Exploration of the Ultracold World

1
Exploration of the Ultracold World

• Ying-Cheng Chen(???), Institute of Atomic
  Molecular Sciences, Academia Sinica
• 12 October, 2009, NDHU

2
Outline

• Overview of Ultracold Atoms
• Introduction to Ultracold Molecules
• Exploration I Molecular cooling
• Exploration II Nonlinear optics with ultracold
  atoms

3
Studying, Research and Life Adventure
Exploration
4
Temperature Landmark
Core of sun
L He
2003 MIT Na BEC
Sub-Doppler cooling
surface of sun
L N2
3He superfluidity
0
(K)
106
103
1
10-3
10-6
10-9
Room temperature
Rb MOT
Typical TC of BEC
5
What is special in the ultracold world?

• A bizarre zoo where Quantum Mechanics governs
• Wave nature of matter, interference, tunneling,
  resonance
• Quantum statistics
• Uncertainty principle, zero-point energy
• System must be in an ordered state
• Quantum phase transition

1µm for Na _at_ 100nk
Superfluid-Mott insulator t Ransition, Max-Planck
Vortex Lattice, JILA MIT
Matter wave interference, MIT
Fermi pressure, Rice
6
Laser Cooling Trapping

• Cooling, velocity-dependent force Doppler effect
• Trapping, position-dependent force Zeeman effect

Laser
fv
7
Magnetic Trapping Evaporative Cooling
Microwave transition
8
Modern Atomic Physics Science Technology
Quantum simulation of condensed-matter
physics BEC/Degenerate Fermi gas Superfluidity/sup
erconductivity Quantum phase transition BEC/BCS
crossover Antiferromagnetism/ high Tc
superconductivity
Precision measurement Atomic clock Test of
particle physics (EDM) Test of nuclear physics
(parity violation) Test of general
relativity Variation of physical constants
Core technology
Atom manipulation
Quantum information science Quantum
control Quantum teleportation Quantum
network Quantum cryptography Quantum computing
Opto-mechanics Nano-photonics Laser cooling of
mirror /mechanical oscillator Coupling of cold
atom with mesoscopic(nano) object Quantum limit
of detection Near field optics
Laser advancement
Weakness Molecule manipulation
Extreme nonlinear optics Atom/molecule under
intense short pulse High harmonic
generation X-ray laser Attosecond laser
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Double Helix of Science Technology
Technology
Better understanding of science helps technology
moving forward
Science
Better technology helps to explore new science
It is a tradition in AMO physics to extend new
technology to explore physics at new regime.
10
Core Technology

• Atom cooling
• Laser technology

Microwave transition
atom trapping /optical lattice
Laser cooling
Magnetic-tuned Feshbach resonance
evaporative cooling
Ultra-short
250 as
Ultra-stable
Ultra-intense
Sub-Hz
100TW
Lasers
Ultra-narrow -linewidth
Non-classical (single photon, entangled photon
pairs)
Sub-Hz
11
Cold Molecules Why ?

• Test of fundamental Physics.
• Search for electron dipole moment
• Quantum Dipolar Gases
• Add new possibility in quantum simulation.
• Cold Chemistry
• Chemistry with clear appearance of quantum
  effects
• Controlled reaction
• Quantum Computation
• Long coherence time and short gate operation time

12
Cold molecules How ?
Coherent transfer from Feshbach molecule
Enhanced PA? Laser cooling? Sympathetic
cooling? Evaporative cooling?
Buffer gas cooling
Electric, magnetic, optical deceleration
Photo- association
Indirect approach
Direct approach

13
Breakthrough in Indirect Approach

• The door to study quantum degenerate dipolar
  gases and quantum information with polar
  molecules is opened by JILAs recent experiment
  with indirect approach.

K.-K. Ni et al Science, 18,1(2008)
14
Laser Cooling of Molecule ?Not so cool !

• Its impractical to implement laser cooling in
  molecules due to the lack of closed transition
  with their complicated internal structures.

