exploration of the ultracold world
DESCRIPTION
IAMS. Exploration of the Ultracold World. Ying-Cheng Chen( 陳應誠 ), Institute of Atomic & Molecular Sciences, Academia Sinica 12 October, 2009, NDHU. Outline. Overview of Ultracold Atoms Introduction to Ultracold Molecules Exploration I: Molecular cooling - PowerPoint PPT PresentationTRANSCRIPT
Exploration of the Ultracold World
Ying-Cheng Chen(陳應誠 ), Institute of Atomic & Molecular Sciences, Academia Sinica
12 October, 2009, NDHUIAMS
Outline• Overview of Ultracold Atoms• Introduction to Ultracold Molecules• Exploration I: Molecular cooling• Exploration II: Nonlinear optics with ultraco
ld atoms
Studying, Research and Life: Adventure & Exploration
Temperature Landmark
106 103 1 10-3 10-6 10-9
0
(K)
Core of sun
surface of sun
Room temperature
L N2
L He3He superfluidity
2003 MIT Na BEC
Typical TC
of BECRb MOT
Sub-Doppler cooling
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
Tmkh B 2 ~1μm for Na @ 100nk
Matter wave interference, MIT Fermi pressure, Rice Vortex Lattice, JILA &MITSuperfluid-Mott insulator tRansition, Max-Planck
Laser Cooling & Trapping
• Cooling, velocity-dependent force: Doppler effect• Trapping, position-dependent force: Zeeman effect
Laser
fv
Atom
v
Magnetic Trapping & Evaporative Cooling
)()( rBrU
Microwave transition
Modern Atomic Physics : Science & Technology
Precision measurementAtomic clock
Test of particle physics (EDM)Test of nuclear physics (parity violation)
Test of general relativityVariation of physical constants
Quantum information scienceQuantum control
Quantum teleportationQuantum network
Quantum cryptographyQuantum computing
Quantum simulation of condensed-matter physics
BEC/Degenerate Fermi gasSuperfluidity/superconductivity
Quantum phase transitionBEC/BCS crossoverAntiferromagnetism/
high Tc superconductivity
Opto-mechanics& Nano-photonicsLaser cooling of mirror/mechanical oscillator
Coupling of cold atom with mesoscopic(nano) objectQuantum limit of detection
Near field optics
Extreme nonlinear opticsAtom/molecule under intense short pulse
High harmonic generationX-ray laser
Attosecond laser
Atom manipulation
Core technology
Laser advancement
Weakness:Molecule manipulation
Double Helix of Science & Technology
Science
Technology Better understanding of science helps technology moving forward
Better technology helps to explore new science
It is a tradition in AMO physics to extend new technology to explore physics at new regime.
Core Technology• Atom cooling
• Laser technology
LasersUltra-intense
Ultra-short
Ultra-stable
Ultra-narrow-linewidth
Non-classical (single photon,
entangled photon pairs)
Laser cooling
100TW
Sub-Hz
250 as
Sub-Hz
atom trapping/optical lattice evaporative cooling
)()( rBrU
Microwave transition
Magnetic-tuned Feshbach resonance
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
TSd
P +-
-+ +
-
rr tt
Cold molecules : How ?
Buffer gas cooling
Electric,magnetic,
optical deceleration
Enhanced PA?Laser cooling?
Sympathetic cooling?Evaporative cooling?
Photo-association
Coherent transfer from Feshbach
molecule
Direct approach
Indirect approach +
Breakthrough in Indirect Approach• The door to study quantum degenerate dipolar gases and quantum information with pola
r molecules is opened by JILA’s recent experiment with indirect approach.
K.-K. Ni et alScience, 18,1(2008)
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. D31,395 (2004) for molecules with nearlyclosed transition.
The ying and yang (dark/bright) sides of molecules. You have to pay the price !
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 ov
ercome mK barrier for direct cooling.• What advantages to take? What disadvantages to live with?
Moleculesprecooling
loadingTrapping
Laser-cooledatoms
loading
sympathetic coolingInelastic collision?Reaction?
Ultracold Molecules
Routes Towards Ultracold Molecules
Buffer gas cooling plus magnetic guiding
Sympathetic cooling in a microwave trap by ultracold cesium atoms.
