  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
$X^1\Sigma^+(v=0, J=1)$ and a middle state $X^1\Sigma^+(v=0, J=2)$, while a
control MW field couples the middle state and a upper state $X^1\Sigma^+(v=0,
J=3)$. In the presence of the control field, the population of middle
rotational states, $X^1\Sigma^+(v=0, J=2)$, 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 Akaike's 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.
