Development of a rotationally magic trap for ultracold RbCs molecules

Ultracold polar molecules offer many exciting opportunities in the fields of quantum computation, quantum simulation and fundamental studies of quantum matter. Many of these applications utilise the rotational states of the molecule and rely on long rotational coherence times. Achieving this in experiments has so far proved challenging, however, owing to the presence of large differential light shifts between rotational levels as a result of the anisotropic molecular polarizability. The solution is to construct a magic-wavelength trap, where the polarizabilities are identical for two (or more) rotational states. Here we report the development of such a magic trap at a wavelength of 1146nm. This wavelength lies between the X¹Σ→b³Π vibronic transitions, allowing the anisotropic component of the polarizability to be tuned to zero. We present spectroscopy of the X¹Σ→b³Π transitions and show that the differential shift of the N=0→N=1 rotational transition can be tuned to be zero in the magic trap for a detuning of approximately 220 GHz from the X¹Σ (v=0,N=0)→b³Π(v'=0,N=1) transition. Using Ramsey spectroscopy we show the absence of dephasing in the magic trap. Based upon reasonable parameters for the laser frequency stability and intensity, we estimate coherence times greater than 10s should be easily achievable in a magic 3D optical lattice.