1 K 1 mK 1 μK
Evaporative cooling in a microwave trap.
hotter molecules
colder molecules
Cs atom
SrF molecule
Radiative damping& trap loading
Recent Ideas
Buffer gas cooling plus magnetic guiding
Direct laser cooling
1 K 1 mK 1 μK
Evaporative cooling in an optical dipole trap.
hotter molecules
colder molecules
A2Π1/2
X2Σ1/2
v’’
012
v’0
ω00
A00A01 A02
4
00
02
2
00
01
10
10
AAAA
What molecule? SrF, Why? • Alkali-like electronic structure with strong transitions at visible wavel
engths. Easy to be detected by convenient diode lasers.• Large electric dipole moment, 3.47 D and many bosonic and fermio
nic 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 cool
ing with reasonable number of lasers. • Suitable for test of fundamental physics and quantum information sc
ience.• Radical molecules. Disadvantages in molecule generation.• What advantages to take? What disadvantages to live with ?
Buffer Gas Cooling
P11(8.5)
P11(7.5)
P11(6.5)P11(5.5)
Q12(7.5)
Q12(6.5)
Q12(5.5)
Q12(4.5)
X2Σ,v=1→A2Π1/2,v’=1
SrF molecules generated by laser ablation of SrF2 solid.
Development of an intense SrF Molecular Beam
2B+3 SrF2(high-temperature~1500K)→BF3+Sr+2SrF+BF
SrF+Sr+
BF+2(neutral BF3)
BF+
N+2 CO+
2
RGA Trace
If one want to work with (cold) molecules then he need to learn some chemistry !
Electron-bombardment heating
SrF Beam Characterization
Brewster window
Light baffle
Residual gas analyzer
Turbo pump
skimmer
Laser beam
PMT
ψ3mm ψ2mm
5cm 13cm 10cm
chopper
oven
ECDL laserNew Focus 6009/6300 Toptica WS-7
Wavelength meter
Setup for laser-induced fluorescence
Even near the congested band edge, all hyperfine lines are well resolved !
Typical Spectrum
(0,0) vibrational band ofA2Π1/2- X2Σ+ transitionof 88SrF
Laser intensity ~5 00mW/cm2
FWHM linewidth ~ 130MHzS/N ratio >200
Laser intensity ~ 5mW/cm2
FWHM linewidth ~ 15 MHzS/N ratio > 50Hyperfine lines resolved (I=1/2 for 19F)
Beam CharacterizationFlux v.s. oven temperature
Flux stability ~ 20% / one hour
Highest flux of 2.1×1015 /(steradian. sec)! Even stronger and more stable beam is possible by resistive heatingand is under development!“An intense SrF radical beam for molecule cooling experiment” submitted to Phys. Rev. A.
Better Spectroscopy of SrF
The rotational/hyperfine lines of (0,0) A2Π1/2- X2Σ+ band 88SrF have been recorded to 10-4 cm-1 precision with a fitting accuracy of ±10-3 cm-1 to the effective Hamiltonian.
Theoretical Modeling• Effective Molecular Hamiltonian
• Better molecular constants have been determined !
],)[2(41],[
41
)(21))(2(
21
)(;],[2
)(;
22222222
222222
2242
NSJeSJeqpNeJeJq
eJeJqSJeSJeqpH
SLNASALHSNNSNHNDNBH
HHHHH
iiDD
iiD
iiiidoublinglambda
zzdzzorbitspinD
rotspinrot
doublinglamdaorbitspinrotspinrot
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 A2Π1/2- X2Σ+ transition of 88SrF ”submitted to J. of Mol. Spec.
Buffer-Gas-Cooled Molecular Beam & Guiding
• On-going work
oven
Dewar
cryostat
Magnetic guide
UHV ChamberSpectroscopyor laser cooling
Helium
SrF
Estimation of Flux (6.6×1015/s) × (9×10-4)x(2.9×10-3)=1.7×1010/s @ ~5K
Already very intense for a radical beam! Higher flux is possible with modified oven.
Turbo pump
Routes Towards Ultracold Molecules
Buffer gas cooling plus ac electric guiding
Sympathetic cooling in a microwave trap by ultracold cesium atoms.