See, however, Di Rosa, Eur.Phys. J. D 31,395
(2004) for molecules with nearly closed
transition.
The ying and yang (dark/bright) sides of
molecules. You have to pay the price !
15
Our approach ? General considerations

• Choose the direct approach to make cold molecules
  in order to have more impacts in other fields as
  well.
• Generate a large number of molecules in the first
  stage.
• Build an AC trap in order to avoid the inelastic
  collision loss.
• Use sympathetic cooling with laser-cooled atoms
  in the ac trap to overcome mK barrier for direct
  cooling.
• What advantages to take? What disadvantages to
  live with?

sympathetic cooling Inelastic collision? Reaction?

loading
Molecules precooling
Trapping
Ultracold Molecules
loading
Laser-cooled atoms
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Routes Towards Ultracold Molecules
1 mK
1 µK
1 K
Buffer gas cooling plus magnetic guiding
Sympathetic cooling in a microwave trap by
ultracold cesium atoms.
Evaporative cooling in a microwave trap.
Radiative damping trap loading
SrF molecule
Cs atom
17
Recent Ideas
1 mK
1 µK
1 K
Buffer gas cooling plus magnetic guiding
Direct laser cooling
Evaporative cooling in an optical dipole trap.
18
What molecule? SrF, Why?

• Alkali-like electronic structure with strong
  transitions at visible wavelengths. Easy to be
  detected by convenient diode lasers.
• Large electric dipole moment, 3.47 D and many
  bosonic and fermionic isotopes . More
  possibilities in the future.
• Microwave trapping consideration. Available
  microwave high power amplifier at its rotational
  transition (2B 15 GHz).
• With nearly diagonal Frank-Condon array that
  allow direct laser cooling with reasonable number
  of lasers.
• Suitable for test of fundamental physics and
  quantum information science.
• Radical molecules. Disadvantages in molecule
  generation.
• What advantages to take? What disadvantages to
  live with ?

19
Buffer Gas Cooling
X2S,v1?A2?1/2,v1
Q12(7.5)
P11(8.5)
P11(7.5)
Q12(6.5)
Q12(5.5)
P11(6.5)
P11(5.5)
Q12(4.5)
SrF molecules generated by laser ablation of
SrF2 solid.
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Development of an intense SrF Molecular Beam
2B3 SrF2(high-temperature1500K)?BF3Sr2SrFBF
Electron-bombardment heating
If one want to work with (cold) molecules then
he need to learn some chemistry !
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SrF Beam Characterization
Laser beam
Light baffle
10cm
13cm
5cm
?3mm
?2mm
skimmer
PMT
oven
Residual gas analyzer
Turbo pump
Brewster window
chopper
ECDL laser New Focus 6009/6300
Toptica WS-7 Wavelength meter
Setup for laser-induced fluorescence
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Typical Spectrum
(0,0) vibrational band of A2?1/2- X2S
transition of 88SrF Laser intensity 5
00mW/cm2 FWHM linewidth 130MHz S/N ratio gt200
Even near the congested band edge, all hyperfine
lines are well resolved !
Laser intensity 5mW/cm2 FWHM linewidth 15
MHz S/N ratio gt 50 Hyperfine lines resolved
(I1/2 for 19F)
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Beam Characterization
Flux v.s. oven temperature
Flux stability 20 / one hour
Highest flux of 2.11015 /(steradian.sec)! Even
stronger and more stable beam is possible by
resistive heating and is under development! An
intense SrF radical beam for molecule cooling
experiment submitted to Phys. Rev. A.
24
Better Spectroscopy of SrF

• The rotational/hyperfine lines of (0,0)
  A2?1/2- X2S band 88SrF have been recorded to
  10-4 cm-1 precision with a fitting accuracy of
  10-3 cm-1 to the effective Hamiltonian.

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Theoretical Modeling

• Effective Molecular Hamiltonian
• Better molecular constants have been determined !

parameter T00 B D A p q
Value(cm-1) 15216.33978(19) 0.2528325(12) 2.5274(28)x10-7 281.46333(34) -0.13353(9) 9.32(3.8)x10-5
High-resolution laser spectroscopy of the (0,0)
band of A2?1/2- X2S transition of 88SrF
submitted to J. of Mol. Spec.
26
Buffer-Gas-Cooled Molecular Beam Guiding

• On-going work

Dewar
cryostat
Magnetic guide
oven
Helium
SrF
Spectroscopy or laser cooling
UHV Chamber
Turbo pump
Estimation of Flux (6.61015/s)
(910-4)x(2.910-3)1.71010/s _at_ 5K Already
very intense for a radical beam! Higher flux is
possible with modified oven.
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Routes Towards Ultracold Molecules
1 mK
1 µK
1 K
Buffer gas cooling plus ac electric guiding
Sympathetic cooling in a microwave trap by
ultracold cesium atoms.
Evaporative cooling in a microwave trap.
Radiative damping trap loading
SrF molecule
Cs atom
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Development of the Microwave Trap
DeMille, Eur.Phys.J D 31,375(2004)

