BEN CATCHPOLE

Towards a three-step laser excitation of rubidium Rydberg states for use in a

microwave CQED single-atom detector

INTRODUCTION

MOTIVATION- Applications for Rydberg states- Single-atom detection (S.A.D.)

THEORETICAL BACKGROUND- Rydberg production- Doppler free spectroscopy- Fine and Hyperfine structure

EXPERIMENTAL TECHNIQUES- Polarisation spectroscopy- Electromagetically induced transparency (EIT)

RESULTS

OVERVIEW

AIM- Three step laser excitation to produce 63P3/2 Rydberg

states for SAD

ACHIEVEMENTS- Doppler-free spectroscopic techniques applied to excite

ground-state rubidium atoms to the 5D5/2 hyperfine excited state.

- Techniques allowed for precise control of laser frequency:- Allan Deviations of 30kHz and 45kHz for first two

transitions over ~ 1hour, this represents as little as 0.0029% and 0.003% of the 10.3±0.1MHz and ~14MHz natural transition linewidths, respectively.

INTRODUCTION

MOTIVATION

Rydberg atoms, with a very high principal quantum number (n), large dipole moments and long transition lifetime can be used in a wide-range of experimental setups…

- One-atom maser (Micromaser)

(1985)- Collapse/revival of VROs

(1987)- Verification of quantised EM field with VROs

(1996)- Production of ‘number’ states on demand – Trapping

- State reduction

- Rydberg ‘blockades’ ~(2000)

- Observation of field state collapse with QND measurements (2007)

- Birth, life and death of a single photon(2007)

- ‘Freezing’ evolution of cavity field with Quantum Zeno effect (2008)

- Sensitive detection of microwave photons(2009)

- Single photon source

- Single Atom Detection (S.A.D.) N atoms (1989), N<10 (1992)

S.A.D.

- Transmission spectrum of cavity split by presence of atom (normal mode splitting), without an atom only a single peak is observed.- When atom and cavity are tuned into resonance mixing of

the states prduces new Eigenfrequencies for the atom-field state

- Demonstrated sufficient sensitivity for SAD

DOPPLER EFFECTS BACKGROUND

DOPPLER BROADENING OF SPECTRAL SIGNAL

- Due to thermal motion of the atomic vapour- Gaseous atoms have a Maxwell-Boltzmann velocity

distribution - Atoms move randomly in all directions- Each velocity component takes a distibution of values- This range of velocities produces a range of Doppler-shifts- Cumulative effect is inhomogeneous line broadening of

spectral signal

DOPPLER-FREE SPECTROSCOPY

- Collimated atomic beam spectroscopy(1942)

- Saturated absorption spectroscopy(1971)

- Polarisation spectroscopy(1976)

- Electromagnetically-induced transparency(1991)

SAS BACKGROUND

SATURATED ABSORPTION SPECTROSCOPY (SAS)

- Velocity-selective saturation of absorption Doppler free signal - Laser divided into a ‘PUMP’ and less intense ‘PROBE’- (Iprobe<<Isat) and (Ipump>Isat) - PUMP ‘burns’ a hole in lower level population density- Means probe encounters less ground state atoms –

reduced absorption

- Pump interacts with atoms in the velocity class:- Far from resonance the counter-propagating beams

interact with a completely different velocity class- Close to resonance the difference between the laser and

the transition is ~0, therefore both beams interact with the same ~0 velocity class

- As the hole is burn into the zero-velocity class of atoms, and only these contribute to the spectral signal, the lamb dip is free from Doppler broadening and the spectra is DOPPLER-FREE

BACKGROUND

FINE AND HYPERFINE STRUCTURE

- GROSS structure described by solutions to the Schrodinger equation

- FINE structure due to SPIN-ORBIT interaction- HYPERFINE structure due to SPIN-SPIN interaction

GROSS STRUCTURE

- As an alkali metal, ground state rubidium has a single valence electron in the outer 5s orbital.

FINE STRUCTURE (due to Spin-Orbit interaction)

- Caused by the splitting of ‘gross structure’ atomic energy levels

- ‘orientation energy’ – as it is determined by the relative orientation of two magnetic vectors.