1 K 1 mK 1 μK
Evaporative cooling in a microwave trap.
hotter molecules
colder molecules
Cs atom
SrF molecule
Radiative damping& trap loading
Development of the Microwave Trap
Rotational transition
Red-detunedmicrowave
AC Stark shift
J=0
J=1
Trapping state
Advantages of microwave trap 1. High trap depth ( ~ 1K)2. Large trap volume (~ 1cm3)3. Good optical access. Allow overlap of MOT with trap for sympathetic coo
ling.4. It can trap molecules in the absolute ground states and thus immune to i
nelastic collisions loss at low enough temperature.
U(x)
x
DeMille, Eur.Phys.J D 31,375(2004)
Observation of standing wave pattern by thermal-sensitive LCD sheet
DDRDzPQE in
)2(4 0
0
Q=11000η=0.87Pin=1060WR=0.217mD=0.2m
E0=0.45 MV/m
Trap depth ~ 0.1 Kfor SrF ground state
“ A high-power microwave Fabry-Perot resonator for molecule trapping experiment”Rev. Sci. Inst. In preparation.
Routes Towards Ultracold Molecules
Buffer gas cooling plus ac electric guiding
Sympathetic cooling in a microwave trap by ultracold cesium atoms.
1 K 1 mK 1 μK
Evaporative cooling in a microwave trap.
hotter molecules
colder molecules
Cs atom
SrF molecule
Radiative damping& trap loading
Sympathetic Cooling of Molecules by Ultracold Atoms
• Conceptually easy but depends on unknown collision properties.
time
T M( t
)
Tm
Ta
Teq
Tempature
τth
• Equilibrium temperature
• Thermalization time
• Collision rate
c: a geometry factor and
ma
mmaaeq NN
TNTN
T
)(23
ma
math NN
NN
2)(2 mama MMMM
),( maampmpa TTcvnn
Larger number of cold atoms,colder atom temperatureand higher atom densityimplieslower molecular temperatureand shorter thermalization time.
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).
“An elongated MOT with high optical density”Optics Express 16,3754(2008)
7cm
Absorption Spectrum
Optical density=105for Cs D2 line F=4 →F’=5
trapping beam
trapping
Coils&cell
trapping
Atom cloud probe
Quest of Second Stage Cooling to overcome the mK Barrier for Direct Approach
• Sympathetic cooling with ultracold atoms– Not so promising due to strong in
elastic loss– AC trap is necessary
• Cavity laser cooling– Haven’t been demonstrated.
• Direct laser cooling – Being demonstrated– Limited to a few species
• Single-photon (information) cooling– In combination with magnetic tra
pping– May be demonstrated soon
• ...
A2Π1/2
X2Σ1/2
v’’
012
v’0
ω00
A00A01A02
4
00
02
2
00
01
10
10
AAAA
M.Raizen
pp ,
N
Scattering rate
acp
atomic linewidthΓ
cavity linewidthκ
cavity-enhanced Rayleigh scattering
Laser Cooling of SrF : to overcome the mK barrier!
• Di Rosa, Eur.Phys. J. D, 31,395 (2004)
state X2Σ,v=0 v=1 v=2 v=3
A2Π, v=0 0.9895 0.0103 1.33x10-4 1.57x10-6
A2Π1/2
X2Σ1/2
v’’
012
v’0
ω00
A00
A01A02
4
00
02
2
00
01
10
10
AAAA
J Phy Chem A, 102,9482,1998
0.9998673600=62%
KTkvm 1,3600~
By repumping the v=1 population back to v=0, the transition is closed to 10-4 level
A2Π1/2,v’=0
X2Σ1/2(v’’=0)663.1nm
X2Σ1/2(v’’=1)685.1nm
N’’
0
1
2
J’
J’’
parity
parity
2.5
1.5
0.5
2.51.51.50.50.5
+-+
+-
---
+++
main repumpingnearestinterference
(0,0
)Q11
(0.5
)
(0,0
)P12
(1.5
)
(0,1
)Q11
(0.5
)
(0,1
)P12
(1.5
)
(0,0
)R12
(1.5
)
(0,0
)Q12
(1.5
)
Nearest>14GHz away
~45GHz
A2Π1/2,v’=0
X2Σ1/2(v’’=0)663.1nm
N’’
0
2
J’
J’’
parity
parity
0.5
1.5 ++
-
(0,0
)Q11
(0.5
)
(0,0
)P12
(1.5
)
0.5
F’’
1
0
26.79MHz80.38MHz
112.19MHz
1
2
N’
0
21.75MHz29.72MHz
F’1
0Small ~ few MHz
Considering to rotational states, four lasers (two @ 663nm and two @685nm ) required to close the transition to 10-4 level.
