Engineering Atom Chips Michael Kraft

1. Engineering Atom Chips Michael Kraft Nano-Scale Systems Integration Group School of Electronics and Computer Science Southampton University

2. Overview • What are Atom Chips? • Building Blocks of Atom Chips • Wires • Cavities • Actuators • Atom Chips Examples • Conclusions

3. Control electrons using wires Control photons using optical fibres How do you control atoms? ATOM CHIPS! Using electromagnetic fields and light to interact with clouds or single atoms Atoms (or clouds) can be trapped in magnetic fields and hover a few um above a chips surface What are Atom Chip?

4. Atom Chips • Devices for trapping and manipulation of atoms on integrated microchips. • Quantum laboratories on chip. • Fundamental research • Quantum behaviour • Low dimensional physics • Entanglement and coupling • New devices – precise sensors • Atom interferometers • Atomic clocks • Accelerometers/Gyroscopes • Quantum information processing • Quantum computers

5. Electrostatic xy comb drive Electrostatic z parallel plate Tuneable optical cavity Silicon Bose-Einstein atom cloud Fibre gold coated at the tip Atom Chip High current density gold wires

6. Multi-Domain MEM System • Electrostatics • 3D Actuator for optical cavity alignment & tuning • Electromagnetic • Confinement field for atom clouds • Optical MEMS • Optical cavity for single atom detection • → INTEGRATION is a key issue!

7. Processing Challenges • Wet and Dry Etching of Silicon • Smooth cavities • DRIE for high aspect ratios • Electroplating and/or etching of Gold • High current density, smooth edged gold wires • Assembly • Multi-level wafer bonding with good alignment • Ultra high vacuum compatible • → Considerable process development necessary • → Applicable to other MEMS devices

8. Atom Guides - Wires • High current density wires allow the creation, trapping and manipulation of cold atoms and BEC’s. • Neutral atoms in a magnetic field feel a potential due their magnetic moment. • V - potential, • µ - magnetic moment of the atom • B - magnetic field. • It is this potential that is used to trap and manipulate the atoms. Atoms accumulate in areas of minimum potential.

9. Splitting Atom Clouds Minimum coalesces Minimum splits

10. Cold Atoms and Bose-Einstein Condensate High temperature Solid balls T = T(crit) =170nK for 87Rb Bose-Einstein condensation Matter waves overlap Low temperature Wave packets T<T(crit) Pure BEC, Single matterwave

11. Laser Cooling Du, PhD thesis, U. of Colorado, 2005 Setup three counter propagating laser beams and a magnetic field MOT on chip: use 3 lasers and a mirror

12. Wire Fabrication: Electroplating Photoresist Silicon oxide Silicon Gold Chromium An electroplating mould is created using photoresist Silicon substrate with 100nm of oxide deposited 100nm of gold is deposited The Cr/Au layer is patterned using a wet etch 5µm of gold is electroplating into the mould The resist is removed creating the finished chip

13. Four Wire Trap

14. Problems With Electroplating • Resist reflow

15. Problems With Electroplating • Mushrooming

16. Problems With Electroplating • Current density

17. Fabrication: Ion Beam Milling Photoresist Silicon oxide Silicon Gold Chromium The Gold is ion beam milled or wet etched Silicon substrate with 100nm of oxide deposited Photoresist is spun and patterned 5µm of Gold is sputtered The resist is removed creating the finished chip

18. Problems With Ion Beam Milling Variable etch rate across the wafer, leading to over etching

19. Fabrication Challenges Corrugation in these wires causes fluctuations in the magnetic field that leads to fragmentation in the atom cloud. Electrochemical deposition into a mould Ion beam milling Gold and chromium wet etch

20. Gold Wires ‘Atom Chip’ Layout 2cm

21. Wire Atom Chip Under Test

22. Atom Interferometer on a Chip 67 mm 23 mm spectacular sensitivity to o EM fields o gravity o other feeble forces 3.5 microns of gold

23. Current cooling techniques Atoms are cooled in a macroscopic magneto-optical trap (MOT). Clouds are then transferred from the macroscopic MOT cloud to the microscopic Atom Chip. Inverted Pyramid: MOT on a chip Pyramids on chip can be used to act as a MOT Simpler system, automatic alignment, arrays of MOTs possible. Pyramidal Micro-cavities

24. Pyramidal Micro-cavities http://www.ic.ac.uk/research/ccm/research/micropyramids.htm KOH etched inverted pyramids with current carrying wires