• Advantages of microwave trap
• High trap depth ( 1K)
• Large trap volume ( 1cm3)
• Good optical access. Allow overlap of MOT with
  trap for sympathetic cooling.
• It can trap molecules in the absolute ground
  states and thus immune to inelastic collisions
  loss at low enough temperature.

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Observation of standing wave pattern by
thermal-sensitive LCD sheet
Q11000 ?0.87 Pin1060W R0.217m D0.2m
E00.45 MV/m
Trap depth 0.1 K for SrF ground state
A high-power microwave Fabry-Perot resonator
for molecule trapping experiment Rev. Sci. Inst.
In preparation.
30
Routes Towards Ultracold Molecules
1 mK
1 µK
1 K
Buffer gas cooling plus ac electric guiding
Sympathetic cooling in a microwave trap by
ultracold cesium atoms.
Evaporative cooling in a microwave trap.
Radiative damping trap loading
SrF molecule
Cs atom
31
Sympathetic Cooling of Molecules by Ultracold
Atoms

• Conceptually easy but depends on unknown
  collision properties.

32
Large-number Ultracold Atom System

• Initially developed for molecule sympathetic
  cooling (with N 1010).
• Found its application in low-light-level
  nonlinear optics based on electromagnetic-induced
  transparency (EIT).

7cm
trapping
Coilscell
Absorption Spectrum
Atom cloud
probe
Optical density105 for Cs D2 line F4 ?F5
trapping
trapping beam
An elongated MOT with high optical
density Optics Express 16,3754(2008)
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Quest of Second Stage Cooling to overcome the mK
Barrier for Direct Approach

• Sympathetic cooling with ultracold atoms
• Not so promising due to strong inelastic loss
• AC trap is necessary
• Cavity laser cooling
• Havent been demonstrated.
• Direct laser cooling
• Being demonstrated
• Limited to a few species
• Single-photon (information) cooling
• In combination with magnetic trapping
• May be demonstrated soon
• ...

M.Raizen
34
Laser Cooling of SrF to overcome the mK barrier!

• Di Rosa, Eur.Phys. J. D, 31,395 (2004)

state X2S,v0 v1 v2 v3
A2?, v0 0.9895 0.0103 1.33x10-4 1.57x10-6
J Phy Chem A, 102,9482,1998
By repumping the v1 population back to v0, the
transition is closed to 10-4 level
0.999867360062
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Considering to hyperfine states, it is necessary
to generate two frequencies differed by 50 or
107 MHz by acousto-optical modulator for each
laser.
Considering to rotational states, four lasers
(two _at_ 663nm and two _at_685nm ) required to close
the transition to 10-4 level.
36

• Nonlinear optics with ultracold atoms
• - Detour of my planned journey but back to my
  old track !

37
Electromagnetically-induced Transparency
Transparent!
Probe laser
Coupling laser
Physical origin destruction interference between
different transition pathways!
3gt
coupling
probe




2gt
1gt
Path ii
Path i
Path iii
38
EIT, Propagation Effect
Vglt17m/s, Hau et.al. Nature397,594,1999
Slow light !

• Large optical density and small ground-state
  decoherence rate are two crucial factors in
  EIT-based application, e.g. optical delay line.

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Nonlinear Optics with Ultracold Atoms

• With on-resonance signal, one can control the
  absorption/transmission of probe photon by signal
  photon.
• Photon switching.
• With off-resonant signal, one can control the
  phase of probe photon by signal photon. Cross
  phase modulation.

