Broadband optical cooling of molecular rotors from room temperature to the ground state

Abstract

Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid spontaneous relaxation. Although atoms are routinely laser-cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational-vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here we cool trapped AlH þ molecules to their ground rotational-vibrational quantum state using an electronically exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH þ . We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3:8 þ 0:9 À 0:3 K, with the groundstate population increasing from B3 to 95:4 þ 1:3 À 2:1 % . This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications and precision tests of fundamental symmetries.

Non-destructive state detection for quantum logic spectroscopy of molecular ions

“…The technique will unfold its full potential in high precision spectroscopy of narrow transitions using an independent spectroscopy laser. However, while in the present work black-body radiation probabilistically populates the detected state, precision spectroscopy will require efficient state preparation schemes 28,29 or ro-vibrational cooling techniques [7][8][9][10][11] . A combination of these powerful tools will enable the realization of optical clocks based on molecular ions approaching the 10 −18 level 30 , where the underlying clock transitions or a combination of transitions can be sensitive to variations of fundamental constants 3 , an electron electric dipole moment (eEDM) 5 or parity violation in chiral molecules.…”

“…While the complexity of molecular structure facilitates these applications, the absence of cycling transitions poses a challenge for direct laser cooling 6 , quantum state control [7][8][9][10][11] , and detection. Previously employed state detection techniques based on photodissociation 12 or chemical reactions 13 are destructive and therefore inefficient, restricting the achievable resolution in laser spectroscopy.…”

Vibronic Spectroscopy of Sympathetically Cooled CaH+

“…Application of methods from the laser-cooling of atomic ions has led to new advances in taming both external and internal quantum states of trapped molecular ions. The new techniques have allowed for the sympathetic sideband cooling of motional modes to the ground state [6,7], preparation of the rotational ground state [8,9,10], and the non-destructive probing of rotational states [11]. The large rotational constants of diatomic hydrides [9,10,8,12] which slows blackbody rethermalization after rotational cooling combined with the availability of atomic coolants of similar masses make these molecular ions particularly well-suited to high precision spectroscopy measurements.…”

“…The new techniques have allowed for the sympathetic sideband cooling of motional modes to the ground state [6,7], preparation of the rotational ground state [8,9,10], and the non-destructive probing of rotational states [11]. The large rotational constants of diatomic hydrides [9,10,8,12] which slows blackbody rethermalization after rotational cooling combined with the availability of atomic coolants of similar masses make these molecular ions particularly well-suited to high precision spectroscopy measurements. Homonuclear diatomic molecular ions allow almost complete decoupling from blackbody radiation and are also a promising route towards tests of fundamental constants [13,14,15].…”

Rotational Cooling of Trapped Polyatomic Molecules

“…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).…”