Extending Rotational Coherence of Interacting Polar Molecules in a Spin-Decoupled Magic Trap

Abstract

Superpositions of rotational states in polar molecules induce strong, long-range dipolar interactions. Here we extend the rotational coherence by nearly one order of magnitude to 8.7(6) ms in a dilute gas of polar 23 Na 40 K molecules in an optical trap. We demonstrate spin-decoupled magic trapping, which cancels first-order and reduces second-order differential light shifts. The latter is achieved with a dc electric field that decouples nuclear spin, rotation, and trapping light field. We observe density-dependent coherence times, which can be explained by dipolar interactions in the bulk gas.Interacting particles with long coherence times are a key ingredient for entanglement generation and quantum engineering. Cold and ultracold polar molecules [1][2][3][4][5][6][7][8][9][10][11] are promising systems for exploring such quantum manybody physics with long-range interactions [12,13] due to their strong and tunable electric dipole moment and long single-particle lifetime [14,15]. The manipulation of their rich internal degrees of freedom has been studied for different molecular species [16][17][18][19]. First observations include ultracold chemistry and collisions [20,21]. Nuclear spin states in the rovibronic ground state further promise exciting prospects for quantum computation due to their extremely long coherence times [22].Rotation is a particularly appealing degree of freedom for molecules because it is directly linked to their dipolar interactions. It can be manipulated by microwave (MW) fields and superpositions of rotational states with opposite parity exhibit an oscillating dipole moment with a magnitude close to the permanent electric dipole moment d 0 . Consequently, using rotating polar molecules has been proposed for quantum computation [23], to emulate exotic spin models [24] or to create topological superfluids [25].In order to make use of the rotational transition dipole in a spatially inhomogeneous optical trap, the coupling of the rotation to the trap field needs to be canceled. To first order this may be achieved by choosing an appropriate angle between the angular momentum of the molecule and the trapping field polarization ε [26] or a special trap light intensity [19] such that the differential polarizability between rotational ground and excited states is canceled. The trap is then referred to as "magic". Coherence times of about 1 ms have been achieved in bulk gases of polar molecules using these techniques [19,27]. However, this is much shorter than the dipolar interaction time, preventing observation of many-body spin dynamics.The coherence time in such a magic trap is limited by the intensity dependence of the molecular polarizabil-ity, which originates from the coupling between rotation, nuclear spins, and the trapping light field. It has been suggested to apply large magnetic [28] or electric fields [29] to reduce these couplings and thus simplify the polarizabilities of the involved states.In this work, we realize a spin-decoupled magic trap, i.e. a magic polarization angle trap wit...

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