Quantum technologies

We present a scheme to directly probe the Wigner function of the motional state of a neutral atom confined in an optical trap. The proposed scheme relies on the well-established fact that the Wigner function at a given point (x,p) in phase space is proportional to the expectation value of the parity operator relative to that point. In this work, we show that the expectation value of the parity operator can be directly measured using two auxiliary internal states of the atom: parity-even and parity-odd motional states are mapped to the two internal states of the atom through a Ramsey interferometry scheme. The Wigner function can thus be measured point-by-point in phase space with a single, direct measurement of the internal state population. Numerical simulations show that the scheme is robust in that it applies not only to deep, harmonic potentials but also to shallower, anharmonic traps.

The mapping of the potential landscape with high spatial resolution is crucial for quantum technologies based on ultracold atoms. However, the imaging of optical dipole traps is challenging because purely optical methods, commonly used to profile laser beams in free space, are not applicable in a vacuum. In this work, we demonstrate precise in situ imaging of optical dipole traps by probing a hyperfine transition with Ramsey interferometry. Thereby, we obtain an absolute map of the potential landscape with micrometer resolution and shot-noise-limited spectral precision. The idea of the technique is to control the polarization ellipticity of the trap laser beam to induce a differential light shift proportional to the trap potential. By studying the response to polarization ellipticity, we uncover a small but significant nonlinearity in addition to a dominant linear behavior, which is explained by the geometric distribution of the atomic ensemble. Our technique for imaging of optical traps can find wide application in quantum technologies based on ultracold atoms, as it applies to multiple atomic species and is not limited to a particular wavelength or trap geometry.

We have designed, built, and characterized a high- resolution objective lens that is compatible with an ultra-high vacuum environment. The lens system ex- ploits the principle of the Weierstrass-sphere solid immersion lens to reach a numerical aperture (NA) of 0.92. Tailored to the requirements of optical lattice experiments, the objective lens features a relatively long working distance of 150 μm. Our two-lens design is remarkably insensitive to mechanical tolerances in spite of the large NA. Additionally, we demonstrate the application of a tapered optical fiber tip, as used in scanning near-field optical microscopy, to measure the point spread function of a high NA optical system. From the point spread function, we infer the wavefront aberration for the entire field of view of about 75 μm. Pushing the NA of an optical system to its ultimate limit enables novel applications in quantum technolo- gies such as quantum control of atoms in optical mi- crotraps with an unprecedented spatial resolution and photon collection efficiency.

This thesis deals with the design and construction of a versatile digital frequency synthesizer for implementation in the state-dependent transport of Cesium atoms. The versatile digital frequency synthesizer consists of a field programmable gate array interfaced with a low noise direct digital synthesizer that will be used for amplitude, phase and frequency modulation. The versatile digital frequency synthesizer provides better flexibility, for generating arbitrary waveforms, and lower phase noise than the previous setup. The measured reduction in phase noise of around 20dB corresponds to an increase in the lifetime of atoms by two orders of magnitude. This improved phase noise specification with the ability to generate arbitrary waveforms opens up possibilities for transporting atoms over macroscopic distances and eventually realizing an atom interferometer with a large space-time area.