Microwave Shielding of Ultracold Polar Molecules

Abstract

We use microwaves to engineer repulsive long-range interactions between ultracold polar molecules. The resulting shielding suppresses various loss mechanisms and provides large elastic cross sections. Hyperfine interactions limit the shielding under realistic conditions, but a magnetic field allows suppression of the losses to below 10 −14 cm 3 s −1 . The mechanism and optimum conditions for shielding differ substantially from those proposed by Gorshkov et al. [Phys. Rev. Lett. 101, 073201 (2008)], and do not require cancelation of the long-range dipole-dipole interaction that is vital to many applications.

Microwave shielding with far-from-circular polarization

“…Furthermore, Gorshkov et al suggested using combined static and microwave fields to achieve shielding by a repulsive second-order interaction, after precisely canceling the first-order interactions due to both fields [29]. While microwave shielding is feasible in static fields, the mechanism was later shown to be different and not reliant on precise cancellation of first-order interactions [27].…”

“…Next, we consider applying a static E-field along z , which lifts the degeneracy of m n = 0 and |m n | = 1 states, where m n is the z projection of the rotational angular momentum n. This Stark shift serves to shift the m n = 0 state, addressed by the spurious σ z polarization component, out of resonance while the microwaves are kept tuned to the m n = ±1 states. Under these conditions, this Hamiltonian effectively reduces to dressing with microwaves that are purely circularly polarized about a static external field, which has previously been demonstrated to realize effective shielding [27]. where the x and y axes are chosen as the semi-major and semi-minor axes.…”

“…(b) A static E-field along z lifts the degeneracy of m n = 0 and m n = ±1 states, such that the n = 1, m n = 0 state is Stark shifted out of resonance and the linear polarization component along z has no effect. Under these conditions, the Hamiltonian reduces to that of polar molecules in the presence of circularly polarized microwaves and a static E-field, which has previously been shown to realize effective shielding [27].…”

“…The latter corresponds to inelastic scattering into lower-lying field-dressed levels, which is referred to as MIL as these channels are not present in the absence of microwave radiation. The role of hyperfine degrees of freedom was addressed previously [27,30], and hyperfine is not included here as its effects can be suppressed by applying a modest magnetic field. Figure 2 shows the probability of RSR and MIL rate for RbCs molecules at 1 µK as a function of the Rabi frequency, Ω, and static field strength, E. This is obtained on resonance, ∆ = 0, and for fixed ellipticity, ξ = π/8, which is half way between σ + circular polarization at ξ = 0 and σ x linear polarization at ξ = π/4.…”

Ultracold collisions of polyatomic molecules: CaOH

“…We are guided here by the results of [25], where the long-range shielding due to an electrostatic field was shown to suppress chemical reactivity as well as long-range inelastic scattering. Going ahead, it will of course be valuable to incorporate the influence of any potential chemical reactivity, for example by means of absorbing boundary conditions [26,27]. Scattering at long range is driven by the dipole-dipole interaction,…”