Danielle Boddy Durham University – Atomic & Molecular Physics group Laser locking to hot atoms.
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Transcript of Danielle Boddy Durham University – Atomic & Molecular Physics group Laser locking to hot atoms.
Danielle Boddy
Durham University – Atomic & Molecular Physics group
Laser locking to hot atoms
The team
First group meeting 18/07/11
Motivation
M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)
Rubidium
Strontium
First group meeting 18/07/11
Motivation
Coulomb: R
qq 1
4
.
0
21
van der Waals (vdW): 66
1
RC
Rydberg:662 4
2
1
2 R
C
is state dependent6C
is the Förster defectCrossover separation
6 6
C
RC
First group meeting 18/07/11
Motivation
At present
First group meeting 18/07/11
Where we
want to be
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
1P1
1S0
λ = 461 nmΓ = 2π x 32
MHz
3P0
3P1
3P2
λ = 689 nmΓ = 2π x 7.5 kHz2nd stage cooling
Doppler temperature = Bk2
TD = 1 mK
Introduce a second stage of cooling on the 3 P1→ 1 S0
transition
Singlet-triplet transitions are characterised by narrow linewidths
Photon recoil limits the minimum temperature to ≈ 460 nK.
First group meeting 18/07/11
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
Simple laser stabilization set-up
Atomic
signal
Red MOT
slow feedback to piezo
689 nm laserFabry-
Perot
cavityslow feedback to piezo
fast feedback to diode
First group meeting 18/07/11
Simple laser stabilization set-up
689 nm laserFabry-
Perot
cavity
Atomic
signal
Red MOT
slow feedback to piezo
fast feedback to diode
slow feedback to piezo
First group meeting 18/07/11
Simple laser stabilization set-up
Atomic
signal
Red MOT
slow feedback to piezo
689 nm laserFabry-
Perot
cavityslow feedback to piezo
fast feedback to diode
First group meeting 18/07/11
Detecting the transition
atomic beam
CCDPD
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
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
Signal recovery: Using a low pas filter
Suppose we follow the amplifier with a bandpass filter
IFQ = 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
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
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 signalModulate the signal external → use an acousto-optic modulator (AOM) in double pass configuration at a large frequency
First group meeting 18/07/11
PD
Generating the error signal
AOM
RF
Lock-in time constant = 10
ms
First group meeting 18/07/11
4
Scanning laser over 100 mHz
Lock-in sensitivity = 1
mV
Error signal
gradient is ~ 0.9 V/MHz
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 cavityRed MOT – easy!
First group meeting 18/07/11
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