Phys 570V: Advanced Topics in Optics and Photonics Professor Tongcang Li Lecture 20: Laser cooling of atoms Course website: http://www.physics.purdue.edu/academic- programs/courses/course_detail.php?SEM=fall2015&c=phys570V Syllabus, Lecture notes, etc. Purdue University Fall 2015 Physics 570V Room: Phys 331 Time: MW 2:30-3:45 PM 1 Take-home exam: due on Wed., 11/11/2015 Final presentations: 11/30-12/9 • 12-minute talk + 3 minute Q&A. • The content of the presentation can be about any recent (after 2000) development in optics and photonics. It can be about one particular paper, an overview of a research topic, or your own research if it is related to this course. • After each presentation, every student (including the speaker) will give a grade (0- 10) to it. The instructor will also grade it. The final grade of the presentation will be the average of the grades given by students (50%) and the instructor (50%). • Presentation sequence: to draw lots on Nov. 11 2 Last lecture: Radiation Pressure ℎ 퐸 Photon momentum: 푝 = = 휆 푐 푑푝 푑 퐸 푃 퐹 = = = for absorbed light 푑푡 푑푡 푐 푐 2푃 퐹 = for reflected light 푐 If we reflect 100W of light, 퐹~7 × 10−7 N 3 Last lecture: optical tweezers Rayleigh approximation 4 Our vacuum optical tweezer at Purdue University Nanodiamond (100 nm) 5 This lecture: laser cooling of atoms 6 Laser cooling 2005 1981 2012 1989 D.J. Wineland, H. Dehmelt: Bull. Am. Phys. SOC. 20, 637 (1975) Neutral atoms Ions Laser cooling/trapping 1997 Steven Chu, Claude Cohen-Tannoudji, William D. Phillips Atomic Bose-Einstein condensation (BEC) Eric A. Cornell, Wolfgang Ketterle, Carl E. Wieman 2001 7 Laser cooling: basic idea Absorption-emission 8 Doppler cooling force (Doppler limit) 9 10 11 Zeeman slower Wikipedia 12 Optical Molasses (Doppler temperature) (240 µK for Na atoms) 13 Sub-Doppler cooling 14 k 2 Recoil Velocity: vR m mL 1 (k)2 Recoil Energy: E mv2 R 2 R 2m = 2.95cm/s For sodium atoms in 589nm laser: vR 1 E = 2 25.0 kHz = k 2.4 µK R 2 B 15 Magneto Optical Trap (MOT) Nature Nanotechnology, 8, 317–318 (2013) 16 Magnetic trap H B (BmF gF )Bz 1 For sodium atoms in the state: Fm1, 1, g F F 2 V Atoms can be trapped in a local minimum of magnetic field. x,y z 17 Evaporative cooling Waferboard / Flickr 18 Bose-Einstein Condensate 19 Vacuum Chamber 1.1∙10-11 Torr 3∙10-9 Torr 800 m/s 30 m/s 6∙10-11 Torr 20 Creation of a Bose-Einstein condensate 800 m/s 30 m/s 24 cm/s Na BEC 21 Signatures of BEC • Bimodal distribution • Anisotropic expansion • Matter interference • superfluidity Cornell/Weiman ketterle 22 Atom Optics & Atom Laser 23 Atoms in an optical lattice 24 Creating a periodic potential Magnetic BEC Waveguide Mirror 532nm Laser Waist: 120 µm Max Power: 1 W Optical Lattice Well Depth: 0 – 22 ER YAG Tweezer (1064nm) Waist ≈ 180 µm Trap Depth ≈ 4 E R 25 Single particle in a periodic potential 2 V(z) V0 sin (kL z) V0 1. Classical: Trapped in a well if V0 Eatom 2. Quantum: Forms a band structure. ikRn (r Rn ) e (r) 26 Expansion in a periodic potential Dilute Thermal Atoms V0 2.24ER 6.5K 11.8 ms 22.8 ms 33.7 ms The expansion of thermal atoms (T=0.52 µK) is in excellent agreement with the single-particle model. 27 Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms Nature 415, 39-44 (3 January 2002) 28 Next lecture: Optical refrigeration of solids 29.