We have realized a fiber-based Fabry-Perot cavity with CO 2 lasermachined mirrors. It combines very small size, high finesse F ≥ 130000, small waist and mode volume, and good mode matching between the fiber and cavity modes. This combination of features is a major advance for cavity quantum electrodynamics (CQED), as shown in recent CQED experiments with Bose-Einstein condensates enabled by this cavity [Y. Colombe et al., Nature 450, 272 (2007)]. It should also be suitable for a wide range of other applications, including coupling to solid-state emitters, gas detection at the single-particle level, fiber-coupled single-photon sources and high-resolution optical filters with large stopband.

When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.

We perform Ramsey spectroscopy on the ground state of ultra-cold 87 Rb atoms magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities over the sample should limit the 1/e contrast decay time to about 3 s, while decay times of 58 ± 12 s are actually observed. We explain this surprising result by a spin self-rephasing mechanism induced by the identical spin rotation effect originating from particle indistinguishability. We propose a theory of this synchronization mechanism and obtain good agreement with the experimental observations. The effect is general and may appear in other physical systems.In atomic clocks and other precision techniques based on atomic spin manipulation [1], a central requirement is to preserve the coherence of a state superposition over long times. Understanding how coherence decays in a given system is important for these applications, and is a touchstone of understanding its dynamics. In trapped ensembles, an inhomogeneous shift ∆(r) of the transition frequency occurs due to the trapping potential and to atomic interactions. Different atoms explore different regions of this shift landscape, and so their spins precess at different rates. This leads to dephasing at a rate determined by the characteristic inhomogeneity ∆ 0 of ∆(r) over the ensemble. Various mechanisms have been exploited to reduce this dephasing. Examples are "magic fields" that strongly reduce the field dependence for a specific transition [2,3], or the mutual compensation scheme successfully employed in ultracold 87 Rb [4], where the trap-induced inhomogeneity can be adjusted to nearly cancel the collisional mean-field inhomogeneity. All such mechanisms however, including the motional narrowing well known in nuclear magnetic resonance, have in common that the dephasing is merely slowed down, but never reversed, and the transverse polarization remains a steadily decreasing function of time.Here we present measurements on a trapped ensemble of 87 Rb atoms with two internal levels equivalent to a spin 1/2. Atomic interactions cause a spontaneous re-phasing of the spins, observed as a much longer decay time and revivals of Ramsey contrast. We are also able to extend the coherence time by more than an order of magnitude beyond the 2 to 3 s previously achieved on this system [3,5]. We explain these remarkable results by a very general mechanism based on the identical spin rotation effect (ISRE) that occurs during collisions in the forward direction between two identical particles [6] -an equivalent description can be given in terms of the exchange mean-field experienced by the atoms [7]. This effect is known to cause transient spin waves [4,[8][9][10][11][12][13][14], a deleterious phenomenon if one is interested in long coherence times. In contrast to those experiments however, we are working in a regime where both (i) the ISRE rate (exchange rate) ω ex /2π = 2 |a 01 |n/m is FIG. 1: Two classes of atoms (red and blue) precess at different rates. Their Bloch vectors were initially parallel, but have started to...