25. Pyramidal Cavities

26. SEM Biggest pyramid in the mask design = 1.2 mm Atomically smooth side walls

27. Pyramid Patterning Reflected gold coating needs to be removed at the edges to avoid disturbing light reflections Pyramid created by process shown previously leaving it with a gold coating Electrophoritic resist is deposited in the pyramids and patterned The gold and chromium is wet etched The resist is removed leaving the flower patterned pyramids

28. Patterned Pyramid With Wires

29. Pyramid Atom Chip Fabrication The gold wires are electroplated, the resist removed and the chips completed 170nm of TEOS oxide is deposited along with 50 nm of chromium and 100nm of gold The front nitride and oxide are stripped The chromium/gold layer is patterned An electroplating mould is created from AZ9260 resist The pyramids are etched in KOH The back alignments are protected with a PECVD nitride layer Openings are etched into the fronts for the pyramidal etch (100) Silicon wafer, 170nm of oxide is deposited followed by 50 nm of Nitride and the alignments etched into the back The alignment marks are etched into the oxide Silicon Chromium Photoresist Silicon oxide Gold Silicon nitride

30. Patterned Pyramid Atom Chip

31. Pyramid Atom Chip in the Lab http://www.ic.ac.uk/research/ccm/research/micropyramids.htm

32. Spherical microfabricated cavities are ideal for making high finesse optical resonators. The aim is to achieve single atom – photon interaction. Light couples directly in and out of the resonator through an optical fibre. Spherical Micro-cavities

33. Spherical Micro-cavities Focal spots clearly visible under microscope

34. Spherical Cavities Fabrication The silicon is etched using an ASE isotropic etch 3µm of Gold is sputtered The silicon nitride is stripped using orthophosphoric acid Photoresist is spun and patterned The resist is removed creating the finished chip A 50nm Chromium and 100nm Gold layer is sputtered 100nm of silicon nitride is deposited and patterned Photoresist is spun and patterned and the gold is ion beam milled The silicon is etched using a HF based solution A silicon substrate with 100nm of oxide deposited and patterned Silicon Chromium Photoresist Silicon oxide Gold Silicon nitride

35. High Finesse Optical Cavity finesse = 5200 optical fibre 74 pm reflection 390 nm 0.9999 bragg stack dielectric coated micro-mirror 0.9999 100 mm

36. Spherical Micro-cavities • Various etch rates can be used to make any radius of curvature • Longer etch rates gives smoother mirrors

37. Atom Chip

38. Actuation Design Guidelines xy actuation Alignment of optical cavity Misalignment between fibre and spherical mirror during fabrication xy translation of 5-10 mm xy actuation accuracy of 0.5-1 mm z actuation Stable and tunable optical cavity z translation of 4-5 mm (coarse tuning) z actuation accuracy of a few nm (fine tuning)

39. Translation in +xy direction(xy-actuator) At 117 V a maximum coverage area of 17.5 by 17.5 mm is achieved.

40. Resonance Frequencies Mode 1: Resonance frequency (fzres=581Hz) in z motion Mode 2: Resonance frequency (fxres=820Hz) in x motion Mode 3: Resonance frequency (fyres=820Hz) in y motion

41. Actuator Chip Prototype

42. Actuator Chip Prototype

43. Fabrication Process XY Actuator Silicon substrate (380 um) Glass substrate (500 um) 1st dry-etch (320 um) 2nd dry-etch (60 um) Anodic bonding

44. In Plane Atom Chip Design Silicon Tuneable optical cavity (spherical cavity and plane mirror ) Bose Einstein atom cloud Fibre with spherical gold coated cavity tip fitted in v-groove Electrostatic x comb drive High current density gold wires

45. Microfabrication is a very suitable approach for manipulating clouds of or single atoms Established a modular ‘toolbox’ for atom chips, including wires, optical cavities and actuators The near future Atom arrays Control over single atoms Further forward Miniaturised atom devices and sensors Far future Quantum computing with neutral atoms? Conclusion

46. Imperial College London E.A.Hinds Pyramids Jonathan Ashmore Fernando Ramirez Martinez Sam Pollock Athanasios Laliotis Cavities Michael Trupke Jon Goldwin Joanna Khunner Athanasios Laliotis Atom guides Stefan Eriksson Rob Sewel Joss Dingjan People Involved Nanoscale Systems Integration Group • University of Southampton • Michael Kraft • Gareth Lewis • Zak Moktadir • Carsten Gollasch