Photons are promising candidates for investigating quantum systems. German researchers demonstrated a new method for generating intense, ultrashort light pulses in the ultraviolet (UV) wavelength region at high repetition rates. These pulses are essential for generating multiple entangled photons. The researchers, from the group of professor Harald Weinfurter at the Ludwig-Maximilians-Universität München and the Max-Planck-Institute for Quantum Optics, Garching, Germany, in the Cluster of Excellence Munich Center for Advanced Photonics, succeeded in their goal, which was to entangle as many photons as possible and to study their properties. Entanglement, or Einstein's “spooky action at a distance,” still fascinates quantum physicists today. Therefore, their focus is not only on realizing the quantum computer, but they would also like to gain deeper insight into the world of quantum physics and to understand how entanglement is distributed over large quantum systems. The ultraviolet light pulses are enhanced inside the four-mirror resonator. The crystal used for the generation of entangled photons is situated in the black box (connected to blue tubes). To generate several entangled photons at once, ultra-short stroboscope-like light pulses of very high power are required. The main challenge for this project was to obtain ultrashort, high energy pulses with a high repetition rate and at UV wavelengths. All these demands had to be fulfilled at the same time. The team succeeded in transferring a method working in the infrared wavelength region to the more powerful ultraviolet region. They implemented a resonator to enhance UV light pulses with a pulse duration in the femtosecond regime (10-15 seconds) at a high repetition rate (82 MHz). Inside the resonator the pulses continuously add up only if the following condition is fulfilled: each incoming pulse has to overlap exactly with the pulses already stored in the resonator. The light intensity created in the resonator exceeds those of comparable commercial laser systems by at least a factor of five. A crystal inside the resonator then allows the generation of entangled photons. Roland Krischek, who co-constructed and characterized the light resonator, sees a lot of potential: “This light resonator allows us to study entanglement of larger quantum systems.” “This resonator can not only be used to generate multiphoton entanglement but also to analyze, for example, molecular formation or carrier dynamics in semiconductors,” said Krischek’s colleague, Witlef Wieczorek. For more information, visit: www.munich-photonics.de