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In cooperation with the Max-Planck-Institute of Quantum Optics and the Universitat Autònoma de Barcelona Cavity cooling of the internal and external motion of molecules M. Kowalewski, G. Morigi, P.W.H Pinkse and R. de Vivie-Riedle Outline Cavity cooling of the internal motion Theoretical approach Results Cavity cooling of the external motion Infrared transitions for molecular cooling Examples CHI3 and MgH+ Conclusion and Outlook Why cold molecules? • cold chemistry e.g. in astrophysics: • spectroscopy • quantum computing with cold molecules ( DeMille, Zoller, dVR) Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Many internal states Example: two normalmode system Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Many internal states Example: two normalmode system with combination band Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Many internal states Example: two normalmode system with rotation 2-Photon spectrocopy of difluorodiazirine (F2CN2) courtesy of Hans Sieber ... and worse and Eberhard Riedle assigned lines: 4748 IR-spectrum of difluorodiazirine courtesy of Hans Sieber and Eberhard Riedle Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Many internal states Spontaneous decay and Raman scattering heats vibrations and rotations → Cooling technique for the internal degrees of freedom Laser cooling and molecules Conventional optical cooling schemes fail for molecules: Absence of closed transitions → doppler cooling not possible Many internal states Spontaneous decay and Raman scattering heats vibrations and rotations → Cooling technique for the internal degrees of freedom (proposals: Tannor, Kosloff, Massnou-Seews …) Coherent scattering into modes of a high finesse cavity Cooling of the internal motion Setup Morigi, Pinkse, Kowalewski, de Vivie-Riedle, Phys. Rev. Lett. 99, 073001 (2007) Shifting the molecular spectrum The spectrum can be shifted by tuning the laser frequency Theoretical approach Coherent scattering into the cavity Spontaneous Raman scattering into free space Heating! Kowalewski, Morigi, Pinkse, de Vivie Riedle, Appl. Phys. B 89, 459, (2007) Ab initio calculations potential energy surfaces of OH: CASSCF/MRCI/cc-pVTZ polarizability functions: OH: Linear response theory for a laser wavelength of 532 nm vibrational wave functions are calculated with imaginary time propagation → polarization matrix elements Rotational cooling of OH Raman selection rules for diatomics: ΔJ=±2 → two independent ladders Desired molecular properties For rotational cooling a large anisotropy is needed Examples for molecules with fast cooling rates: COS (approx. 250 times faster) CS2 (approx. 1332 times faster) => cooling in few ms possible Applications: Keep the sample internally cool (protection for black body radiation) Efficient for precooled samples with few rotational states occupied Cavity-mediated Sideband cooling Using vibrational transitions to cool the external motion in a harmonic trap Sideband cooling scheme in the IR Cooling of the external motion of molecules in a harmonic trap Use vibrational states to form a two level system S. Zippilli, G. Morigi, Phys. Rev. Lett. 95, 143001 (2005) S. Zippilli, G. Morigi, W.P. Schleich J. Mod. Opt. 54, 1595 (2007) Sideband cooling scheme in the IR Cooling of the external motion of molecules in a harmonic trap Use vibrational states to form a two level system →Focus on the sideband cooling condition (Δ=-) S. Zippilli, G. Morigi, Phys. Rev. Lett. 95, 143001 (2005) S. Zippilli, G. Morigi, W.P. Schleich J. Mod. Opt. 54, 1595 (2007) Parameter regime Homogenous line widths in the IR: few Hz up to ~1000Hz → Long lifetimes Trap frequencies > 100kHz → Strong confinement, well resolved sidebands Choose vibrational transitions between 1000 – 4000 cm-1 → Lamb-Dicke Regime Loss channels are slow and manageable Only few rotational substates due to spontaneous decay Cavity – Trap Setup Possible traps: optical potentials for neutral molecules ion traps Criteria for the choice of molecules Desired molecular properties: “Good” transition dipole moment Transitions above 2500 cm-1 (tunable laser sources) → C-H, O-H, N-H Bonds For optical traps: large polarizability is needed → heavy atoms Iodoform in an optical potential properties calculated with ab intio methods (B3LYP/aug-cc-pVTZ) → C-H stretch is chosen as cooling transition → well localized vibration CHI3 in a far off resonant trap: Iodoform in an optical potential Example: MgH+ MgH+ in an Ion trap: Transition frequency at 1609 cm-1 transition frequency at 1609 cm-1 → not reachable for the laser → use the overtone at 3152 cm-1 (large anharmonicity!) maximum rate is still 1500 s-1 Conclusions Cavity cooling offers a variety of methods for molecules Vibrational and rotational levels can be addressed Cooling of the external motion seems possible Polarizability is one decisive molecular quantity for internal cooling as well as for trap depths Outlook Strategies for larger and more complex molecules Different parameter regimes: Cavity mediated Doppler cooling? Can collective enhancement increase the effeciency? Introduce optimal control theory to cavity cooling Thanks to: Markus Kowalewski Giovanna Morigi Pepijn W.H. Pinkse Funding: ESF: EUROQUAM/CMMC Munich Centre of Advanced Photonics