Understanding and controlling collisions is crucial to the burgeoning field of ultracold molecules. All experiments so far have observed fast loss of molecules from the trap. However, the dominant mechanism for collisional loss is not well understood when there are no allowed 2-body loss processes. Here we experimentally investigate collisional losses of nonreactive ultracold 87 Rb 133 Cs molecules, and compare our findings with the sticky collision hypothesis that pairs of molecules form long-lived collision complexes. We demonstrate that loss of molecules occupying their rotational and hyperfine ground state is best described by second-order rate equations, consistent with the expectation for complex-mediated collisions, but that the rate is lower than the limit of universal loss. The loss is insensitive to magnetic field but increases for excited rotational states. We demonstrate that dipolar effects lead to significantly faster loss for an incoherent mixture of rotational states.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Polar molecules offer a new platform for quantum simulation of systems with long-range interactions, based on the electrostatic interaction between their electric dipole moments. Here, we report the development of coherent quantum state control using microwave fields in 40 Ca 19 F and 87 Rb 133 Cs molecules, a crucial ingredient for many quantum simulation applications. We perform Ramsey interferometry measurements with fringe spacings of ∼ 1 kHz and investigate the dephasing time of a superposition of N = 0 and N = 1 rotational states when the molecules are confined. For both molecules, we show that a judicious choice of molecular hyperfine states minimises the impact of spatially varying transition-frequency shifts across the trap. For magnetically trapped 40 Ca 19 F we use a magnetically insensitive transition and observe a coherence time of 0.61(3) ms. For optically trapped 87 Rb 133 Cs we exploit an avoided crossing in the AC Stark shifts and observe a maximum coherence time of 0.75(6) ms.

We present the design, construction, and characterisation of longitudinal-and transverse-field Zeeman slowers, based on arrays of permanent magnets, for slowing thermal beams of atomic Sr. The slowers are optimised for operation with deceleration related to the local laser intensity (by the parameter ), which uses more effectively the available laser power, in contrast to the usual constant deceleration mode. Slowing efficiencies of up to ≈ 18 % are realised and compared to those predicted by modelling. We highlight the transverse-field slower, which is compact, highly tunable, light-weight, and requires no electrical power, as a simple solution to slowing Sr, well-suited to spaceborne application. For 88 Sr we achieve a slow-atom flux of around 6 × 10 9 atoms s −1 at 30 ms −1 , loading approximately 5 × 10 8 atoms in to a magneto-optical-trap (MOT), and capture all isotopes in approximate relative natural abundances.

We propose and demonstrate the laser cooling and trapping of Rydberg-dressed Sr atoms. By offresonantly coupling the excited state of a narrow (7 kHz) cooling transition to a high-lying Rydberg state, we transfer Rydberg properties such as enhanced electric polarizability to a stable magneto-optical trap operating at < 1 μK. Simulations show that it is possible to reach a regime where the long-range interaction between Rydberg-dressed atoms becomes comparable to the kinetic energy, opening a route to combining laser cooling with tunable long-range interactions. DOI: 10.1103/PhysRevLett.120.183401 The strong interactions between Rydberg atoms have led to numerous experimental breakthroughs in many-body quantum simulation [1][2][3][4], quantum information processing [5][6][7], and quantum optics [8][9][10][11][12]. To take advantage of coherent dynamics, these realizations have focused on timescales shorter than the lifetime of the Rydberg state. However, there is growing interest in extending the investigation time of Rydberg ensembles for applications in quantum simulation and metrology [13][14][15]. A promising method is to off-resonantly couple the ground state to a Rydberg state, resulting in the controlled admixture of some interacting Rydberg character [16][17][18][19][20]. This Rydberg dressing approach could enable the realization of supersolids [21][22][23][24], frustrated or topological quantum magnetism [25][26][27][28], or spin squeezing for metrology [15,17]. Experimentally, Rydberg dressing has been demonstrated for two atoms [29] and in optical lattices [3,30], but it seems to be more challenging in randomly distributed ensembles due to uncontrolled loss mechanisms [31][32][33][34].In this Letter we introduce a new scheme where Rydberg dressing is applied to an excited state undergoing spontaneous emission (Fig. 1). We show that Rydberg-dressed atoms can be laser cooled to sub-microkelvin temperatures and trapped in a magneto-optic trap (MOT), while simultaneously acquiring properties of the Rydberg state such as enhanced sensitivity to dc electric fields. The result is a hybrid magneto-electro-optical trap controllable by electric as well as magnetic fields. We show that the Rydbergdressed MOT can operate in a regime where the interaction strength is comparable to the dissipation and the kinetic energy, and with a lifetime that exceeds that of the Rydberg state by a factor of ∼70. Although laser cooling of Rydberg-dressed atoms has been proposed to protect crystalline phases against dissipative effects [20], active cooling of Rydberg gases is a relatively unexplored area [35,36] where interesting physics could arise from the presence of cooling and the mechanical effect of the interactions. Examples could include Sisyphus-like cooling [37,38] induced by the Rydberg-dressed potential or, in the spirit of antiblockade experiments [39], cooperative cooling where the collective scattering of multiple photons by groups of atoms dominates over single particle cooling.The laser cooling and Rydberg dre...