At the Weizmann Institute of Science, researchers have managed to “pluck” a single photon – one particle of light – out of a pulse of light. The findings of this research, which appeared recently in Nature Photonics, bear both fundamental and practical significance: Light is the workhorse of today’s communication systems, and single photons are likely to be the backbone of future quantum communication systems. In addition, say the scientists, the apparatus they have devised will spur further research into the fundamental particle nature of light.“Once we move over to quantum communication, information will have to be encoded in single photons,” said Dr. Barak Dayan, head of the Rehovot institite’s quantum optics group. “Each photon will then represent a single ‘qubit’ – a quantum bit that can exist in more than one state at the same time (for example, an equal combination of both 1 and 0).”Dayan and his research team, led by Dr. Serge Rosenblum and Orel Bechler, set out to demonstrate a scheme for pulling just one photon out of a stream, on demand.Their mechanism relies on a physical effect that they call single-photon Raman interaction (SPRINT), which is based on a single atom or atom-like system.“The advantage of SPRINT,” said Dayan, “is that it is completely passive – it does not require any control fields, just the interaction between the atom and the optical pulse.”In previous research, he and his team had employed SPRINT as a switch for single photons that sent them down different pathways, effectively turning the apparatus into a photonic router. In this work, the atom becomes a tap rather than a switch, snatching one photon from the flow and then turning itself off.“It is not trivial,” says Dayan, “to have a mechanism that continues to function even in high fluxes of photons and to remove just one photon.” The experimental setup of Weizmann’s quantum optics group relies on state-of-the-art technologies – laser cooling and trapping of atoms (in this case rubidium), the fabrication of chip-based, ultrahigh-quality glass microspheres, and optical nanofibers.“The ability to divert a single photon from a flow could be harnessed for various tasks,” says Dayan, “from creating non-classical states of light that are useful for basic scientific research, through eavesdropping on imperfect quantum-cryptography systems that rely on single photons, to increasing the security of your own quantum-communication systems.”The existence of photons was first suggested by Einstein in 1905, yet many of their properties are just now coming to light. Dayan believes the new method will expand our capabilities to study and control them as individual particles.NO MORE BROWN APPLES? After you slice an apple, it loses its appetizing white color very quickly; this is not harmful, but as many people are unwilling to eat “old-looking” fruit, huge quantities of fresh products are thrown out each year.The reason for this ugly coloring is a chemical reaction caused by catalysis by the enzyme tyrosinase. Actually, this browning is a defense mechanism of plants.“There are caterpillars with a full stomach that are actually starving to death because they are no longer able to digest the material that is changed by the tyrosinase,” said Dr. Matthias Pretzler of the biophysical chemistry department at the University of Vienna, who was involved in the structure elucidation of the first plant tyrosinase together with his colleagues. The research results have now been published in the wellknown journal Angewandte Chemie (Applied Chemistry).Tyrosinase, which is a metal-containing enzyme catalyzing the oxidation of phenols, has been explored by Dr. Annette Rompel for over 20 years. It is also involved in the synthesis of human melanin and therefore responsible for human “browning” as well. “In the 1990s we were far away from handling the enzyme, but with the development of modern chromatographic and crystallographic methods there are better opportunities available to us today,” said Rompel, who already tried to isolate and purify the enzyme from walnut leaves. “The leaves make the skin become brown, which proves that they contain a high concentration of the enzyme and represent an attractive source for our research,” she explained.To find a harmless method to control the tyrosinase, they have to find out how the enzyme is actually activated.“We want to find the enzyme that switches the tyrosinase from latent to active,” she explained. “If we are successful in suppressing the first activating step, this will be an enormous success for science,” added Rompel. “It would mean that a banana would not turn brown anymore if you squeezed it in your bag.”Still, it would rot probably even faster.