A dipolar Bose-Bose mixture of Dysprosium isotopes with controllable interspecies interactions

Technical Summary: A Dipolar Bose-Bose Mixture of Dysprosium Isotopes with Controllable Interspecies Interactions

Problem and Motivation
Ultracold gases of highly magnetic lanthanide atoms, specifically dysprosium (Dy), offer a unique platform for investigating quantum gases with long-range, anisotropic dipole-dipole interactions. While single-species dipolar Bose-Einstein condensates (BECs) have enabled the observation of dipolar supersolids, extending these studies to multi-component systems is challenging. Previous attempts at spinor mixtures of lanthanides have been hindered by strong inelastic losses due to dipolar relaxation. Furthermore, realizing stable mixtures with tunable interactions is essential for exploring exotic regimes such as multi-component supersolidity and density-modulated phases. The specific challenge addressed in this work is the creation of a stable, quantum-degenerate mixture of two Dy isotopes that suppresses dipolar relaxation while retaining the large magnetic dipole moment and offering controllable interspecies interactions.

Methodology
The experiment utilizes a dual-isotope magneto-optical trap (DI-MOT) to simultaneously trap $^{162}\text{Dy}$ and $^{164}\text{Dy}$, the two most abundant bosonic isotopes of dysprosium. By employing phase-modulated sidebands on the 421-nm slowing and 626-nm trapping lasers, the researchers create spatially overlapping MOTs without additional laser systems. The atoms are prepared in their lowest absolute Zeeman sublevel ($m_J = -8$), which naturally suppresses dipolar relaxation.

Following the MOT phase, the atoms are transferred to a crossed-beam optical dipole trap (ODT) formed by 1064-nm laser beams. Forced evaporative cooling is performed by ramping down the optical power. A key feature of this system is the near-identical electronic polarizability of the two isotopes at the trapping wavelength, ensuring they experience nearly identical trapping potentials. The population imbalance between the isotopes is controlled by tuning the radio-frequency drive of the electro-optic modulator (EOM) generating the 626-nm light, which alters the radiation pressure and loading efficiency into the ODT.

To characterize interspecies interactions, the team performs atom-loss spectroscopy as a function of magnetic field to identify interspecies Feshbach resonances. They further probe the miscibility of the mixture by ramping the magnetic field to target values near a broad resonance and measuring the center-of-mass separation of the two expanding clouds after time-of-flight.

Key Results

1. Realization of a Quantum-Degenerate Mixture: The authors successfully realized a stable Bose-Bose mixture of $^{162}\text{Dy}$ and $^{164}\text{Dy}$. The mixture thermalizes efficiently, with evaporation trajectories closely following those of single-isotope cases. The evaporation efficiency ($\gamma$) was found to be nearly identical for single-species and mixture operations ($\approx 2.4-2.7$), indicating efficient thermalization between the species.
2. Collisional Properties and Losses: While the mixture is stable, $^{164}\text{Dy}$ exhibits significantly shorter lifetimes ($\sim 0.6$ s) compared to $^{162}\text{Dy}$ ($\sim 14$ s) at the production field of 9.7 G, suggesting an isotope-specific three-body loss mechanism. Consequently, pure $^{162}\text{Dy}$ condensates can reach higher atom numbers ($>1.3 \times 10^5$) than $^{164}\text{Dy}$ ($\sim 8.0 \times 10^4$).
3. Interspecies Feshbach Resonances: The study identified multiple interspecies Feshbach resonances. Most notably, a broad resonance was found centered at $B_0 = 10.8(3)$ G, extending from approximately 10.1 G to 11.95 G. This is a relatively broad resonance by lanthanide standards. A narrower resonance was also identified at 9.05(5) G.
4. Miscible-Immiscible Transition: By tuning the magnetic field across the broad resonance at 10.8 G, the researchers observed a field-dependent center-of-mass separation between the two isotopes. Below the critical field ($B < 9.7$ G), the clouds overlap (miscible). As the field increases toward the resonance, the clouds separate, with the heavier $^{164}\text{Dy}$ settling below the $^{162}\text{Dy}$, providing the first evidence of a miscible-immiscible transition in a Dy BEC mixture.

Significance and Claims
The paper claims to present the first realization of a quantum-degenerate Bose-Bose mixture of $^{162}\text{Dy}$ and $^{164}\text{Dy}$. The significance of this work lies in establishing a platform that combines the large magnetic dipole moment of dysprosium (10 $\mu_B$) with the tunability of a dual-species system via interspecies Feshbach resonances.

The authors state that this system is "well suited for exploring dipolar effects in ultracold mixtures," specifically citing the potential to investigate:

• Multi-component supersolidity.
• Dipolar superfluids.
• Exotic density-modulated phases.

The work provides the necessary tools—specifically the broad interspecies resonance and the stable degenerate mixture—to control the interspecies scattering length ($a_{12}$) and explore the phase diagram of dipolar mixtures, including the transition between miscible and immiscible states. The authors note that while they have observed the onset of phase separation, a definitive demonstration of full phase separation and the resolution of density-modulated structures remain subjects for future investigation.