would then like to turn the one-way radio communication into a real exchange. "We want to bounce a photon to and fro several times between two molecules," he explains. This would interlink the properties of the molecules and the exchanged photons very firmly: they would be entangled. Such entangled systems are again useful for the data exchange in the quantum computer or via quantum encrypted connections, because every partner in the entangled system contains information about the other partners. The physicists would then create a permanent connection in the quantum radio.