Ul tra-c ol d atoms Jean Dalibard Laboratoire Kastle r Brossel, Depa rtement de Physique, Ecole Norma le Superieure , Paris, Fran ce Tuesday, 3 May- 2:30 p.m, Laser light is often associated with thenotionof heat. However, when properly adjusted,laser beams can cool atoms at e xtremely low temperatures , onebilliontimeslower than room temperature , opening thusthe pathto a wea lth of novel phenomena . Mytalk willfirst present the physica l principles of laser cooling,whicharebased onthee xchange of momentum and energy between lightand matter. I will then provide some illustratio nsofthis very active field of research , rangingfrom metrology to collect ive behaviours , like interference of matter w aves and superfluidity. The metrol ogical applications have been theinitial drivingforceofthedomain, with the simple idea thatslowatoms canbe probed fora longtime in anatomic clock, hence leadingto an increased preci sionofthe device . The research on co llective phenomena has been boosted bythe success of evaporativecooling,whichhasprovided the ultimate stepto bring a laser-cooled atom ic assemb ly down to the quantum regime. The quantum gases that are produced inthis way areeitherBose-Einstein condensates orde generate Fermi fluids(or mi xtures ofboth), depend ingonthestatistical nature oftheatomic species . Inthelast part ofthe talk I will describehowthiscold quantum mattercan pr ovide answersto questions that are still open for otherphys ical systems, such as superconducting ma teria ls or neutron stars.
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Ultra-coldatomsJean DalibardLaboratoire Kastler Brossel, Departement dePhysique,Ecole NormaleSuperieure, Paris, France
Tuesday, 3 May- 2:30 p.m,
Laser light isoften associatedwith thenotionof heat. However, when properly adjusted,laserbeamscan coolatoms at extremely lowtemperatures, onebilliontimeslower than room temperature, openingthusthepathto awea lth ofnovel phenomena .
Mytalkwill first present the physica l principles of laser cooling,whicharebased ontheexchange ofmomentum andenergy between lightandmatter. Iwill then provide some illustrationsof thisvery activefieldof research , rangingfrom metrology to collective behaviours, likeinterference of matter wavesand superfluidity.Themetrological applicationshave been theinitial drivingforce ofthedomain, withthesimple idea thatslowatoms canbeprobed fora longtime in anatomic clock, hence leadingtoanincreased precisionofthe device.The research oncollective phenomena hasbeen boosted bythesuccess of evaporativecooling,whichhasprovided theultimate stepto bring a laser-cooled atomicassemblydown to thequantum regime.
The quantum gases thatare produced inthisway areeitherBose-Einsteincondensates ordegenerateFermi fluids(ormixtures ofboth), dependingonthestatisticalnature oftheatomicspecies. Inthelastpartofthe talk Iwilldescribehowthiscold quantummattercan provide answersto questions thatarestill open forotherphysicalsystems, such assuperconducting materia lsorneutron stars.
Ultra-cold atoms
Jean DalibardEcole normale supérieureCNRS and UPMC
How one can use light to generate novel states of matter, or simulate already existing states
photo NIST
Light = source of information on matter
Spectroscopy gives us insights on the composition of stars, chemical elements in flames, gases in an electric discharge...
Can we use light to “act” on matter?
The elementary process in light-matter interaction
g: ground state
e: excited statelifetime τ quasi-resonant
photon:
In a photon absorption or emission process, the velocity of the centre-of-mass of the atom changes by
recoil velocity: 3 cm/s for sodium atoms (m = 23 u.m.a.), 3 mm/s for cesium atoms (m = 133 u.m.a.)
Photon momentum:
Internal energy levels of an atom
m: mass of the atom
The radiation pressure force
With a laser, the repetition rate is limited only by the lifetime of the electronic excited state (typically 10-8 s)
acceleration in the range 104 to 105 gatomresonant photons
The repetition of these elementary “collisions” creates a force on the atoms
The velocity drops from 100 m/s to 0 m/s on a distance ≈
cm.
Kepler : orientation of comets tails with respect to the sun
Outline of the talk
1. Laser cooled atoms and metrology
2. From optical molasses to Bose-Einstein condensates
3. Quantum gases: superfluidity and quantum simulators
The optical molasses
atoms
~ 2 cm
106 sodium atoms
NIST
cooling
Doppler cooling (Hänsch-Schawlow): an atom travelling to the right (resp. left) interacts more with the wave travelling to the left (resp. right)
Characteristic energy scale:
Choose
laserbeams
atomone-dimension model:
How to measure the temperature of the atoms?
The temperature is a measurement of the disordered motion of atoms in a gas, a liquid or a solid.
Cesium Δv = 1 cm/s
T = 2 μK
C. Salomon, J. Dalibard, W. D. Phillips, A. Clairon, and S. Guellati, Europhys. Lett. 12, 683 (1990) : Laser cooling of cesium atoms below 3 microKelvins
Sisyphus cooling
Y. Castin and J. Dalibard, Europhys. Lett. 14, 761 (1991) :Quantization of atomic motion in optical molasses.
The temperature scale for laser cooled gases
1 K 100 K 104K 106K1 mK1 μK T
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sun (surface)
sun (centre)
liquidnitrogen
BoseEinsteincondensates
Nobel 2001E. Cornell, W. Ketterle, C. Wieman
opticalmolasses
Nobel 1997S. Chu, C. Cohen-Tannoudji, W. Phillips
superfluidhelium
Principle of an atomic clock
Definition of the unit of time (‘second’):
a
b a and b are the two lowest energy levels of the cesium atom (isotope 133)
By definition, the electromagnetic wave that is resonant with the a-b transitionperforms 9 192 631 770 oscillations in a time interval of one second.