Using an optical circuit, the interaction of a single photon with a single molecule — one of the most fundamental processes in nature — was successfully observed. The experiment is valuable to our understanding of fundamental physics and has possible applications in quantum communication and optical computing. Optical phenomena such as the blueness of the sky, the green color of grass and reflections in water can all be traced to elementary collisions between countless particles of light and matter, or photons and molecules. The probability of such a collision between one photon and one molecule is extremely small, as the area on the molecule capable of absorbing the photon — known as the absorption cross section — is usually far smaller than the molecule itself. Only in very strongly absorbing molecules does the absorption cross section approach the actual size of the molecule. Experimental setup. On the left, a dye molecule is located in a cryostat at about -272º C. With a system of mirrors and lenses, green laser light is targeted at the source molecule. As a result, the molecule emits a stream of orange photons, which are captured by the same lens system. These photons are transmitted to the target molecule via an optical fiber and a semitransparent mirror. A photon detector measures the reflected light. Figure 2. Graph of the measurement. The frequency of the orange photons is varied by applying a potential difference to the source molecule. When the frequency of the photons matches the resonance frequency of the target molecule (this happens at about -27 V), the photons are absorbed, leading to a lower number of photons sensed by the detector. (Images: FOM Institute for Atomic and Molecular Physics) Reminiscent of finding the proverbial needle in a haystack, an international team at ETH Zurich succeeded in observing just that by using a system of mirrors and lenses, optical fiber and green laser light. The optical circuit the researchers constructed allowed them to transmit photons between two single dye molecules separated by several meters. To observe the molecules' interaction with one photon, the scientists still had to fire approximately 10 million photons at a single molecule to hope to see the absorption occur. The disruptive effect of collisions between the single molecule and the molecules in its surroundings — which perturb the molecule in its interaction with light — were eliminated by cooling the dibenzanthanthrene (DBATT) molecule to one degree above absolute zero (-272º C). The cooling also increases the absorption cross section dramatically by causing the single molecule to behave as though it's an opaque disk that's a million times larger than the molecule itself. The trade-off is that the frequency of the photons needs to match the resonance frequency of the molecule with extreme precision — to within one part in 100 million. Artist's impression of a single photon interacting with a DBATT molecule. In their experiment, FOM Institute for Atomic and Molecular Physics (AMOLF) group leader Yves Rezus and colleagues used a DBATT molecule trapped in a transparent organic crystal. To make sure the photons had exactly the right frequency for interaction, they used a second DBATT molecule as a light source in a second crystal. Illuminating the source crystal with green laser light caused the source molecule to emit a stream of orange photons, which were collected in an optical fiber and sent to the target molecule. There, a photon detector measured the reflection. Artist's view of a single molecule sending a stream of single photons to a second molecule at a distance, in quantum analogy to the radio communication between two stations. (Image: Robert Lettow) A key aspect of the setup was that the researchers could tune the frequency of the photons by applying an electric field to the source molecule. When the frequency of the photons exactly matched the resonant frequency of the DBATT molecule, a decrease in the reflection was observed — unambiguous proof of photon absorption by the target molecule. "While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons," the physicists said in their paper, published Feb. 27 in Physical Review Letters. For more information, visit: www.ethlife.ethz.ch