Progress towards laser cooling strontium atoms on the intercombination transition Danielle Boddy Durham University – Atomic & Molecular Physics group

The team Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

Motivation: Rydberg physics Ionization threshold Energy States of high principal quantum number n. Exaggerated size and lifetimes. Can be prepared through laser excitation. Greatly enhanced inter-atomic interactions. Strong, tunable, long-range dipole-dipole interactions among the atoms. Applications include quantum computation. M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)Rev. Mod. Phys. 82, 2313 (2010) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

Motivation: Dipole blockade Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Y Miroshnychenko et al., Nat. Phys. 5, (2009)Nat. Phys. 5, (2009) E Urban et al., Nat. Phys. 5, (2009)Nat. Phys. 5, (2009)

Sr 88 is an alkaline earth metal with no hyperfine structure. Two valence electrons permits two electron excitation. Motivation: An introduction to strontium Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Ground state Rydberg state Doubly excited state Two-electron excitation of an interacting cold Rydberg gas J. Millen, G. Lochead and M. P. A. Jones Phys. Rev. Lett. 105, (2010) Phys. Rev. Lett. 105, (2010) Spectroscopy of strontium Rydberg states using electromagnetically induced transparency S. Mauger, J. Millen and M. P. A. Jones J. Phys. B: At. Mol. Opt Phys. 40, F319 (2007) J. Phys. B: At. Mol. Opt Phys. 40, F319 (2007) At present, we’re investigating the spatial excited state distribution.

Motivation: Dipole blockade regime Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 rBrB T ~ 5 mK Density ~ 1 x 10 9 cm -3 T ~ 400 nK Density ~ 1 x cm -3 No blockade How do we enter the dipole blockade regime? Blockaded

Motivation: Laser cooling of strontium Progress towards laser cooling strontium atoms on the intercombination transition - May P11P1 3P03P0 3P13P1 3P23P2 1S01S0 λ = 689 nm Γ = 2π x 7.5 kHz 2 nd stage cooling λ = 461 nm Γ = 2π x 32 MHz 1 st stage cooling 1 S 0 → 3 P 1 intercombination transition → T D ≈ 180 nK. Photon recoil limits T D → T min ≈ 460 nK. Introduce two stages of cooling: First cool on the (5s 2 ) 1 S 0 → (5s5p) 1 P 1. Second cool on the narrow-line (5s 2 ) 1 S 0 → (5s5p) 3 P 1.

Outline Simple laser stabilization set-up Laser system Pound-Drever-Hall (PDH) Locking to an atomic transition Fluorescence Electron shelving Summary Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

Simple laser stabilization set-up Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Laser system Fabry-Perot cavity Atomic signal Red MOT

Simple laser stabilization set-up Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Laser system Fabry-Perot cavity Atomic signal Red MOT Laser system

Laser system Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Compared old and new designs.

Laser system Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 time frequency OLD NEW 2 Wavemeter 10 s 1 OLD NEW Wavemeter 1 time frequency 2

Laser system Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Old New New design fluctuates more in the short term. Little difference between the long term stability.

Simple laser stabilization set-up Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Laser system Fabry-Perot cavity Atomic signal Red MOT Fabry-Perot cavity

Pound-Drever-Hall (PDH) technique Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Require the laser linewidth < 7.5 kHz. Noise broadens the linewidth to the MHz regime. Uses Fabry-Perot cavity as a frequency reference. Cavity peaks are spaced by the free spectral range :

Pound-Drever-Hall (PDH) technique Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Phase modulator adds sidebands to the laser. High-finesse Fabry-Perot cavity measures the time-varying frequency of the laser input. An electronic feedback loop works to correct the frequency error and maintain constant optical power. Laser Phase modulator FPD Lock Box Etalon Piezo Current modulation Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001)

Pound-Drever-Hall (PDH) technique Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 FPD PS Slow feedback to piezo Fast feedback to diode Feedback to cavity piezo Atomic signal Laser Lock Box A crystal oscillator phase modulates the 689 nm beam at a frequency of 10 MHz.

