A High Phase-Space-Density Gas of Polar Molecules (0808.2963v2)

Abstract: A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, would not only enable explorations of a large class of many-body physics phenomena, but could also be used for quantum information processing. We report on the creation of an ultracold dense gas of 40K87Rb polar molecules. Using a single step of STIRAP (STImulated Raman Adiabatic Passage) via two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 10^12 cm^-3, and an expansion-determined translational temperature of 350 nK. The polar molecules have a permanent electric dipole moment, which we measure via Stark spectroscopy to be 0.052(2) Debye for the triplet rovibrational ground state and 0.566(17) Debye for the singlet rovibrational ground state. (1 Debye= 3.336*10^-30 C m)

• The paper achieves efficient STIRAP transfer of Feshbach-associated KRb molecules to their rovibrational ground state at a density of 10¹² cm⁻³.
• It utilizes advanced laser stabilization and ab initio intermediate state selection to ensure coherence and minimal heating during the transfer.
• The results pave the way for exploring dipolar interactions in ultracold systems, offering new avenues for quantum simulation and computation.

A High Phase-Space-Density Gas of Polar Molecules

In the paper "A High Phase-Space-Density Gas of Polar Molecules," the authors present the creation and characterization of an ultracold dense gas of heteronuclear polar molecules, specifically $^{40}$K$^{87}$Rb, which are transitioned to their rovibrational ground states through a coherent technique referred to as STIRAP (Stimulated Raman Adiabatic Passage). This work expands the exploration opportunities in many-body quantum physics due to the unique interaction characteristics of polar molecules.

Experimental Implementation and Results

The primary step involved in this paper is the creation of ultracold KRb molecules. Initially, a Feshbach resonance is used to associate fermionic potassium ($^{40}$K) and bosonic rubidium ($^{87}$Rb) atoms into weakly bound molecules. The authors then employ a precise laser technique utilizing STIRAP to transfer these Feshbach molecules into the rovibrational ground state, achieving a peak density of 10$^{12}$ cm$^{-3}$ at a translational temperature of 350 nK. This high phase-space density is crucial for realizing observable dipole-dipole interactions, which underlie the profound implications for quantum simulation and information processing.

Successful transfer to the ground state is evidenced by both density and electric dipole measurements. The electric dipole moments are characterized using Stark spectroscopy, revealing the values of 0.052(2) Debye for the triplet ground state and 0.566(17) Debye for the singlet ground state, aligning with theoretical predictions and showing substantial polar character.

Technical Insights and Methodology

The STIRAP process used here is central to achieving the high-efficiency state transfer required to maintain the integrities of an ultracold gas. Crucial considerations include selecting appropriate intermediate states through ab initio calculations and transition dipole moment evaluations—achieving coherent state transfer with minimal heating. The experimental setup ensures phase coherence and optimal transition probabilities, minimizing perturbations such as spontaneous emission.

The authors utilize advanced laser stabilization techniques, involving frequency combs for maintaining the coherence necessary for STIRAP. This ensures a robust transfer mechanism across the excited and electronic ground-state potentials, crucial for achieving the desired phase-space conditions.

Implications and Future Directions

The properties of an ultracold gas of polar molecules allow for the investigation of complex phenomena, including quantum phase transitions and quantum simulations of condensed matter systems. This paper lays the groundwork for subsequent experiments exploring dipolar interactions and entanglement, essential for developing quantum computation protocols.

Additionally, the research provides a versatile framework for molecules beyond KRb, suggesting pathways to paper a variety of heteronuclear diatoms, each with unique dipolar interactions suitable for emulating specific quantum models.

In conclusion, this work demonstrates a significant advancement in the field of ultracold molecules, creating new venues for explorations in quantum many-body physics. Future work may involve manipulating and enhancing interaction effects, potentially employing optical lattices or other confinement techniques, aiming towards the realization of exotic quantum states that enrich the field of quantum simulation and processing.