Precision atom interferometry in a 10 meter tower

Transcription

1 Precision atom interferometry in a 10 meter tower Leibniz Universität Hannover RTG 1729, Lecture 1 Jason Hogan Stanford University January 23, 2014

2 Cold Atom Inertial Sensors Cold atom sensors: Laser cooling; ~10 8 atoms, ~uk (no cryogenics) Atom is freely falling (inertial test mass) Lasers measures motion of atom relative to sensor case Accelerometers, gravimeters, gyroscopes, gradiometers Technology evolution: Image: AI gyroscope (1997) AI compact gyroscope (2008) AOSense commercial AI gravimeter (2011)

3 Testing fundamental physics with AI Some physics motivation for increasing sensitivity Testing general relativity Equivalence principle Short range gravity/fifth forces Post-Newtonian effects Gravitational wave astronomy Terrestrial and space detectors Tests of QED Photon recoil (alpha measurements) Tests of quantum mechanics Macroscopic superposition states Decay of coherence

4 Large wavepacket separation Long interferometer time (>2 seconds) Large momentum transfer beam splitters (>10 hk) 4 cm

5 Outline Brief review of light pulse atom interferometry Long time atom interferometry results Rotation compensation Optical lattice launch Large momentum transfer atom optics Delta kick cooling

6 Light Pulse Atom Interferometry 1D (vertical) atomic fountain Atom is freely falling Lasers pulses are atom beamsplitters & mirrors (Raman or Bragg atom optics) pulse sequence

7 Stimulated two photon process Rabi oscillations Beamsplitter (p/2) and mirror (p) pulses possible Atom Optics

8 Position Light Pulse Atom Interferometry p/2 pulse beamsplitter p pulse mirror p/2 pulse beamsplitter Time p > Interior view p+k>

9 Inertial Force Sensitivity Atom-Light interaction: The local phase of the laser is imprinted on the atom at each interaction point. Laser phase encodes the atom s position as a function of time Motion of the atom is measured w.r.t. a wavelength-scale laserruler (~0.5 micron) Example: Free-fall gravitational acceleration, (p/2 p p/2) sequence Df = (f D f B ) (f C f A ) ~ / l

10 Apparatus Ultracold atom source >10 6 atoms at 50 nk 3e5 at 3 nk Optical Lattice Launch 13.1 m/s with 2372 photon recoils to 9 m Atom Interferometry 2 cm 1/e 2 radial waist 500 mw total power Dynamic nrad control of laser angle with precision piezo-actuated stage Detection Spatially-resolved fluorescence imaging Two CCD cameras on perpendicular lines of sight Current demonstrated statistical resolution, ~ g in 1 hr ( 87 Rb)

11 Ultra-cold atom source BEC source in TOP trap, then diabatic steps in strength of trap to further reduce velocity spread: < 3 nk Atom cloud imaged after 2.6 seconds free-fall No apparent heating from lattice launch

12 Interference at long interrogation time Wavepacket separation at apex (this data 50 nk) Interference (3 nk cloud) 2T = 2.3 sec Near full contrast g/shot (inferred) Dickerson, et al., arxiv: (2013)

13 Phase shifts Gravity Coriolis Timing asymmetry Curvature, quantum Gravity gradient Wavefront (T ij, gravity gradient; v i, velocity; x i, initial position; a, wavefront curvature; g, acceleration; T, interrogation time; k eff, effective propagation vector) Observe velocity dependent shifts with spatial imaging (useful when atoms expand from a point source)

14 Rotation Compensation System In-vacuum nanopositioning stage & mirror < 1 nrad measured precision ~ 1 nrad repeatability Piezoresistive position sensors Rigidly anchored to quiet floor nanopositioner (x3) mirror Coarse alignment Anchor plate

15 Observing velocity-dependent phase Point Source Interferometry (PSI) Long time of flight position-velocity correlation Velocity-dependent phase spatial phase gradient Spatially resolved detection point source Final size much larger than initial size

16 Coriolis phase shift Side view Coriolis phase shift: Expansion from point source: No gradient Phase gradient F=2 F=2 F=1 F=1 Dickerson, et al., PRL 111, (2013)

