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