progress towards laser cooling strontium atoms on the intercombination transition

Progress towards laser cooling strontium atoms on the

intercombination transition

Danielle BoddyDurham University – Atomic & Molecular Physics group

The team

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

Motivation: Rydberg physics Ionization

threshold

Ener

gy

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)


http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=RSINAK000080000001013101000001&idtype=cvips



Motivation: Dipole blockade


Y Miroshnychenko et al., Nat. Phys. 5, 115-118 (2009)E Urban et al., Nat. Phys. 5, 110-114 (2009)



Sr88 is an alkaline earth metal with no hyperfinestructure.

Two valence electrons permits two electron excitation.

Motivation: An introduction to strontium


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, 213004 (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)

At present, we’re investigating the spatial excited state distribution.

http://prl.aps.org/abstract/PRL/v105/i21/e213004

http://dx.doi.org/10.1088/0953-4075/40/22/F03

http://dx.doi.org/10.1088/0953-4075/40/22/F03

http://dx.doi.org/10.1088/0953-4075/40/22/F03

Motivation: Dipole blockade regime


rB

T ~ 5 mKDensity ~ 1 x 109 cm-3

T ~ 400 nKDensity ~ 1 x 1012 cm-3

No blockade

How do we enter the dipole blockade regime?

Blockaded

Motivation: Laser cooling of strontium


1P1

3P0

3P1

3P2

1S0

λ = 689 nmΓ = 2π x 7.5 kHz2nd stage cooling

λ = 461 nmΓ = 2π x 32

MHz1st stage cooling

1S0 → 3P1 intercombination transition → TD ≈ 180 nK.

Photon recoil limits TD → Tmin ≈ 460 nK.

Introduce two stages of cooling:

First cool on the (5s2) 1S0 → (5s5p) 1P1.

Second cool on the narrow-line (5s2) 1S0 → (5s5p) 3P1 .

Outline

Simple laser stabilization set-up

Laser system

Pound-Drever-Hall (PDH)

Locking to an atomic transition

Fluorescence

Electron shelving

Summary




Laser system

Fabry-Perot

cavity

Atomic signal

Red MOT



Laser system

Fabry-Perot

cavity

Atomic signal

Red MOT

Laser system

Laser system


Compared old and new designs.

Laser system


time

frequency

OLD

NEW2 Wavemeter

10 s

1

10 s

OLD

NEW

Wavemeter1

time

frequency2

Laser system


Old New

New design fluctuates more in the short term.Little difference between the long term stability.



Laser system

Fabry-Perot

cavity

Atomic signal

Red MOT

Fabry-Perot

cavity

Pound-Drever-Hall (PDH) technique


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 :

Lc

FSR 2



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 modulato

r

FPD

Lock Box

Etalon4

PiezoCurrent

modulation

Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)






4

FPD

PS

2

Slow feedback to

piezo

Fast feedback to

diode

Feedback to cavity piezo

Atomic signal

Laser

2

Lock Box

A crystal oscillator phase modulates the 689 nm beam at a frequency of 10 MHz.



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 Ps = 0.42 Pc

Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)




Pound-Drever-Hall (PDH) summary


Generate PDH signal

Gradient of error signal → strength of lock and laser linewidth

NEXT STEP: Finish high bandwidth servo

IMPROVEMENTS: Build high-finesse cavity



Laser system

Fabry-Perot

cavity

Atomic signal

Red MOT

Atomic signal

Locking to an atomic transition


CHALLENGE: Detecting the transition.

Two detection methods:

1. Electron Shelving

2. Fluorescence

Electron shelving


Excite atoms to the 3P1 and measure the rate at which these atoms decay out of the state.

Photon scattering rate is proportional to the linewidth of the transition.

1P1

3P1

1S0

λ = 689 nm

λ = 461 nm

Electron shelving


1P1

3P1

1S0

λ = 461 nm

atomic beam

photodiode

The amount of scattered light is proportional to the

number of atoms initially in the 1S0 ground state.

Electron shelving


1P1

3P1

1S0

λ = 689 nm

λ = 461 nm

atomic beam

photodiode

Electron shelving: Experiment


atomic beam

photodiode

Electron shelving: Experiment


≈ 32 MHz

Electron shelving: Lifetime measurement


Using a velocity of 500 ms-1

Lifetime of 3P1 is (23 ± 1) μs

Gradient: (8.9 ± 0.2) x 10-2 mm-1

Electron shelving: Crossed beams


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


Detected the transition indirectly via electron shelving.

Determined the lifetime of the 3P1 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


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.



(a) Slice along direction of laser beam → absorption and decay.

(b) Slice along direction of atomic

beam → transverse velocity distribution.

(b)(a)



Gradient: (9.0 ± 0.3) x 10-2 mm-1

Using a velocity of 500 ms-1

Lifetime of 3P1 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)




BUT what about the absorption?

Fluorescence: The model


Solves optical bloch equations (OBEs) for a two level atom as a function of

distance.

Velocity distribution of atoms α Tk

mv

Bev 23

2

)(vf

Randomly selects a value of and .

If the value of is kept.

)(vf 'v

)'()( vfvf 'v



x

Assuming the laser is on resonance the only other unknown in the OBEs is

the Rabi frequency.

Top hat pulse: Ed.

x

Gaussian pulse: 2

2)(2. waistwaistx

eEd



0 .0 002 0 .0 004 0 .0 006 0 .00 08 0 .0 010 0 .00 12D istan cem0 .2

0 .4

0 .6

0 .8

1 .0

E xci ted po pu lat io n state 22

Top hat pulse Gaussian pulse

Velocity of 500 ms-1



0 .00 1 0 .00 2 0 .0 03 0 .00 4 0 .00 5 0 .0 06D is tancem0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

E xcited pop u lat ion state 22

0 .001 0 .0 02 0 .003 0 .00 4 0 .00 5 0 .00 6D is tancem0 .2

0 .4

0 .6

E xcited po pu latio n state 22

2 x waist

Fluorescence: Summary


Detected the transition directly.

Determined the lifetime of the 3P1 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


689 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 3P1 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?


Thanks for listening Any questions?

progress towards laser cooling strontium atoms on the intercombination transition

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