We demonstrate one- and two-dimensional transverse laser cooling and magneto-optical trapping of the polar molecule yttrium (II) oxide (YO). In a 1D magneto-optical trap (MOT), we characterize the magneto-optical trapping force and decrease the transverse temperature by an order of magnitude, from 25 to 2 mK, limited by interaction time. In a 2D MOT, we enhance the intensity of the YO beam and reduce the transverse temperature in both transverse directions. The approach demonstrated here can be applied to many molecular species and can also be extended to 3D.

Atomic physics was revolutionized by the development of forced evaporative cooling: it led directly to the observation of Bose-Einstein condensation 1, 2 , quantum-degenerate Fermi gases 3 , and ultracold optical lattice simulations of condensed matter phenomena 4 . More recently, great progress has been made in the production of cold molecular gases 5 , whose permanent electric dipole moment is expected to generate rich, novel, and controllable phases 6-8 , dynamics [9][10][11] , and chemistry 12-14 in these ultracold systems. However, while many strides have been made 15 in both direct cooling and cold-association techniques, evaporative cooling has not yet been achieved due to unfavorable elastic-to-inelastic ratios 13 and impractically slow thermalization rates in the available trapped species. We now report the observation of microwave-forced evaporative cooling of hydroxyl (OH) molecules loaded from a Starkdecelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least an order of magnitude in temperature and three orders in phasespace density, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and sufficiently large initial populations, much colder temperatures are possible, and even a quantum-degenerate gas of this dipolar radical -or anything else it can sympathetically cool -may now be in reach.Evaporative cooling of a thermal distribution 16 is, in principle, very simple: by selectively removing particles with much greater than the average total energy per particle, the temperature decreases. In the presence of elastic collisions, the high-energy tail is repopulated and so may repeatedly be selectively trimmed, allowing the removal of a great deal of energy at low cost in particle number. This process may be started as soon as the thermalization rate is fast enough to be practical and continued until its cooling power is balanced by the heating rate from inelastic collisions. It generally yields temperatures deep into the quantum-degenerate regime (far below the recoil limit of optical cooling).The key metric for evaporation is therefore the ratio of two timescales. The first is the rate of elastic collisions, which rethermalize the distribution, while the second is the rate at which particles are lost from the trap for reasons other than their being deliberately removed, e. g. the rates of inelastic scattering and background gas collisions. Both theoretical 14,17,18 13,19,20 have seemed to show a generically poor value of this ratio across multiple molecular systems; this has led to a general belief that evaporative cooling is unfavorable in molecules 15 . As no trapped molecular system has achieved sufficiently rapid thermalization, there has been a lack of experiments to test this expectation.Hydroxyl would not, at first glance, seem to be a promising candidate for evaporative cooling. Its open-shell 2 Π 3/2 ground state and its propensity towards hydrogen bonding create a large anisotrop...

Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities to explore fundamental physics and enable new measurements. However, their size and use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 10 13 . These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks. Main Text:Optical atomic clocks, which rely on high-frequency, narrow-linewidth optical transitions to stabilize a clock laser, outperform their microwave counterparts by several orders of magnitude due to their inherently large quality factors (1). Optical clocks based on laser-cooled and latticetrapped atoms have demonstrated fractional instabilities at the 10 -18 level (2), setting stringent new limits on tests of fundamental physics (3, 4) and may eventually replace microwave clocks in global timekeeping, navigation and the definition of the SI second (5). Despite their excellent performance, optical clocks are almost exclusively operated by metrological institutions and universities due to their large size and complexity.Although significant progress has been made in reducing the size of laser-cooled atomic clocks to fit inside a mobile trailer (6), applications of these clocks are still limited to metrological clock comparisons and precision geodesy (7). In contrast, optical oscillators referenced to thermal atomic or molecular vapors can be realized in small form factors and still reach instabilities below 10 -14 (8,9). A fully integrated optical clock would benefit many of the applications (10) that currently utilize compact or chip-scale (11) microwave atomic clocks but, until recently, techniques for on-chip laser stabilization to atoms (12) and optical frequency division (13) were not available. Here, we propose and demonstrate an architecture for an integrated optical clock, based on an atomic vapor cell implemented on a silicon chip and a