Transmission phase shift across a Kondo correlated quantum dot

We report on measurements of the transmission phase across a quantum dot embedded in an original two-path interferometer both in the strong and weak Kondo regime. The Kondo effect is a well known many-body phenomenon, which is characterized by a single energy scale, the Kondo temperature $T_{\mathrm{K}}$. In the strong Kondo regime at low temperatures ($T$/$T_{\mathrm{K}}$ \textless 1) we found that the transmission phase is locked to $\pi $/2 in the Kondo valley when the single level spacing $\delta $is significantly larger than the level broadening $\Gamma $. When $\Gamma $ is relatively large, on the other hand, the phase smoothly shifts by $\pi $ across two peaks on both ends of the Kondo valley without showing any plateau. As the temperature is increased exceeding $T_{\mathrm{K}}$, the Kondo correlation becomes lifted and then the phase shift looks similar to that in the Coulomb blockade regime, where the phase evolves $\pi $ across a Coulomb peak followed by a $\pi $-phase lapse in the Coulomb valley. In such a weak Kondo regime ($T$/$T_{\mathrm{K}}$ \textgreater 1) we observed asymmetric phase evolution about the valley center, which is linked to the orbital parity relation between the levels of interest.