17 Coriolis phase shift Side view Coriolis phase shift: Expansion from point source: No gradient Phase gradient F=1 F=1 F=2 F=2 Dickerson, et al., PRL 111, (2013)

18 Spatial fringes versus rotation rate Interference patterns for rotating platform: Spatial frequency increases with increased rotation Imaging the fringe pattern improves contrast Fringe contrast Integrated contrast Dickerson, et al., PRL 111, (2013)

19 CCD2 Dual-axis gyroscope Measurement of rotation rate near null rotation operating point. Rotation phase shift: CCD1: CCD2: CCD1 F = 1 Ellipse Fits CCD 1 CCD 2 F = 2 (pushed) y z x

20 CCD2 Dual-axis gyroscope Measurement of rotation rate near null rotation operating point. Rotation phase shift: CCD1: CCD2: Precision: Noise Floor: CCD1 CCD2 CCD1 y z x

21 Phase shear readout (PSR) Observing phase shear does not require PSI or uniform rotation Consider an atom s phase when changing the angle of just one pulse: Atom Absolute phase depends on the path of the laser Atoms illuminate the phase prescribed by the laser sequence Laser phase: Phase depends on position; no need for position-velocity correlation.

22 Phase shear readout (PSR) Tilt angle of final pulse to introduce a phase shear: y Fluorescence image

23 Phase shear readout Tilt angle of final pulse to introduce a phase shear: -80 µrad 0 µrad 80 µrad -40 µrad 40 µrad Enables simultaneous readout of contrast and phase Sugarbaker, et al., PRL 111, (2013)

24 Phase shear readout Phase Shear Readout (PSR) F = 2 (pushed) F = 1 F = 1 g g 1 cm 4 mm/s F = 2 (pushed) 1 cm Mitigates noise sources: Pointing jitter and residual rotation readout Laser wavefront aberration in situ characterization Single-shot interferometer phase measurement

25 Phase shear readout Phase Shear Readout (PSR) F = 2 (pushed) F = 1 F = 1 g g 1 cm 4 mm/s F = 2 (pushed) 1 cm Mitigates noise sources: Pointing jitter and residual rotation readout Laser wavefront aberration in situ characterization Single-shot interferometer phase measurement

26 Phase shear readout Phase Shear Readout (PSR) F = 2 (pushed) F = 1 F = 1 g g 1 cm 4 mm/s F = 2 (pushed) 1 cm Mitigates noise sources: Pointing jitter and residual rotation readout Laser wavefront aberration in situ characterization Single-shot interferometer phase measurement

27 Phase shear readout Phase Shear Readout (PSR) F = 2 (pushed) F = 1 F = 1 g g 1 cm 4 mm/s F = 2 (pushed) 1 cm Mitigates noise sources: Pointing jitter and residual rotation readout Laser wavefront aberration in situ characterization Single-shot interferometer phase measurement

28 Deterministic Phase Extraction Demonstrate single shot phase using short T interferometer (less sensitive to seismic noise) T = 25 ms, 60 μrad misalignment at final pulse Sugarbaker, et al., PRL 111, (2013)

29 Measuring small phase shears with PSR Can be difficult to measure small phase gradients Use PSR: Applied shear Residual Coriolis Alternate sign of tilt Trial 1 Trial 2 Applied shear adds or subtracts from residual Coriolis. Analogous to a heterodyne measurement in the time domain.

30 Gyrocompass demonstration using phase shear Use phase shear to determine true North Vary rotation compensation direction, measure phase shear 0.01 deg resolution in 1 hr. g Sugarbaker, et al., PRL 111, (2013)

31 PSR with timing asymmetry Vertical shear due to asymmetric pulse spacing: -240 µs -160 µs 0 µs 160 µs 240 µs Beam tilt + timing asymmetry: -160 µs -80 µs 0 µs 80µs 160 µs

32 LMT Atom Optics

33 Sequential Multiphoton Bragg Sequence of 6hk Bragg pulses used to realize a 102hk atom interferometer. Sample interferometer pulse sequence Chiow, PRL, 2011

34 Lasers for LMT in the 10 m Tower 1560 nm Master 30 W Amplifiers PPLN Doubling Crystals 7 W per beam at 780 nm Chiow, et al., Opt. Lett. 37, 18 (2012).

