Quantentechnologie

Transforming an initial quantum state into a target state through the fastest possible route—a quantum brachistochrone—is a fundamental challenge for many technologies based on quantum mechanics. Here, we demonstrate fast coherent transport of an atomic wave packet over a distance of 15 times its size—a paradigmatic case of quantum processes where the target state cannot be reached through a local transformation. Our measurements of the transport fidelity reveal the existence of a minimum duration—a quantum speed limit—for the coherent splitting and recombination of matter waves. We obtain physical insight into this limit by relying on a geometric interpretation of quantum state dynamics. These results shed light upon a fundamental limit of quantum state dynamics and are expected to find relevant applications in quantum sensing and quantum computing.

We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the lossless-cavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive two-atom case (N=2) is almost equal to the single-emitter case (N=1), which is in contrast to free-space where one would expect an N^2 scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of two-atom entanglement schemes involving optical cavities becomes realistic.

We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the two-photon carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap, that acts as one field for the two-photon Raman coupling. The improved ratio between cooling and heating processes in this configuration enables a five times lower fundamental temperature limit for resolved sideband cooling. We apply this method to perform Raman cooling to the two-dimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.

The demand for fiber cavities for studying light matter interaction increased in the recent years. The ablation setup in Paris is used as a basic tool for fabricating fiber mirrors by ablating material on a fiber end facet with a CO2 laser. In this work we improved the controllability and the efficiency of the setup. A better quality of the CO2 laser beam was achieved by the minimisation of the astigmatism caused be the dichroic mirror. This enables the adjustment of a certain beam radius by moving the fiber end facet along the beam direction. Based on the experiences in Paris, we built up an improved ablation setup in Bonn. The dichroic mirror and the microscope are substituted by a new alignment method of the fiber end facet. A 1550nm laser is overlapped with the CO2 laser beam and coupled into the fiber. By the amount of light coupled into the fiber, the position of the fiber is determined. As a new approch, a Michelson profilometer is implemented into the setup to determine the structure profiles on the fiber end facet. All electrical components in the ablation setup are controlled centrally with a MBED microcontroller which will enable an automated alignment of the fiber and ablation process in the future. In a further experiment, the properties of a fiber cavity are investigated in Paris. Elliptically polarized light at a wavelength of 900nm is coupled into the cavity and from the transmission signal as well as the camera pictures, the free spectral range, the finesse and the spectral width is determined. From this, the reflectivity of the mirrors, the mode waist and the cavity field decay rate are roughly estimated. Especially the birefringence of the mirror coating was also investigated in this measurement and is observed as a resonance splitting in the transmission signal.