Two-frequency population dynamics in a low-barrier double-well BEC

The dynamics of a a $^{87}$Rb Bose-Einstein condensate (BEC) are studied in a RF-dressed double-well. As we deform the trap from a single to a double well, we bias the system to prepare a population imbalance, $z$, between the wells. After suddenly removing the bias we observe population and phase dynamics. For $\mu \la V_b$, where $\mu$ is the chemical potential and $V_b$ the barrier height, we observe plasma oscillations in $z$ about $z = 0$, as predicted in the two-mode model of the Josephson junction. Additionally, we observe a second, higher frequency component in these oscillations. Through a comparison to three-dimensional GPE calculations, we find that the two-mode frequency dynamics are due to nonlinear mixing of the dipole mode with an octupole mode.