Danielle Boddy Durham University – Atomic & Molecular Physics group

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

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

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

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

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

6. Motivation How can we enter the dipole blockaded regime in strontium? 1P1 3P2 3P1 Doppler temperature = 3P0 λ = 461 nm Γ = 2π x 32 MHz TD = 1 mK λ = 689 nm Γ = 2π x 7.5 kHz 2nd stage cooling Two electrons → Form singlet and triplet states 1S0 LS coupling breakdown→ weakly allowed singlet-triplet transitions Singlet-triplet transitions are characterised by narrow linewidths Introduce a second stage of cooling on the 3 P1→1 S0 transition 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 fast feedback to diode Atomic signal Fabry-Perot cavity 689 nm laser slow feedback to piezo slow feedback to piezo Red MOT First group meeting 18/07/11

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

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

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

12. 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

13. Signal recovery: Using a low pas filter Noise tends to be spread over a wider spectrum than the signal. 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? First group meeting 18/07/11

14. 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

15. Lock-in amplifier Require a reference frequency Multiplies the input signal by the reference signal Integrates over a specific time (ms to s) 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 The lock in is reference to the operating frequency of the AOM. First group meeting 18/07/11

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

17. What next? Immediate future Finish slow lock circuit Try locking laser using this slow lock and set-up red MOT optics Try electron shelving experiment on main experiment 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

18. 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