The rich internal structure and long-range dipole-dipole interactions establish polar molecules as unique instruments for quantumcontrolled applications and fundamental investigations. Their potential fully unfolds at ultracold temperatures, where a plethora of effects is predicted in many-body physics [1,2], quantum information science [3,4], ultracold chemistry [5,6], and physics beyond the standard model [7,8]. These objectives have inspired the development of a wide range of methods to produce cold molecular ensembles [9][10][11][12][13][14]. However, cooling polyatomic molecules to ultracold temperatures has until now seemed intractable. Here we report on the experimental realization of opto-electrical cooling [15], a paradigm-changing cooling and accumulation method for polar molecules. Its key attribute is the removal of a large fraction of a molecule's kinetic energy in each step of the cooling cycle via a Sisyphus effect, allowing cooling with only few dissipative decay processes. We demonstrate its potential by reducing the temperature of about 10 6 trapped CH 3 F molecules by a factor of 13.5, with the phase-space density increased by a factor of 29 or a factor of 70 discounting trap losses. In contrast to other cooling mechanisms, our scheme proceeds in a trap, cools in all three dimensions, and works for a large variety of polar molecules. With no fundamental temperature limit anticipated down to the photon-recoil temperature in the nanokelvin range, our method eliminates the primary hurdle in producing ultracold polyatomic molecules. The low temperatures, large molecule numbers and long trapping times up to 27 s will allow an interaction-dominated regime to be attained, enabling collision studies and investigation of evaporative cooling toward a BEC of polyatomic molecules.The ability to prepare ultracold molecular ensembles has an application potential akin to that of ultracold atoms some decades ago. In fact, the association of KRb dimers [16] as well as the laser cooling of SrF [17] has brought fascinating physics within reach. However, both approaches are restricted to a highly specialized set of purely diatomic molecule species. In order to investigate fundamental physics based on relativistic effects near heavy nuclei or parity violation effects in chiral molecules, or to study molecules of astrophysical, biological, or chemical interest, a more general approach to preparing ultracold molecular ensemble is imperative. This holds in particular for the rich chemical variety of carbon-, nitrogen-, or oxygen-based molecules for which the constituent atoms have not even been laser cooled. Devising a dissipative process to cool such molecules into the ultracold regime has been an exceedingly challenging problem. The standard approach for atoms, laser cooling, is in general impossible for molecules due to the lack of suitable cycling transitions. Creating an artificial cycling transition via cavity cooling [18] has not been demonstrated despite substantial experimental [19] and theoretical [20][21][...

We demonstrate direct cooling of gaseous formaldehyde (H2CO) to the microkelvin regime. Our approach, optoelectrical Sisyphus cooling, provides a simple dissipative cooling method applicable to electrically trapped dipolar molecules. By reducing the temperature by 3 orders of magnitude and increasing the phase-space density by a factor of ∼10(4), we generate an ensemble of 3×10(5) molecules with a temperature of about 420  μK, populating a single rotational state with more than 80% purity.

Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multiphoton ionization or laser-induced fluorescense do not work. As this is the case for most polyatomic molecule species, in this paper we investigate an alternative based on state-selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge, as the frequency differences between various transitions are easily substantially less than the Stark broadening in an electric trap. However, by using a unique trap design that provides homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M-substate.substate alone is only possible for M = 1, for > M 1 it is at least theoretically possible to deplete two sets of rotational states, which differ by the single M-substate of interest.As an example, we discuss the depletion of the single M-substate | + = + 〉 J K M J 1; ;1 , using MWD and IRD (see figures 1 and 2). In both cases, the MW coupling,, is the only one that addresses the | + = + 〉 J K M J 1; ;1 state. The population of this state is detected by running the whole experimental sequence twice: the depletion (MWD or IRD) is applied once with driving the1; ; 1 transition, and once without. The difference of both measurements yields the population in the | + = + 〉 J K M J 1; ;1 state. An extension to other single M-substates is possible, but at the same time more challenging due to experimental limitations. Using MWD, any single M-substate can be detected by coupling all M-substates up to the desired single M-substate and measuring twice, as explained above. To be able to implement this scheme, a sufficient spectral resolution is needed to resolve all individual transitions. In the case of IRD, one has to keep in mind that spontaneous decay can lead to population transfer to the state of interest. J = 3 and J = 5 states used for our depletion schemes, as shown in figure 4.IR transition frequencies of the v 1 vibrational mode are provided in [47] and were verified via saturated absorption spectroscopy in a room-temperature cell. The transition frequencies were determined with sub-MHz resolution using a frequency comb. We found that, luckily, the Q-branch IR transition from the = | | = J K 3, 3 state is well isolated, with the closest relevant transition from a different state being several GHz away (see figure 6). Our spectral resolution is clearly better than this, and thus the driving of this transition is K and J selective. The IR transition from the = | | = J K 5, 3 state is less isolated. Here, within several GHz we find also the IR transitions from the = | |= J K 7, 8, 2 states and = | |= J K 8, 9, 10, 1 states. However, inside our trap, these states are in sum populated by at most a few percent, and can...

Controlling the internal degrees of freedom is a key challenge for applications of cold and ultracold molecules. Here, we demonstrate rotational-state cooling of trapped methyl fluoride molecules (CH3F) by optically pumping the population of 16 M -sublevels in the rotational states J=3, 4, 5, and 6 into a single level. By combining rotational-state cooling with motional cooling, we increase the relative number of molecules in the state J=4, K=3, M =4 from a few percent to over 70%, thereby generating a translationally cold (≈ 30 mK) and nearly pure state ensemble of about 10 6 molecules. Our scheme is extendable to larger sets of initial states, other final states and a variety of molecule species, thus paving the way for internal-state control of ever larger molecules.Motivated by a multitude of applications ranging from quantum chemistry to many-body physics [1][2][3][4], recent years have witnessed an immense effort to generate cold and ultracold ensembles of polar molecules [5][6][7][8][9][10]. Much of this attention has focused on diatomic molecules, despite unique possibilities for polyatomic molecules [11][12][13][14]. The latter possess additional rotational and vibrational degrees of freedom which could be used for various applications. For example, symmetric-top molecules have been suggested to be ideally suited to simulate quantum magnetism [11,12]. Moreover, precision tests of physics based on chirality require molecules with at least four atoms [13]. In addition, single large molecules have been suggested for the realization of an entire quantum computer, using different vibrational modes to encode individual qubits [14]. Last but not least, the high vapor pressure for many polyatomic molecule species, even at room temperature, allows the efficient generation of high-density initial ensembles [15].A key challenge for obtaining cold and ultracold molecular ensembles has been gaining and maintaining control of the internal molecular state. While this is true for molecules in general, it is particularly problematic for larger, polyatomic molecules. Thus, even for the relatively light molecule CH 3 F discussed here, several thousand rotational states are populated at room temperature. For larger molecules, a huge number of states is populated even at liquid-helium temperatures. Gaining quantum-state control of such molecules requires some form of internal-state cooling. While internal-state cooling has been demonstrated for bialkali dimers [16][17][18] as well as for a number of diatomic molecular ions [19][20][21][22][23], its implementation for polyatomic molecules is lacking.In this Letter, we demonstrate comprehensive internalstate control of the polyatomic molecule methyl fluoride (CH 3 F). In a two-step process, molecules in 16 rotational M -sublevels in the lowest four rotational J states in the |K|=3 manifold are optically pumped into a single rotational M -sublevel (J, K, M being the usual symmetrictop rotational quantum numbers). As a first step, we demonstrate rotational-state cooling (RSC) b...