Strong atom-cavity coupling observed for trapped single atoms and Bose-Einstein condensates on an atom chip

We have used a fiber Fabry-Perot cavity on an atom chip to obtain strong, extremely stable atom-cavity coupling for single atoms and Bose-Einstein condensates trapped inside the cavity. The coherent atom-cavity coupling parameter, g₀/2pi ~ 209 MHz, is larger than both cavity decay, kappa/2pi ~ 53 MHz and spontaneous emission gamma/2pi ~ 3 MHz. Due to the small mode volume, these values, well inside the strong coupling regime of cavity QED, are realized with a cavity finesse F ~ 36000, which is a moderate value compared to usual strong-coupling cavity QED experiments. The cavity also achieves a very high cooperativity parameter, C=g₀ ²/2kappagamma ~ 130.The use of trapped Bose-Einstein condensates (BECs) is a fundamentally new situation for cavity QED, which should enable a new class of experiments, where atoms that share a well-defined internal and external quantum state collectively interact with a quantized mode of the cavity. Besides, as the spatial extension of the BEC assumes the fundamental limit that can be attained for an atom cloud - much smaller than an optical wavelength in our case - an extremely strong, stable and uniform coupling to the cavity mode is achieved. This makes the system interesting for applications such as quantum memories. For single atoms, our combination of high resonator cooperativity with extreme positioning precision enables qubit detection with near-unit efficiency. This realizes a long-awaited milestone for quantum information processing on atom chips.