1. Danielle Boddy Durham University – Atomic & Molecular Physics group Laser locking to hot atoms

2. The team First group meeting 18/07/11

3. Motivation M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)Rev. Mod. Phys. 82, 2313 (2010) Rubidium Strontium First group meeting 18/07/11

4. Motivation Coulomb: van der Waals (vdW): Rydberg: is state dependent is the Förster defect Crossover separation First group meeting 18/07/11

5. Motivation At present First group meeting 18/07/11 Where we want to be

6. Motivation How can we enter the dipole blockaded regime in strontium? Two electrons → Form singlet and triplet states LS coupling breakdown → weakly allowed singlet-triplet transitions 1P11P1 1S01S0 λ = 461 nm Γ = 2π x 32 MHz 3P03P0 3P13P1 3P23P2 λ = 689 nm Γ = 2π x 7.5 kHz 2 nd stage cooling Doppler temperature = T D = 1 mK Introduce a second stage of cooling on the 3 P 1 → 1 S 0 transition Singlet-triplet transitions are characterised by narrow linewidths Photon recoil limits the minimum temperature to ≈ 460 nK. First group meeting 18/07/11

7. Outline Simple laser stabilization set-up Detecting the transition Signal recovery Lock-in amplifier Generating the error signal What next? Summary First group meeting 18/07/11

8. Simple laser stabilization set-up Atomic signal Red MOT slow feedback to piezo 689 nm laser Fabry-Perot cavity slow feedback to piezo fast feedback to diode First group meeting 18/07/11

9. Simple laser stabilization set-up 689 nm laser Fabry-Perot cavity Atomic signal Red MOT slow feedback to piezo fast feedback to diode slow feedback to piezo First group meeting 18/07/11

10. Locking the laser to the cavity A crystal oscillator phase modulates the 689 nm beam at a frequency of 10 MHz. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Employ the Pound-Drever-Hall (PDH) technique Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001) PS Laser

11. Locking the laser to the cavity Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001) FPD PS Laser Lock to central feature

12. Locking the laser to the cavity Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001) FPD PS Slow feedback to piezo Fast feedback to diode Lock Box Laser Increasing the gradient of the error signal strengthens the lock and reduces the linewidth Gradient is maximum when P s = 0.42 P c

13. Locking the cavity to the atoms Progress towards laser cooling strontium atoms on the intercombination transition - May 2011 Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)Am. J. Phys. 69 (1) 79 (2001) FPD PS Slow feedback to piezo Fast feedback to diode Lock Box Laser Increasing the gradient of the error signal strengthens the lock and reduces the linewidth Gradient is maximum when P s = 0.42 P c

14. Simple laser stabilization set-up Atomic signal Red MOT slow feedback to piezo 689 nm laser Fabry-Perot cavity slow feedback to piezo fast feedback to diode First group meeting 18/07/11

15. Detecting the transition atomic beam CCD PD Used a CCD camera to take spatially resolved images of the fluorescence Tried both an indirect and direct method of detection the transition Photodiode detector (PD) is a transimpedance, high gain, low noise circuit PD sits at end of a sealed 1:1 telescope Focus is at the centre of the of atomic beam Photodiode has an active area of 3.8 mm x 3.8 mm PD is contained within a Faraday cage First group meeting 18/07/11

16. Signal recovery Suppose our signal is a 10 nV sine wave at 10 kHz. Amplification is required to bring the signal above noise Our PD has 11 nV/√Hz of input noise at 10 kHz (according to datasheet) IF Amplifier bandwidth = 100 kHz Output = 10 μV (10 nV x 1000) Amplifier gain = 1000 Noise = 3.5 mV (11 nV/√Hz x √100 kHz x 1000) Signal-to-noise (SNR) ~ 3 x 10 -3 Need to single out the frequency of interest! First group meeting 18/07/11

17. Signal recovery: Using a low pas filter Suppose we follow the amplifier with a bandpass filter IF Q = 100 (a very good filter) Signal detected in 100 Hz bandwidth (10 kHz/Q) Centre frequency = 10 kHz Noise = 110 μV (11 nV/√Hz x √100 Hz x 1000) SNR ~ 0.01 This is still not good enough! How do we overcome this problem? Noise tends to be spread over a wider spectrum than the signal. First group meeting 18/07/11

18. Signal recover: Using a lock-in amplifier Lock-in amplifiers are used to detect and measure very small AC signals Singles out the component of the signal at a specific reference frequency and phase Lock-in can detect the signal at 10 kHz with a bandwidth of 0.01 Hz! Noise = 1.1 μV (11 nV/√Hz x √0.01 Hz x 1000) The signal is still 10 μV SNR ~ 9 Accurate measurement of the signal is possible! First group meeting 18/07/11

19. Lock-in amplifier Require a reference frequency Multiplies the input signal by the reference signal Integrates over a specific time (ms to s) The lock in is reference to the operating frequency of the AOM. Resulting signal is a DC signal, where signal not at the reference frequency is attenuated to zero Since the signal is slowly varying, then 1/f noise overwhelms the signal Modulate the signal external → use an acousto-optic modulator (AOM) in double pass configuration at a large frequency First group meeting 18/07/11

20. PD Generating the error signal AOM RF Lock-in time constant = 10 ms First group meeting 18/07/11 Scanning laser over 100 mHz Lock-in sensitivity = 1 mV Error signal gradient is ~ 0.9 V/ MHz

21. PD noise Dark current is inherent is all photosensitive devices Ideally, want the noise of the PD to be limited by dark current Dark current has shot noise Photodiode dark current is 1 nA & R = 40 MΩ Noise voltage e.g if Δf = 1 Hz, then σ v = 10 -6 V First group meeting 18/07/11

22. What next? Finish slow lock circuit Try locking laser using this slow lock and set-up red MOT optics Try electron shelving experiment on main experiment Immediate future Long(ish) term future Finish Pound-Drever-Hall fast lock Long term future Build high-finesse cavity Red MOT – easy! First group meeting 18/07/11

23. Summary Reproduced Saffman’s rubidium Rydberg plot for strontium The interactions between ground state atoms and Rydberg atoms for strontium is at least 7 orders of magnitude greater than for Rubidium Generated slow lock error signal via fluorescence spectroscopy First group meeting 18/07/11