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Exploration of the Ultracold World

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

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Exploration of the Ultracold World

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  1. IAMS Exploration of the Ultracold World Ying-Cheng Chen(陳應誠), 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 Sub-Doppler cooling 2003 MIT Na BEC surface of sun L N2 3He superfluidity 0 (K) 103 106 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 @ 100nk Superfluid-Mott insulator t Ransition, Max-Planck Vortex Lattice, JILA &MIT Fermi pressure, Rice Matter wave interference, MIT

  6. Atom v 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/superconductivity 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

  9. 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. S d + P T - - + + - 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 JILA’s 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

  16. hotter molecules colder molecules Routes Towards Ultracold Molecules 1 μK 1 mK 1 K Buffer gas cooling plus magnetic guiding Radiative damping & trap loading Sympathetic cooling in a microwave trap by ultracold cesium atoms. Evaporative cooling in a microwave trap. SrF molecule Cs atom

  17. hotter molecules colder molecules A2Π1/2 v’ 0 ω00 A00 A01 A02 v’’ 2 1 0 X2Σ1/2 Recent Ideas 1 μK 1 mK 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 X2Σ,v=1→A2Π1/2,v’=1 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.

  20. BF+2(neutral BF3) N+2 CO+2 BF+ Sr+ SrF+ RGA Trace Development of an intense SrF Molecular Beam 2B+3 SrF2(high-temperature~1500K)→BF3+Sr+2SrF+BF Electron-bombardment heating If one want to work with (cold) molecules then he need to learn some chemistry !

  21. SrF Beam Characterization Laser beam Light baffle 10cm 13cm 5cm ψ3mm ψ2mm 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

  22. Typical Spectrum (0,0) vibrational band of A2Π1/2- X2Σ+ transition of 88SrF Laser intensity ~5 00mW/cm2 FWHM linewidth ~ 130MHz S/N ratio >200 Even near the congested band edge, all hyperfine lines are well resolved ! Laser intensity ~ 5mW/cm2 FWHM linewidth ~ 15 MHz S/N ratio > 50 Hyperfine lines resolved (I=1/2 for 19F)

  23. Beam Characterization Flux 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 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) 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.

  25. Effective Molecular Hamiltonian Better molecular constants have been determined ! Theoretical Modeling “High-resolution laser spectroscopy of the (0,0) band of A2Π1/2- X2Σ+ 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 (6.6×1015/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.

  27. hotter molecules colder molecules Routes Towards Ultracold Molecules 1 μK 1 mK 1 K Buffer gas cooling plus ac electric guiding Radiative damping & trap loading Sympathetic cooling in a microwave trap by ultracold cesium atoms. Evaporative cooling in a microwave trap. SrF molecule Cs atom

  28. J=1 U(x) Red-detuned microwave Rotational transition AC Stark shift x J=0 Trapping state 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.

  29. Observation of standing wave pattern by thermal-sensitive LCD sheet Q=11000 η=0.87 Pin=1060W R=0.217m D=0.2m E0=0.45 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. hotter molecules colder molecules Routes Towards Ultracold Molecules 1 μK 1 mK 1 K Buffer gas cooling plus ac electric guiding Radiative damping & trap loading Sympathetic cooling in a microwave trap by ultracold cesium atoms. Evaporative cooling in a microwave trap. SrF molecule Cs atom

  31. Tempature Tm TM( t) Teq Ta time τth Larger number of cold atoms, colder atom temperature and higher atom density implies lower molecular temperature and shorter thermalization time. • Equilibrium temperature • Thermalization time • Collision rate • c: a geometry factor and 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 Coils&cell Absorption Spectrum Atom cloud probe Optical density=105 for Cs D2 line F=4 →F’=5 trapping trapping beam “An elongated MOT with high optical density” Optics Express 16,3754(2008)

  33. cavity-enhanced Rayleigh scattering Scattering rate cavity linewidthκ atomic linewidthΓ A2Π1/2 v’ 0 ω00 A00 A01 A02 v’’ 2 1 0 X2Σ1/2 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 • Haven’t 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. Di Rosa, Eur.Phys. J. D, 31,395 (2004) v’ A2Π1/2 0 ω00 A00 A01 A02 v’’ 2 1 0 X2Σ1/2 Laser Cooling of SrF : to overcome the mK barrier! J Phy Chem A, 102,9482,1998 By repumping the v=1 population back to v=0, the transition is closed to 10-4 level 0.9998673600=62%

  35. parity A2Π1/2,v’=0 J’ ＋ 2.5 parity － F’ J’ N’ A2Π1/2,v’=0 Nearest>14GHz away － 1 1.5 ＋ 0.5 0 Small ~ few MHz － ＋ 0.5 0 － nearest interference main repumping (0,0)P12(1.5) (0,0)Q11(0.5) (0,0)P12(1.5) (0,0)Q11(0.5) (0,0)Q12(1.5) (0,1)P12(1.5) (0,0)R12(1.5) (0,1)Q11(0.5) N’’ J’’ parity F’’ N’’ J’’ parity ＋ 2.5 2 112.19MHz 2 1 ＋ 21.75MHz 1.5 29.72MHz ＋ 1.5 2 1.5 － 1 0.5 － ~45GHz 1 ＋ 0.5 0 0.5 26.79MHz ＋ 0 X2Σ1/2(v’’=0) 663.1nm 80.38MHz X2Σ1/2(v’’=1) 685.1nm X2Σ1/2(v’’=0) 663.1nm 0 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 @ 663nm and two @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. |3> 2> |1> Electromagnetically-induced Transparency Transparent! Probe laser Coupling laser Physical origin: destruction interference between different transition pathways! |3> coupling probe ＋… ＋ ＋ ＝ |2> |1> Path ii Path i Path iii

  38. EIT, Propagation Effect Vg<17m/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.

  39. 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 γ Schmidt & Imamoglu Opt. Lett. 21,1936,1996

  40. 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 陳易馨&余怡德 CPS Physics Bimonthly, 524, Oct. 2008

  41. ~10Hz 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 λ/2 Probe DL Without compensation coupling ~9GHz FFT VCSEL frequency probe Beatnote between coupling & probe laser With compensation δB<2mG limited by 60Hz AC magnetic field!

  42. Good EIT Spectrum Obtained EIT with ~50% transmission at 200kHz width for OD~ 60 for Cs D2 F=3 →F’=3 transition.

  43. The Slow Light 10μs for ~2cm atomic sample ! Vg~2000m/s

  44. 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. mF= 0 1 2 3 4 F=4, gF=4/15 P2 C2 P2 F=3, gF=0 P1 (a) P1 F=4, gF=1/4 C1 F=3, gF=-1/4 Cs 6S1/2 -6P3/2 (D2-line) 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. cold atom Coupling& probe Signal beam 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 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.

  48. The Setup

  49. 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)

  50. Keep walking !Molecule cooling Nonlinear optics with ultrcold atomsWelcome to join us !Ultracold Atom and Molecule Lab IAMS, Academia Sinica

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