35 LMT in 10 m apparatus LMT using sequential Raman transitions with long interrogation time. >98% contrast! 4 cm 8 cm wavepacket separation (!!) LMT demonstration at 2T = 2.3 s (unpublished). Contrast loss consistent with spontaneous emission

36 Path to increasing LMT Spontaneous emission Increase detuning Loss probability: Pulse area held constant Inhomogeneous transition probability Big laser beams, cold atoms E.g., non uniform Rabi frequency due to Gaussian spatial profile of laser.

37 Delta Kick Cooling

38 Delta kick cooling in a harmonic trap At the end of evaporation BEC: Atom number: Cloud diameter: Temperature: ~10 6 atoms μm ~1 μk (from chemical potential) Harmonic Lens: t = 0 t < t Lens t = t Lens t = t Lens + ε v x C. Monroe et al., PRL 65, Ammann & Christensen, PRL 78, 1997

39 Magnetic lens in a harmonic trap Trap turned off

40 Magnetic lens in a harmonic trap Trap turned off

41 Magnetic lens in a harmonic trap Trap turned off x 0 Residual velocity is x 0 ω

42 Oscillations in a TOP trap Absorptive images of atoms released into weak TOP potential (3.7 G & 25 G/cm) Cloud width oscillations (breathing modes) Radial Vertical anisotropy

43 Isotropic turning points in a TOP trap Tune radial and vertical trap frequencies of gravity + TOP trap using field gradient. Absorptive images of atoms released into weak TOP potential (3.7 G & 22.9 G/cm) Cloud width oscillations (breathing modes) Radial Vertical TOP turn-off time

44 z Lattice Solution to Anisotropy Another solution: optical lattice confinement Lattice locks atoms vertically x

45 z Lattice Solution to Anisotropy Another solution: optical lattice confinement Radial (two-dimensional) expansion x x

46 z Lattice Solution to Anisotropy Another solution: optical lattice confinement Lattice turns off -- Expansion in three dimensions x x x

47 z Lattice Solution to Anisotropy Another solution: optical lattice confinement Turn off trap x x x x

48 Lattice-Aided Lens Cooling Lattice turn-off time TOP turn-off time Cloud launched to 9 meters Radial Vertical 5 mm 20x colder (50 nk)

49 Extending to colder temperatures Delta kick in micro gravity: weaker potential, more expansion Multiple lens sequences Apply additional kick at the fountain turning point Apply optical potential for radial delta kick (high power AI laser)

50 Lens AC Stark Lens Apply transient optical potential ( Lens beam ) to collimate atom cloud in 2D Lens beam Lens beam parameters: 3 mm radial waist, ~3 Watts, 1 THz red detuned, ~30 ms duration (typical)

51 2D AC Stark Lens No Lens With Lens North View Demonstrated refocusing in 2D West View Atom cloud radius <200 microns (resolution limited) after 2.6 seconds drift!

52 Focusing atom cloud with AC Stark lens Vary effective focal length of AC Stark lens by changing lens pulse duration Collimating lens: Refocusing lens: Time Time Apply lens near top of fountain (~ 1 s TOF) With collimating lens, delta kick theory suggests temperature < 100 pk (!) Challenge: Measure pk temperature when TOF expansion is small compared to size. Work ongoing!

53 Lens Initial application: Relaunch sequence Launch Lens Relaunch Detect Lens beam Launched to meters Relaunched to 6 meters 5 seconds total free fall time Not possible without lens

54 Collaborators Stanford Mark Kasevich (PI) Susannah Dickerson Alex Sugarbaker Sheng-wey Chiow Tim Kovachy NASA GSFC Babak Saif Bernard D. Seery Lee Feinberg Ritva Keski-Kuha Theory: Peter Graham Savas Dimopoulos Surjeet Rajendran Former members: David Johnson Visitors: Philippe Bouyer (CNRS) Jan Rudolph (Hannover)