Experimental setup for fast BEC generation and number-stabilized atomic ensembles

Abstract

Ultracold atomic ensembles represent a cornerstone of today’s modern quantum experiments. In particular, the generation of Bose-Einstein condensates (BECs) has paved the way for a myriad of fundamental research topics as well as novel experimental concepts and related applications. As coherent matter waves, BECs promise to be a valuable resource for atom interferometry that allows for high-precision sensing of gravitational fields or inertial moments as accelerations and rotations. In general, the sensitivity of state-of-the-art atom interferometers is fundamentally restricted by the Standard Quantum Limit (SQL). Multi-particle entangled states (e.g. spin-squeezed states, Twin-Fock states, Schrödinger cat states) generated in BECs can be employed to surpass the SQL and shift the sensitivity limit further towards the more fundamental Heisenberg Limit (HL). However, in current real-world atom interferometric applications, ultracold but uncondensed atomic clouds are employed, due to their speed advantage in the sample preparation. The creation of a BEC can take up several tens of seconds, while standard high-precision atom interferometers operate with a cycle rate of several Hz. In addition, the pursued entangled states can be only beneficial if technical noise sources, such as magnetic field or detection noise are not dominating the measurement resolution. These challenges need to be overcome in order to fully exploit the potential sensitivity gain offered by a quantum-enhanced atom interferometer.

This thesis describes the design and implementation of a new experimental setup for Heisenberg-limited atom interferometry, which incorporates a high-flux BEC source and the manipulation and detection of atoms at the single-particle level.

The presented fast BEC preparation includes a high-flux atom source in a double magneto-optical trap (MOT) configuration that allows to collect 87Rb atoms in a 3D-MOT, which is supplied by a 2D+-MOT with 2×10^10 atoms/s. Forced evaporative cooling of the atoms is divided into two stages, which is sequentially carried out in a magnetic quadrupole trap (QPT) and a crossed-beam optical dipole trap (cODT). The high-flux atom source together with the hybrid evaporation scheme allows to consistently produce BECs with an average of 2×10^5 atoms within 3.5 s.

The capabilities of the single-particle resolving detection are demonstrated by realizing a feedback control loop to stabilize the captured number of atoms in a small MOT. A proof-of-principle measurement is demonstrated for the successful stabilization of a target number of 7 atoms with sub-Poissonian fluctuations. The number noise is suppressed by 18 dB below shot noise, which corresponds to a preparation fidelity of 92%.

Based on this success, the thesis presents an even improved single-particle resolution. The system comprises a six-channel fiber-based optical setup, which provides independent intensity stabilization and frequency detuning, improved pointing stability as well as a better spatial overlap of the MOT beams. The presented high-speed BEC production combined with accurate atom number preparation and detection, as the two main features of the experimental apparatus, pave the way for a future entanglement-enhanced performance of atom interferometers.