Experiments on enhancement of inter-stage transfer efficiency in laser cooling of strontium atoms

Abstract

Laser cooling is a matter of paramount significance, particularly when dealing with atoms from the alkaline earth metal group. These atoms offer ultra-narrow optical transitions, which can be utilized for quantum information processing, extremely precise optical atomic clocks and various other experiments including the detection of dark matter and gravitational waves. However, all of these experiments generically suffer from different sorts of noises arising from loss of atoms during different stages of experiments. The loss of atoms increases the various noise levels in different quantum experiments, including but not limited to atom number shot-noise in optical atomic clocks and atom interferometry, frequency instability in atomic clocks attributable to fluctuations in atom numbers etc. Additionally, the number of atoms is extremely important in atom-interferometry based gravitational wave (GW) detection. In a nutshell, the significance of the number of atoms becomes paramount in achieving a good signal-to-noise ratio for these experiments. However, the number of atoms is affected by various factors. The conventional method of cooling alkaline earth metals involves a two-tier Doppler cooling approach to make the atoms adequately cold for various quantum experiments. For instance, for strontium atoms, the lowest achievable temperature (Doppler limited temperature) in the first stage is 760 μK. This temperature, though relatively low, is unsuitable for experiments involving quantum phenomena. Consequently, a second stage of cooling is necessary, with its Doppler-limited temperature (proportional to the linewidth of the optical transition) reaching lower than the single photon recoil temperature (460 nK). Therefore, the two stages of cooling of alkaline earth atoms can be referred to as the initial magneto-optical trap (MOT) cooling phase and the narrow-line MOT cooling phase. in the aforementioned two-tier cooling method, there are two significant reasons for having a lower number of atoms:

Firstly, the suboptimal capture of atoms during the initial stage of cooling is often due to the poor optimization of experimental parameters. Manual tuning of multiple parameters can often fail to discover the optimal set of parameters that would result in the maximum number of atoms.

Secondly, the temperature and velocity distribution of atoms within a MOT are directly related to the linewidth of the corresponding laser cooling transition used in the MOT. The number of atoms interacting with light inside a MOT depends on both the linewidth of the transition and the laser detuning. A smaller linewidth leads to fewer interacting atoms within the MOT with a fixed light detuning. Therefore, during the transfer of alkaline atoms from the initial MOT with a broader linewidth to the narrow-line MOT, a significant number of atoms fail to interact with the laser, which is precisely locked to the narrow-line transition of the secondary MOT. This results in a substantial loss of atoms from the trap during the transfer process, despite artificially broadening the laser detuning through temporal modulation. This thesis outlines the methodologies used to address these challenges in experiments with strontium 88 atoms. Specifically, it discusses the methods to enhance the number of atoms by:

1. Implementing machine learning techniques to optimize the parameters of the initial stage of cooling. 2. Employing a novel three-level intermediate cooling method, which reduces the temperature of the first stage cooling by approximately 75%, thereby narrowing the velocity distribution and enhancing the transfer efficiency to around 85%, as opposed to the conventional method that offers a transfer efficiency of around 40%. Notably, these methods can be applied across all the elements belonging to the alkaline earth metal group as they share similar level structures. By applying this approach, we can effectively increase the number of atoms in the experiments, consequently elevating the SNR for various applications.