towards a three-step laser excitation of rubidium rydberg states for use in a microwave cqed...
TRANSCRIPT
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.