Pound-Drever-Hall (PDH) technique Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Laser locks to the central feature of the PDH error signal Increasing the gradient of the error signal strengthens the lock and reduces the linewidth. (a) (b) (c) (d) Gradient depends on sideband power: carrier power ratio. Gradient steepest when P s = 0.42 P c Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001)

Pound-Drever-Hall (PDH) summary Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Generate PDH signal Gradient of error signal → strength of lock and laser linewidth NEXT STEP: Finish high bandwidth servo IMPROVEMENTS: Build high-finesse cavity

Simple laser stabilization set-up Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Laser system Fabry-Perot cavity Atomic signal Red MOT Atomic signal

Locking to an atomic transition Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 CHALLENGE: Detecting the transition. Two detection methods: 1.Electron Shelving 2.Fluorescence

Electron shelving Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Excite atoms to the 3 P 1 and measure the rate at which these atoms decay out of the state. Photon scattering rate is proportional to the linewidth of the transition. 1P11P1 3P13P1 1S01S0 λ = 689 nm λ = 461 nm

Electron shelving Progress towards laser cooling strontium atoms on the intercombination transition - May P11P1 3P13P1 1S01S0 λ = 461 nm atomic beam photodiode The amount of scattered light is proportional to the number of atoms initially in the 1 S 0 ground state.

Electron shelving Progress towards laser cooling strontium atoms on the intercombination transition - May P11P1 3P13P1 1S01S0 λ = 689 nm λ = 461 nm atomic beam photodiode

Electron shelving: Experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 atomic beam photodiode

Electron shelving: Experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 atomic beam photodiode

Electron shelving: Experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 atomic beam photodiode

Electron shelving: Experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 ≈ 32 MHz

Electron shelving: Lifetime measurement Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Using a velocity of 500 ms -1 Lifetime of 3 P 1 is (23 ± 1) μs Gradient: (8.9 ± 0.2) x mm -1

Electron shelving: Crossed beams Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 atomic beam photodiode FWHM crossed beams is ≈ 20 MHz. Linewidth has reduced by 1/3. This is not narrow enough for the Fabry-Perot to lock to!

Electron shelving: Summary Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Detected the transition indirectly via electron shelving. Determined the lifetime of the 3 P 1 state. And the lineshape? Tried crossing the beams:  Did the linewidth reduce?  Is this narrow enough for the laser to lock to?  Work in progress Try a direct method of detection.

Fluorescence: The experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Strontium has negligible vapour pressure at room temperature → heated to 900 K. atomic beam CCD camera takes spatially resolved images of the fluorescence. Exposure length set to 65.5 ms.

Fluorescence: The experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 (a)Slice along direction of laser beam → absorption and decay. (b)Slice along direction of atomic beam → transverse velocity distribution. (b) (a)

Fluorescence: The experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Gradient: (9.0 ± 0.3) x mm -1 Using a velocity of 500 ms -1 Lifetime of 3 P 1 is (22.2 ± 0.7) μs Other time resolved fluorescence detection: τ = (21.3 ± 0.5) μs See R Drozdowski., Phys. D. 41:125 (1997)Phys. D. 41:125 (1997)

Fluorescence: The experiment Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 BUT what about the absorption?

Fluorescence: The model Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Solves optical bloch equations (OBEs) for a two level atom as a function of distance. Velocity distribution of atoms α Randomly selects a value of and. If the value of is kept.

Fluorescence: The model Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 x Assuming the laser is on resonance the only other unknown in the OBEs is the Rabi frequency. Top hat pulse: x Gaussian pulse:

Fluorescence: The model Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Top hat pulse Gaussian pulse Velocity of 500 ms -1

Fluorescence: The model Progress towards laser cooling strontium atoms on the intercombination transition - May x waist

Fluorescence: Summary Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Detected the transition directly. Determined the lifetime of the 3 P 1 state. Written code to model absorption and decay. Data and theory don’t quite agree. NEXT: Try locking to this fluorescence signal. Need to find source of problem.

Summary Progress towards laser cooling strontium atoms on the intercombination transition - May nm laser built and tested. Need to finish PDH high bandwidth servo circuit. Build high-finesse cavity. Tested an indirect and direct method to detect the transition. Measured lifetime of 3 P 1 state from both methods. Try locking to fluorescence signal. If this works….GREAT! If it doesn’t work….try pump-probe spectroscopy Red MOT → colder atoms

Questions? Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Thanks for listening Any questions?