Optical Production of Ultracold Polar Molecules

Abstract

We demonstrate the production of ultracold polar RbCs molecules in their vibronic ground state, via photoassociation of laser-cooled atoms followed by a laser-stimulated state transfer process. The resulting sample of X 1 Σ + (v = 0) molecules has a translational temperature of ∼ 100 µK and a narrow distribution of rotational states. With the method described here it should be possible to produce samples even colder in all degrees of freedom, as well as other bi-alkali species.PACS numbers: 33.80. Ps, 39.25.+k, 33.80.Wz, 33.70.Ca Samples of ultracold, polar molecules (UPMs) can provide access to new regimes in many phenomena. Ultracold temperatures allow trapping, and polarity can be used to engineer large, anisotropic, and tunable interactions between molecules. These features make UPMs attractive as qubits for quantum computation [1], as building blocks for novel types of many-body systems [2], and for the study of chemistry in the ultracold regime [3]. Furthermore, UPMs can be used as uniquely sensitive probes of phenomena beyond the Standard Model of particle physics [4].Methods such as buffer-gas cooling [5], Stark-slowing [6], billiard-like collisions [7], and velocity filtering [8] have produced samples of polar molecules at temperatures of ∼ 10-100 mK. Formation of heteronuclear molecules from pre-cooled atoms via photoassociation (PA) [9,10,11,12] or Feshbach resonance techniques [13] promises access to much lower temperatures; however, these processes leave molecules in highly excited vibrational levels, which have vanishingly small polarity [14] and are unstable to collisions [15,16]. The possibility of transferring such molecules to their vibronic ground state, via optical processes such as stimulated Raman transitions, has been discussed extensively (see e.g. [1,17,18,19,20]); however, insufficient data on the structure of experimentally accessible molecules has made it difficult to identify specific pathways for efficient transfer.Here we report the production of UPMs via PA of laser-cooled Rb and Cs atoms, followed by a two-step stimulated emission pumping (SEP) process. This yields RbCs molecules in their absolute vibronic ground state X 1 Σ + (v = 0). These polar molecules (calculated electric dipole moment µ ≈ 1.3 D [21]) have a translational temperature of ∼ 100 µK. The distribution of rotational states is also quite narrow, so the resulting sample of X 1 Σ + state molecules is cold in all degrees of freedom. Figure 1 shows the methods by which we produce and detect UPMs. A pair of colliding, ultracold Rb and Cs atoms is photoassociated, i.e., the pair absorbs a photon and is driven to an electronically excited molecular level [22]. This level decays rapidly, with branching fraction of ∼ 7% into the long-lived a 3 Σ + (v = 37) level [9]. After a period of PA, a resonant laser pulse ("pump" pulse) transfers these metastable, vibrationally excited molecules to an intermediate, electronically excited state (i). The population of state i is monitored by applying an intense laser pulse ("ionizat...

Fourier-transform spectroscopy and coupled-channels deperturbation treatment of theA 1Σ+–b 3Πcomplex of KCs

“…The experimental data field starts from the lowest vibrational level v * A = 0 of the singlet and nonuniformly covers rotational quantum numbers J ∈ [7,225] in the energy range E J ∈ [10040, 13250] cm −1 . Overall 42 adjusted fitting parameters have been required to reproduce more than 3400 rovibronic termvalues of the A ∼ b complex with a standard deviation of 0.004 cm −1 which is absolutely consistent with the uncertainty of the experiment of 0.003-0.01 cm −1 .…”

“…Due to proximity of K(4p) and Rb(5p) energies, the energy of low-lying electronic states of KCs is similar to the ones in the RbCs molecule, however, KCs possess larger permanent electric dipole moment. The RbCs molecule has been already studied in ultracold conditions [7,8,9]. In particular, the recent quantum dynamics simulation of the mixed A 1 Σ + and b 3 Π Ω states of RbCs in a time-dependent wave-packet approach [6] demonstrated that the pump -dump picosecond pulsed laser scheme could be efficient to form ultracold heteronuclear diatomic molecules in highly bonded ground X 1 Σ + state.…”

“…Overall more than 200 collisionally enhanced LIF spectra were rotationally assigned to 39 K 133 Cs and 41 K 133 Cs isotopomers yielding with the uncertainty of 0.003-0.01 cm −1 more than 3400 rovibronic term values of the strongly mixed singlet A 1 Σ + and triplet b 3 Π states. Experimental data massive starts from the lowest vibrational level vA = 0 of the singlet and nonuniformly cover the energy range E J ∈ [10040, 13250] cm −1 with rotational quantum numbers J ∈ [7, 225]. Besides of the dominating regular A 1 Σ + ∼ b 3 ΠΩ=0 interactions the weak and local heterogenous A 1 Σ + ∼ b 3 ΠΩ=1 perturbations have been discovered and analyzed.…”

Three-body interactions with cold polar molecules

“…11,12,13,14,15,16 and cooling of polar molecules with the goal to create quantum degenerate ground state molecules are currently developed in several laboratories. 17,18,19,20,21,22,23 We show below that for an appropriate choice of static electric and microwave fields the effective interaction reduces to the form as in Eq. (1) with tunable two-body and the three-body interactions.…”

Preparation of a quantum state with one molecule at each site of an optical lattice

“…A promising strategy for the creation of such molecules is based on the association of ultracold atoms using a Feshbach resonance, or photoassociation and subsequent transfer to a much lower rovibrational level using Raman transitions 16 . If the moleculemolecule interactions are predominantly elastic and effectively repulsive, then a state with one molecule per lattice site can finally be obtained using a quantum phase transition from a superfluid to a Mott insulator by ramping up the depth of an optical lattice 6 .…”