Considering to hyperfine states, it is necessary to generate two frequencies differed by ~50 or 107 MHz by acousto-optical modulator for each laser.
Nonlinear optics with ultracold atoms
- Detour of my planned journey but back to my old track !
Electromagnetically-induced Transparency
Coupling laser
Probe laser
Transparent!
0,5.0,0 3 cc
|1>
|3>
p32
2>
c32
probe coupling
= + + +…
Path i Path iiiPath ii
2totA
|1>
|2>
|3>
Physical origin: destruction interference between different transition pathways!
EIT, Propagation Effect
00 |)/(
|
p
pppp
pg ddnn
cdkd
v
• Large optical density and small ground-state decoherence rate are two crucial factors in EIT-based application, e.g. optical delay line.
)()()
23()11(
312
31
312
312
cc
g
gd ODLN
cLn
cvL
Slow light !
Vg<17m/s, Hau et.al. Nature397,594,1999
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.
3
21
4
couplingprobe signal
Without signal
With signal beam
Schmidt & Imamoglu Opt. Lett. 21,1936,1996
γ
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
For a good introductory article, see 陳易馨 & 余怡德 CPS Physics Bimonthly, 524, Oct. 2008
Truth table for CNOT gateTCTCTCTC
TCTCTCTC
0111;1101
1010;0000
Signal
Probe
PBS PBS
Atoms
Control qubit
Target qubit
1
0
10 or
Reduction of Ground-state decoherence rate
Coupling ECDL
VCSEL
ProbeDL
λ/2PBS
frequency
coupling
VCSELprobe
~9GHz
Bias-TeeIdc
RF
Reduction of mutual laser linewidth
~10Hz
Beatnote between coupling & probe laser
Reduction of inhomogeneity of straymagnetic field
Faraday rotationas diagnosis tool.Three pairs of coilsfor compensation.
350kHz/Gauss for Cs
BL
FFT
δB<2mG limited by 60Hz AC magnetic field!
Without compensation
With compensation
Good EIT SpectrumObtained EIT with ~50% transmission at 200kHz width for OD~ 60 for Cs D2 F=3 →F’=3 transition.
The Slow Light
10μs for ~2cmatomic sample !Vg~2000m/s
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).
3
2
4
couplingprobe signal
1
signal
Atom A
Atom B
medium
probe
signal
Double EIT Spectrum
• Photon-switching with on-resonance signal field has been observed. • XPM work is underway !
mF= 0 1 2 3 4
F=4, gF=4/15
F=3, gF=0
F=4, gF=1/4
F=3, gF=-1/4
C1
P1
C2 P2P2
P1
(a)
Cs 6S1/2 -6P3/2 (D2-line)
Matching the Group Velocity
Td(P1)
Td(P2)
Group velocity matched !
Probe 1 Probe 2
No atoms
IC1
fixed
decreaseIC2
Future Work : Cavity Enhanced Cross Phase Modulation
• A “holy grail” in nonlinear optics is to realize a mutual phase shift of πradian 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 intera
ction 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.
cold atom
Signal beam
Coupling&probe
The Setup
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 Germany’s Univ.)– P.- H. Sun (now in industry)
Keep walking !Molecule cooling
Nonlinear optics with ultrcold atomsWelcome to join us !
Ultracold Atom and Molecule Lab IAMS, Academia Sinica
Slow Light : Dark-State Polariton
2
2
21
cos
0),(]cos[
;tan
),(sin),(cos),(
cv
tzz
ct
kkkn
etzNtzEtz
g
pcg
kzip
coupling
1>
|2>
|3>
coupling
1>
|2>
|3>
probecoupling
1>
|2>
|3>
probe
Lukin&Fleischhauer, PRL 84,5094,2000
Light component
Matter component: atomic spin coherence
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 Yu’s team.
coupling
probe
Hau et.al. Nature, 409,490,2001 Y.F. Chen et.al. PRA 72, 033812, 2005
Q-Value Measurement Under High-Power Operation
0
/)0()(
QeUtU t
microwave OFF
Quality-factor
Coupling efficiency
PLocked
PUnlocked
Unlocked
Locked
PP
1
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
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
Observed Line narrowing effect for large OD gas
Increasing theOD of atom cloud