With signal beam
Without signal
probe
signal
coupling
?
Schmidt Imamoglu Opt. Lett. 21,1936,1996
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XPM Application Controlled-NOT gate for Quantum
Computation

• CNOT and single qubit gates can be used to
  implement an arbitrary unitary operation on n
  qubits and therefore are universal for quantum
  computation.
• Single photon XPM can be used to implement the
  quantum phase gate and CNOT gate

Truth table for CNOT gate
PBS
PBS
Signal
Control qubit
Atoms
Probe
Target qubit
For a good introductory article, see ?????? CPS
Physics Bimonthly, 524, Oct. 2008
41
Reduction of Ground-state decoherence rate
Reduction of mutual laser linewidth
Reduction of inhomogeneity of stray magnetic field
Coupling ECDL
Faraday rotation as diagnosis tool. Three pairs
of coils for compensation. 350kHz/Gauss for Cs
RF
Bias-Tee
Idc
VCSEL
PBS
?/2
Probe DL
Without compensation
coupling
9GHz
FFT
VCSEL
frequency
probe
Beatnote between coupling probe laser
With compensation
dBlt2mG limited by 60Hz AC magnetic field!
42
Good EIT Spectrum
Obtained EIT with 50 transmission at 200kHz
width for OD 60 for Cs D2 F3 ?F3 transition.
43
The Slow Light
10µs for 2cm atomic sample ! Vg2000m/s
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XPM with Group-Velocity-Matched Double Slow Light
Pulses

• Both probe signal pulses becoming
  group-velocity-match slow light in a high OD gas
  for longer interaction time. M. Lukin Phys. Rev.
  Lett. 84, 1419 (2000).

probe
signal
signal
Atom B
coupling
Atom A
medium
probe
signal
45
Double EIT Spectrum

• Photon-switching with on-resonance signal field
  has been observed.
• XPM work is underway !

46
Matching the Group Velocity
Probe 1
Probe 2
No atoms
Group velocity matched !
IC1 fixed
Td(P1)
Td(P2)
decrease IC2
47
Future Work Cavity Enhanced Cross Phase
Modulation

• A holy grail in nonlinear optics is to realize
  a mutual phase shift of pradian with two light
  pulses containing a single photon.
• It can be applied to the implement of
  controlled-NOT gate for quantum computation and
  to generate quantum entangled state.
• Few-photon-level XPM is challenging !
• Large Kerr Nonlinearity
• Low loss
• Strong focusing to increase the atom-laser
  interaction strength
• Long atom-laser interaction time
• We are working on cavity-enhanced XPM. The
  technology may also be applied to cavity laser
  cooling of molecules in the future.

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The Setup
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Acknowledgement

• Financial support from NSC, IAMS.
• Helps from many colleagues,
• WY Cheng, KJ Song, J Lin, K Liu, SY Chen
• Current member
• Chih-Chiang Hsieh
• Ming-Feng Tu
• Jia-Jung Ho
• Wen-Chung Wang
• Former member
• S. -R. Pan (now in Colorado state University)
• H.-S. Ku (now in Univ. of Colorado/JILA)
• T.-S. Ku (now in Univ. of Colorado/JILA)
• Prashant Dwivedi (now in Germanys Univ.)
• P.- H. Sun (now in industry)

50
Keep walking !Molecule cooling Nonlinear optics
with ultrcold atomsWelcome to join us
!Ultracold Atom and Molecule Lab IAMS, Academia
Sinica
51
Slow Light Dark-State Polariton
3gt
3gt
3gt
coupling
coupling
coupling
probe
probe
2gt
2gt
2gt
1gt
1gt
1gt
Light component
Matter component atomic spin coherence
LukinFleischhauer, PRL 84,5094,2000
52
EIT and the Photon Storage

• By adiabatically turn off the coupling light, the
  probe pulse can completely transfer to atomic
  spin coherence and stored in the medium and can
  be retrieved back to light pulse later on when
  adiabatically turn on the coupling.
• This effect can be used as a quantum memory for
  photons.
• The photon storage and retrieved process has been
  proved to be a phase coherent process by Yus
  team.

coupling
probe
Hau et.al. Nature, 409,490,2001
Y.F. Chen et.al. PRA 72, 033812, 2005
53
Q-Value Measurement Under High-Power Operation
Quality-factor
PUnlocked
microwave OFF
Coupling efficiency
PLocked
54
Cavity Frequency Locking

• Pound-Drever-Hall Scheme to obtain error signal
• Feedback by vacuum linear translation stage
• Locked to better than 50 kHz (linewidth 700kHz)

Locked
55
Fabry-Perot Cavity Coupling

• Coupling by a circular horn through mirror with
  mesh.
• Obtained optimum coupling through systematic
  study by varying mesh parameters.

Reflection signal
56
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