Rotational laser cooling of vibrationally and translationally cold molecular ions

Abstract

Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology 1-3 , quantum computing 4,5 and manybody quantum mechanics 6,7 , whereas in chemistry, stateprepared molecular targets are of interest for uni-molecular reactions with coherent light fields 8,9 , for quantum-stateselected bi-molecular reactions 10-12 and for astrochemistry 12. Here, we demonstrate rotational ground-state cooling of vibrationally and translationally cold MgH + ions, using a lasercooling scheme based on excitation of a single rovibrational transition 13,14. A nearly 15-fold increase in the rotational ground-state population of the X 1 + electronic groundstate potential has been obtained. The resulting ground-state population of 36.7 ± 1.2% is equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature setups. At present there is a strong interest within the scientific community to produce and experiment with cold molecules. As standard laser-cooling schemes developed for atomic species cannot be applied to molecules, in the recent past tremendous effort has been put into developing schemes to cool molecules by other means. Several schemes for producing translationally cold and strongly bound neutral molecules in a single quantum state have been demonstrated. These include buffer-gas cooling of magnetic dipolar molecules in magnetic traps 15 , Stark deceleration and trapping of molecules with permanent 16,17 or induced 18 electric dipole moments, formation of molecules through Feshbach resonances followed by transfer to the vibrational ground state of electronic molecular potentials by stimulated Raman adiabatic passage schemes 19 and photoassociative molecule formation directly in the rovibrational ground state 20 or by transfer to the vibrational ground state through the application of shaped femtosecond laser pulses 21. With respect to molecular ions, the well-established technique of buffer-gas cooling is the only method that has so far led to the production of translationally as well as internally cold molecular ions 22. Although buffer-gas cooling is simple and generally applicable, it limits both the effective internal and translational temperature to the kelvin range, and owing to frequent collisions with the buffer-gas atoms it prevents strong spatial localization as well as utilization of internal coherences. Sympathetic cooling of trapped molecular ions, through the Coulomb interaction with laser-cooled atomic ions, has, on the other hand, proven highly effective for reaching translational temperatures in the millikelvin range, where the ions become spatially localized in the form of so-called Coulomb crystals 23. The sympathetic cooling scheme does, however, not significantly influence the molecular ion's internal degrees of freedom, owing to the very distant Coulomb interactions within the crystalline structures. ...

Surface-electrode point Paul trap

“…Originating from mass spectrometry [1], they have then been applied in fields such as metrology [2], quantum information science [3,4], and cold molecular physics [5,6], to mention but a few.…”

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.…”

Optically induced structural phase transitions in ion Coulomb crystals

“…Ty, 37.10.Vz, 64.70.kp, 36.40.Ei When an ensemble of confined ions with the same sign of charge is cooled to a sufficiently low temperature, the ionic system forms a crystalline structure [1], often referred to as an ion Coulomb crystal. Since the first experimental realizations of ion Coulomb crystals through laser cooling of atomic ions into the milli-Kelvin regime in electromagnetic traps [2,3], there has been growing theoretical [4][5][6][7][8][9][10][11][12][13][14] and experimental [15][16][17][18][19][20][21][22][23][24] interest in studying the structural and dynamic properties of these crystals under different trapping conditions and for various ion compositions.The unique localization and isolation of the individual ions constituting the crystals have already led to a large number of amazing results within precision measurements [25], cavity quantum electrodynamics (CQED) [26][27][28][29][30], quantum information science [31][32][33][34][35], and cold molecular science [36][37][38][39]. For experiments involving larger three-dimensional ion Coulomb crystals, such as CQED related experiments [26,27] with the interesting prospect of creating quantum memories and other quantum devices, ...…”

“…The unique localization and isolation of the individual ions constituting the crystals have already led to a large number of amazing results within precision measurements [25], cavity quantum electrodynamics (CQED) [26][27][28][29][30], quantum information science [31][32][33][34][35], and cold molecular science [36][37][38][39]. For experiments involving larger three-dimensional ion Coulomb crystals, such as CQED related experiments [26,27] with the interesting prospect of creating quantum memories and other quantum devices, full structural control of the crystal structures is still in need for optimizing the coupling between the ions and the cavity modes.…”