Download Cavity cooling of the internal and external motion of molecules

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