Fano effect in an ultracold atom-molecule coupled system

The Fano resonance, with a unique asymmetric line shape, is a well-known manifestation of quantum interference with numerous promising applications for light-harvesting devices. Here we report an experimental observation of the Fano effect in an ultracold atom-molecule coupled system realized by photoassociation near a magnetically tunable $d$-wave Feshbach resonance. Our result shows that, compared to ubiquitous single-peak Fano-type line shapes, the coupled Fano resonances feature a two-peak profile. We develop a theory of coupled Fano resonances which agrees very well with the experimental results. This work may open a new perspective for tuning ultracold collisional interaction using the Fano resonance and studying bound states in continuum.

Figure 1

Experimental details and schematic of energy levels and their couplings. (a) Experimental geometry and configuration of both lasers and magnetic coil pairs for generating Fano resonances. (b) A schematic diagram of molecular potential. A pair of Cs atoms in the open channel is coupled to a $d$-wave bound molecular state in the closed channel $|c\rangle$ via Feshbach resonance. The PA laser near resonant with the free-bound transition can also induce the bound-bound transition and forms electronically excited molecular bound states in the outer well of the $0_g^{-}$ potential below the $6S_{1/2}+6P_{3/2}$ threshold. (c) Scheme for coupled dual Fano resonances. The same PA laser can induce couplings of both $\mid E\rangle$ and $\mid b_c\rangle$ to two quasidegenerate excited molecular states, $\mid b_1\rangle$ and $\mid b_2\rangle$, having the same vibrational and rotational quantum numbers but different molecular hyperfine quantum numbers.

Figure 2

Two-dimensional spectra of atom-loss strength. The number of atoms remaining in the trap after PA is recorded as a function of PA laser frequency at different $B$ around the $d$-wave Feshbach resonance. The color map is inverted to show that the PA-induced trap loss increases with a shift from blue to red.

Figure 3

$K_{\mathrm{av}}$ (spheres with error bars) as a function of $B$ near the $d$-wave Feshbach resonance for the rovibrational levels (a) $v=10,J=0$, (b) $v=17,J=0$, and (c) $v=10,J=2$. By combining the observed Fano minimum in $B$ with other chosen parameters, we have (a) $q_1=-0.3$ and $q_2=21.69$, (b) $q_1=0.3$ and $q_2=21.69$, and (c) $q_1=3.37$ and $q_2=7.82$. The solid line shows the prediction of the analytical theory from the coupled dual Fano resonances model with (a) ${\mathrm{\Gamma }}_1=15.5\mathrm{MHz}$ and ${\mathrm{\Gamma }}_2=0.04\mathrm{MHz}$, (b) ${\mathrm{\Gamma }}_1=10.5\mathrm{MHz}$ and ${\mathrm{\Gamma }}_2=0.001\mathrm{MHz}$, and (c) ${\mathrm{\Gamma }}_1=6.2\mathrm{MHz}$ and ${\mathrm{\Gamma }}_2=0.3\mathrm{MHz}$.