We have demonstrated microwave-assisted coherent control of ultracold $^{85}$Rb$^{133}$Cs molecules with a ladder-type configuration of rotational states. A probe microwave (MW) field is used to couple a lower state $X^1Sigma^+(v=0, J=1)$ and a middle state $X^1Sigma^+(v=0, J=2)$, while a control MW field couples the middle state and a upper state $X^1Sigma^+(v=0, J=3)$. In the presence of the control field, the population of middle rotational states, $X^1Sigma^+(v=0, J=2)$, can be reduced by a control MW field. Broadening of spectral splitting and shift of central frequency in this coherent spectrum are observed to be dependent on Rabi frequency of the control MW field. Applying Akaikes information criterion, we conclude that our observed coherent spectra happen through the crossover range of electromagnetically induced transparency and Aulter-Townes splitting as Rabi frequency of control field increases. Our work is a significant development in microwave-assisted quantum control of ultracold polar molecules with multilevel configuration, and also offers a great potential in quantum information based on ultracold molecules.
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