Realization of repulsive polarons in the strongly correlated regime

TL;DR

This work reports the stable realization of repulsive polarons in a strongly correlated, quasi-2D Bose bath formed by $^6$Li dimers. By promoting a small fraction of dimers to excited transverse levels, the authors engineer synthetic-spin impurities that dress themselves with a strongly repulsive bath, enabling measurements of polaron energy, quasiparticle residue, and an unusually large effective mass, including values exceeding $2$ times the bare dimer mass. The spectral properties are probed via trap modulation and tilted Bragg spectroscopy, with results—energy shifts, lifetimes, and mass enhancements—well captured by both T-matrix theory and quantum Monte Carlo simulations, and exhibiting clear beyond-mean-field effects. A key theoretical insight is that the $n_z=1$ impurity shows no zero-momentum energy shift due to parity-related cancellation, while the $n_z=2$ branch reveals strong dressing. Overall, the work establishes a robust platform for studying impurity physics in low-dimensional, strongly correlated Bose systems and introduces the synthetic-spin polaron as a versatile spectroscopic probe.

Abstract

Mobile impurities interacting with a quantum medium form quasiparticles known as polarons, a central concept in many-body physics. While the quantum impurity problem has been extensively studied with ultracold atomic gases, repulsive polarons in the strongly correlated regime have remained elusive. Typically, the impurity atoms bind into molecules or rapidly decay into deeper lying states before they can acquire an appreciable dressing cloud. Here, we report on the realization of polarons in a strongly repulsive quasi-two-dimensional quantum gas. Using a superfluid of $^6$Li dimers, we introduce impurities by promoting a small fraction of the dimers into higher levels of the transverse confining potential. These novel synthetic-spin polarons give access to the strongly repulsive regime where common decay channels are suppressed. We extract key polaron properties - the energy, quasiparticle residue, and effective mass - using trap modulation and Bragg spectroscopy. Our measurements are well captured by a microscopic T-matrix approach and quantum Monte Carlo simulations, revealing deviations from mean-field predictions. In particular, we measure a significant enhancement of the polaron mass, with values exceeding twice the free dimer mass. Our demonstration of a stable repulsive Bose polaron establishes a platform for studying impurity physics in low-dimensional and strongly correlated systems.

Figures (6)

• Figure 1: Experimental realization of repulsive synthetic-spin polarons. a) An impurity (blue) is immersed in a homogeneous 2D Bose-Einstein condensate (green). The atomic cloud is confined to a single 2D plane of a blue-detuned optical lattice in combination with a ring trap (box potential in the radial plane). b) The eigenstates of the harmonic trapping potential along the $z$ direction play the role of synthetic spins, allowing us to realize a 2D repulsive polaron. Here, the bath corresponds to a superfluid of density $n$ in the ground state $n_z=0$, while the impurity is created by populating a higher level such as $n_z=2$. Without interactions, the energy $\hbar\omega_\textrm{r}$ to address this transition is $2\hbar\omega_z$, the harmonic oscillator level spacing. With increasing repulsion $na_\textrm{2D}^2$, $\hbar\omega_\textrm{r}$ exhibits an interaction shift arising from the difference between the bath chemical potential $\mu_\mathrm{B}$ and the repulsive polaron energy $\mu_\mathrm{P}$. In contrast to previous Bose polaron realizations, there is no underlying attractive branch.
• Figure 2: Polaron energies from trap modulation spectroscopy. a) Response $R(\omega_{\textrm{m}})$ measured via trap modulation spectroscopy. A well-defined mode is visible, corresponding to the creation of polarons in the $n_z=2$ state. b) Energy shift $\Delta$, extracted from Lorentzian fits to the spectra shown in panel (a). The error bars of the experimental data are small compared to the symbol size. A comparison with QMC theory shows excellent agreement up to $na_\textrm{2D}^2\lesssim 0.05$ in the strongly interacting regime. Both T-matrix and QMC theories deviate for large interactions, where the details of the scattering potential become relevant, while MF theory fails already for moderate interactions. For small interactions, all theories agree well with the experiment (inset).
• Figure 3: Spectral function at different interaction strengths. a) Ridge plot showing the response for different $na_\textrm{2D}^2$, directly connected to the spectral function $A_2(\omega_\mathrm{m}+\mu_\mathrm{B})$. From Lorentzian fits (blue dashed lines), the full width at half maximum $\gamma$ is extracted (inset), as well as the integral, shown as blue shaded areas, which determines the quasiparticle residue $Z$. As impurity-bath interactions increase, the spectral response shifts and broadens, reflecting strong correlations with the medium. b) Quasiparticle residue $Z$, quantifying the overlap between the dressed polaron and a non-interacting particle. For increasing $na_\textrm{2D}^2$, the extracted $Z$ (blue circles) steadily decreases, signaling enhanced dressing by the bath, which is supported by QMC (red crosses) and T-matrix (purple line) calculations.
• Figure 4: Effective mass of the $n_z=1$ and $n_z=2$ polaron. a) Creation of a polaron via Bragg spectroscopy: By overlapping two beams onto the atoms, a selected energy of $\omega_\textrm{m}=\omega_1-\omega_2$ and momentum $\mathbf{q}_\textrm{tot}=\mathbf{k}_1-\mathbf{k}_2$ can be transferred. Slightly tilting the beams allows for excitations into the $n_z=1$ and $n_z=2$ impurity states due to a small transverse momentum $q_z\ll q_\textrm{tot}$ imparted in addition to an in-plane momentum $q\approx q_\textrm{tot}$ (inset). b) Bragg spectrum of a bosonic superfluid, displaying the phonon ($n_z=0$) and the impurity modes ($n_z=1$ and $n_z=2$). The inverse of the curvature of the impurity dispersion directly gives the effective mass $m^*$. c) Bragg spectrum taken in the strongly interacting regime. $E_\textrm{P}(q)$ extracted from Lorentzian fits to the momentum slices are displayed for the $n_z=1$ and $n_z=2$ mode, including the quadratic fits. d) Effective masses extracted from the quadratic fits to the Bragg spectra, including a comparison with QMC and T-matrix theory. The bottom panel shows the zero-momentum energies $\omega_\textrm{r}=E_\textrm{P}(q=0)$ for $n_z=1$ extrapolated from the fit. Remarkably, the energy shift from the bare transition remains zero for all gas parameters $na_{2D}^2$ due to symmetry.
• Figure S1: Extraction of the response. Density images after a $9ms$ T/4-TOF expansion for a system before (left) and after (right) resonant modulation. The imaging is slightly tilted to avoid fringes caused by atoms that have expanded out of focus from entering the central region. The smoothed vertical slices from the sum between the black dashed lines is shown in the lower panels. A clear condensate peak is visible for both slices, but with different maximum values (red dashed lines).