- S: total ELECTRONIC SPIN angular momentum- L: total ORBITAL angular momentum- J represents the TOTAL ELECTRONIC angular

momentum, which takes values of |L-S| to L+S.

- Both L and S sum to zero for closed orbitals, consequently only the valence electron contributes…

FINE STRUCTURE BACKGROUND

(2s+1)LJ

(1s22s22p63s23p63d104s24p65s)

J=L+S

5s state(L=0),(S=1/2)5S1/25p state(L=1),(S=1/2)5P1/2 5P3/2

(due to Spin-Spin interaction)HYPERFINE STRUCTURE

- Caused by splitting of fine structure energy levels- Also an ‘orientation energy’, due to two magnetic dipoles

in different orientations.- NUCLEAR SPIN angular momentum (I) is proportional to

nuclear structure – determines magnetic moment of nucleus- F represents the TOTAL ATOMIC angular momentum which takes values |J-I| to J+I and determines the HYPERFINE energy levels.

HYPERFINE STRUCTURE BACKGROUND

5s state(L=0),(S=1/2)5S1/2

5P3/2

5p state(L=1),(S=1/2)5P1/2

F=I+J

(due to Spin-Spin interaction)HYPERFINE STRUCTURE

- The most abundant (72%) isotope, 85Rb has a total nuclear

spin I=5/2- Consequently have 2 possible values of angular

momentum for the 5S1/2 ground state and 4 for the excited 5P3/2 state, due to non-zero L value.

- Only specfic transitions are allowed due to dipole selection rules (∆F=0,±1) – indicated by arrows. (red arrows are enhanced transitions)

- Each level is further divided into multiple ZEEMAN MAGNETIC SUBLEVELS (mF=2F+1)

HYPERFINE STRUCTURE BACKGROUND

F=I+J

5S1/2 (5/2)±1/2 (2,3)

5P1/2 (5/2)±1/2 (2,3)

5P3/2 (5/2)±1/2 (2,3)

(5/2)±3/2 (1,4)

~1260 nm

EXPERIMENTAL TECHNIQUES

- The specific excitation pathway utilised during the experiment

- 780.24 nm transition: 5S1/2 (F=3) and 5P3/2 (F=4) hyperfine levels

- 775.98 nm transition: 5P3/2 (F=4) and 5D5/2 (F=5) hyperfine levels

- From 5D5/2 (F=5), mF=5, dipole selection rules (∆l=±1 and ∆j=0,±1) dictate that nP3/2, nF5/2 and nF7/2 Rydberg states are attainable

EXPERIMENTAL TECHNIQUES

POLARISATION SPECTROSCOPY (5S1/25P3/2 transition)

- Form of SAS, based on light-induced BIREFRINGENCE and DICHROISM

- Circularly polarised PUMP used to generate OPTICAL ANISTROPY, which is interrogated by the linearly polarised PROBE- Linearly polarised PROBE can be decomposed into 2

circularly polarised beams, rotating in opposite directions, these encounter different refractive indicies and absorption coefficients

- Beam splitter used to seperate |H> and |V> components - Intensity difference provides polarisation spectroscopy

signal

IMPROVED SNR & NO NEED FOR FREQUENCY MODULATION!

ELECTROMAGNETICALLYINDUCEDTRANSPARENCY (EIT)

EXPERIMENTAL TECHNIQUES

- The second 776nm transition is much weaker (longer atomic lifetime) – therefore utilised QUANTUM AMPLIFICATION (QA), whereby a lifetime difference was used to create an EIT.

- Produced an enhanced first step transmission signal which is a function of second step detuning- QA: detection of a weak resonance signal via the response of a strong atomic transition when the transitions share a common state

- Excitation to the 5D5/2 state hinders multiple absorption-emission cycles on the 5S-5P transition, leading to a visibly enhanced first-step transmission peak known as a ‘reduced absorption peak’

- Lifetimes: (5D5/2=238.5ns) and (5P3/2=26.24ns)

- Therefore EIT peak ~10x larger than optically available

- Typical 5P3/2 (F=4) to 5D5/2 (F=5) spectral feature detected using QA, consequently the signal represents a first-step reduced absorption peak and dispersion-shaped error signal generated through frequency modulation of the spectral feature.