Creation of an Ultracold Gas of Ground State Dipolar ⁶Li⁴⁰K Molecules

Creation of dipolar molecules in the ro-vibrational ground state is a long-standing scientific goal and has been achieved. Such molecules are promising platforms for precision measurement, quantum simulations of many-body systems, and quantum bit manipulation based on the dipole-dipole interaction. Ultracold ⁶Li⁴⁰K molecules in the deeply-bound ro-vibronic states possess a large electric dipole moment (3.6 Debye). This makes them a suitable candidate for exhibiting strong effects of the long-range and anisotropic nature of the dipole-dipole interactions.

Firstly, we demonstrate a high-effient STIRAP (~98% one-way transfer efficiency) and Raman process (~93% one-way transfer efficiency) that transfers Feshbach molecules to the ro-vibrational ground state molecules in the optical dipole trap. Specifically, the diode lasers was found to have high integrated phase noise around 200 mrad (4% of the total laser power), which is considered as the main reason for the failure of previous attempts. To reduce the noise, we extend the diode lasers' cavity from 3 cm to 20 cm. 75% reduction of the integrated phase noise was achieved for the Pump and Stokes lasers. The effect of laser polarization impurity on the STIRAP efficiency is estimated based on numerical methods and our far-detuned Raman transfer result. The loss can be suppressed by increasing the single photon detuning Δ. Our transfer regime involves only singlet states by utilizing deeply bound vibrational states of the ??1Σ+ excited potential as intermediate states, therefore the hyperfine structure of the excited state is unresolved. Stark shift has also been measured to indicate the induced dipole moment.

Secondly, we perform the dark resonant spectroscopy on the J=2 rotational state, and minimize the error of rotational constant B₀ toward ~20 kHz. Then, we conduct the rotational microwave spectroscopy on the ground state molecules to transfer them into J=1 rotational hyperfine states. Rabi oscillation between J=0 ground state and J=1 excited state is also observed.