Observation of Coherent Multiorbital Polarons in a Two-Dimensional Fermi Gas

We report on the experimental observation of multiorbital polarons in a two-dimensional Fermi gas of ${}^{173}\mathrm{Yb}$ atoms formed by mobile impurities in the metastable ${}^3{P}_0$ orbital and a Fermi sea in the ground-state ${}^1{S}_0$ orbital. We spectroscopically probe the energies of attractive and repulsive polarons close to an orbital Feshbach resonance and characterize their coherence by measuring the quasiparticle residue. For all probed interaction parameters, the repulsive polaron is a long-lived quasiparticle with a decay rate more than 2 orders of magnitude below its energy. We formulate a many-body theory, which accurately treats the interorbital interactions in two dimensions and agrees well with the experimental results. Our work paves the way for the investigation of many-body physics in multiorbital ultracold Fermi gases.

Figure 1

(a) Numerical calculation of the repulsive polaron (blue line), attractive polaron (red line), and bound dimer (dashed gray line) energies in quasi-2D across the OFR. (b) Relevant nuclear spin states in our measurement: The background Fermi sea is in the $m_F=+5/2$ state (blue, $|g,\uparrow ⟩$), while impurities are in the weakly interacting $m_F=-3/2$ ground state (blue, $|g,0⟩$) or the strongly interacting $m_F=-5/2$ “clock” state (yellow, $|e,\downarrow ⟩$).

Figure 2

Spectral response of the spin-imbalanced Fermi gas across the OFR. (a) Number of atoms transferred into $|e,\downarrow ⟩$ as a function of detuning $E/{\epsilon }_F$ and interaction strength $\ln ({\kappa }_F{a}_{2\mathrm{D}})$. Each data point is the average of two or three individual measurements. The blue and white (red and white) dashed line denotes the repulsive (attractive) polaron energy predicted from our theory. Additionally, we plot the dimer binding energy (solid magenta line) which saturates to $E=0$. (b)–(d) Raw spectra recorded with the clock laser at interaction parameters (b) $\ln ({\kappa }_F{a}_{2\mathrm{D}})=-0.72(4)$, (c) 0.26(4), and (d) 4.07(6). Here, zero detuning corresponds to the $|g,0⟩\to |e,\downarrow ⟩$ transition in the absence of a background Fermi sea. Error bars denote the standard error of the mean.

Figure 3

Coherent Rabi oscillations into attractive and repulsive polarons. (a) Measured quasiparticle residue $Z=(\mathrm{\Omega }/{\mathrm{\Omega }}_0{)}^2$ extracted from fits to Rabi oscillations. Blue circles (red squares) correspond to the repulsive (attractive) polaron. The theoretical prediction for $Z$ is shown as dashed lines. In the inset, we plot Rabi oscillations at $\ln ({\kappa }_F{a}_{2\mathrm{D}})=-0.41(5)$ (green), 0.84(5) (blue), and 7.52(10) (yellow) with fits (solid lines). The gray points and solid line correspond to a reference measurement without a background Fermi sea. (b) Damping rates ${\mathrm{\Gamma }}_R$ of the Rabi coupling into the repulsive (attractive) polaron state are shown as blue circles (red squares). The blue dashed line is the width of the repulsive polaron peak from our theory, while the red dashed line corresponds to $(1-Z)+{\gamma }_0$ with the fitted parameter ${\gamma }_0=0.06(2)$. Empty markers refer to data points binned with distinct ${\mathrm{\Omega }}_0$ and error bars denote the fit parameter error.

Figure 4

Repulsive polaron decay rate (blue circles). The error bars denote the fit error in ${\mathrm{\Gamma }}_{\text{rep}}$ and uncertainty in $\ln ({\kappa }_F{a}_{2\mathrm{D}})$. The blue dash-dotted line corresponds to the decay rate from three-body recombination in 2D for the mean kinetic energy in our system [37]. The inset shows sample population time traces recorded at $\ln ({\kappa }_F{a}_{2\mathrm{D}})=-0.94(4)$ (yellow), $-0.59(4)$ (green), and 0.05(4) (blue), as well as the fitted exponential curves (solid lines).