This thesis presents work on experiments performed using electromagnetically induced transparency (EIT) in Rubidium vapour cells. Starting with a simple three-level Λ-system, extra complexity is introduced by adding an additional control beam. This makes the switching from narrow transmitting resonances to absorbing resonances possible. Rydberg EIT is then introduced and is used to shown how the switching of such resonances using external electric fields in possible. The work then progresses towards the use of thin vapour cells to overcome the limitations of conventional systems. In the transition to thin vapour cells, the role of dipole interactions in Rydberg atoms motivates the idea of an atomic quantum dot. The Kerr effect is then characterised and used to perform phase modulation in a gas. The Rydberg state itself is then phase modulated and this is shown to give rise to frequency sidebands which offer a promising way of determining dc electric fields.
This thesis presents work on experiments performed using electromagnetically induced transparency (EIT) in Rubidium vapour cells. Starting with a simple three-level Λ-system, extra complexity is introduced by adding an additional control beam. This makes the switching from narrow transmitting resonances to absorbing resonances possible. Rydberg EIT is then introduced and is used to shown how the switching of such resonances using external electric fields in possible. The work then progresses towards the use of thin vapour cells to overcome the limitations of conventional systems. In the transition to thin vapour cells, the role of dipole interactions in Rydberg atoms motivates the idea of an atomic quantum dot. The Kerr effect is then characterised and used to perform phase modulation in a gas. The Rydberg state itself is then phase modulated and this is shown to give rise to frequency sidebands which offer a promising way of determining dc electric fields.
We initially discuss the theory of three-level systems using the dressed state formalism. One of the dressed states, containing a ground state and a Rydberg state, does not couple with the probe laser; thus, the medium becomes transparent to the latter. This phenomenon is known as electro- magnetically induced transparency (EIT) and this dressed state is known as a Rydberg dark state. We show that EIT can be used to extract the reduced dipole matrix element for transitions to a Rydberg state. However, a prob- lem with three-level Rydberg EIT in a vapour cell is the occurrence of space charges caused by photoelectric ionisation of Rb metal deposited inside the cell. To avoid this problem, we consider adding a third laser resonant with a fourth level. This is to avoid using the laser whose wavelength is less than the threshold wavelength. In cold atoms, the effect of the third laser is to split the usual EIT resonance into a doublet. In thermal atoms, we observe narrow features due to electromagnetically induced absorption and electro- magnetically induced transparency in the Doppler-free configuration. Next we consider the action of a far off-resonance radio frequency (rf) field in the three-level system. We demonstrate the formation of rf-dressed EIT reso- nances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field in- dependent of laser frequency fluctuations. We further investigate the theory of rf-dressed Rydberg EIT using the Floquet approach in order to understand the formation of the sideband structure of the Rydberg state. We find that if the time scale of the rf interaction is much shorter than that of the system evolution and decoherence, the sideband structure is well resolved. We also show that the intermediate state exhibits a sideband structure, induced by the Rydberg state, when the Rabi frequency of coupling laser is larger than twice the modulation frequency. Finally we consider resonant microwave cou- pling between the Rydberg states which leads to an Autler-Townes splitting of the EIT resonance in cold atoms. This splitting can be employed to vary the group index by ±10^5 allowing independent control of the absorptive and dispersive properties of the medium, i.e., one can switch the transparency of the medium or control the group velocity of a pulse propagation by tuning on and off the microwave field.
This thesis set out to investigate light storage based on dynamic electromagnetically induced transparency (EIT) in a room-temperature atomic ensemble of rubidium as a means to provide a quantum memory for single-photons created by a single rubidium atom coupled to a high-finesse optical resonator. Setting up the light storage medium presented a new addition to the research group's portfolio of experimental techniques and led to investigations of EIT, slow light and stored light in warm rubidium-87 vapour. Lambda level schemes connecting Zeeman or hyperfine substates on the D1 and D2 lines were addressed in rubidium vapour cells containing different buffer gases and different isotopic fractions of rubidium-87 and rubidium-85. Single beam spectroscopy with a weak probe was used to characterise the vapour cells. A numerical method to fit the D line spectrum to a theoretical model to include isotopic fractions and collisional broadening of a buffer gas has been implemented. Temperature and isotopic fractions could be reliably extracted from the fit parameters. For an offset-stabilisation of two lasers to address a lambda level scheme connecting the two different hyperfine groundstates in rubidium a phase locked loop including a frequency divider has been designed and implemented. Light storage and retrieval has been demonstrated using a Zeeman scheme on the D1 line. Two microsecond long classical light pulses containing one million photons on average were stored and retrieved with an efficiency of 15% after a delay of one microsecond. Several methods of attenuating the strong co-propagating control laser beam to allow for lowering the signal pulse intensity in future experiments are discussed.
This thesis describes the development of an experiment to study coherent population transfer between ground states, and between ground and Rydberg states, in ultracold atoms. In order to study coherent transfer between hyperfine ground states a pair of phase stable Raman beams is required. Both beams are derived from a single master laser before being spatially separated into individual components using a novel Faraday filtering technique. The frequency dependent Faraday effect in an isotopically pure thermal vapour is exploited to rotate the plane of polarisation of each Raman component such that they may be separated using a polarising beam splitter. The Raman beams are applied to a sample of ultracold atoms and evidence of coherent population transfer is observed. Rydberg states offer an ideal tool for electrometry; the electric field induced Rydberg energy level shift scales with the seventh power of the principle quantum number. Electromagnetically induced transparency (EIT) is used to map Rydberg energy level shifts onto a ground state transition. EIT in a thermal vapour cell also provides a novel method of stabilising the Rydberg coupling laser. The Rydberg energy level shift is highly sensitive to the electric field produced by adsorbates bonded to a nearby dielectric surface. These effects are found to be time dependent and can be eliminated if the electric field is applied transiently. The measured electric field is compared to that calculated by numerical solution of Laplace's equation; the bulk dielectric is found to have a strong effect on the local electric field experienced by the atoms. The exaggerated properties of Rydberg states make these systems ideal for quantum information processing and precision electrometry.
The subject of this thesis is the study of coherent interaction of light with matter (atoms) to improve the precision measurements and techniques. Special attention is drawn to get the narrow subnatural electromagnetically induced transmission (EIT), electromagnetically induced absorption (EIA) and nonlinear magneto-optic rotation (NMOR) caused by alkali atoms contained in a vapor cell. Subnatural polarization rotation introduces by a strong circularly polarized light in the absence of any external magnetic field was also studied. A detailed theoretical treatment, given in this dissertation, allows to associate each of the features of the spectra with a special physical mechanism. Many quantum phenomena related to interferences, coherences, optical pumping etc. experiments are studied using home-built diode lasers. This thesis also describes laser cooling and trapping of rubidium atoms using two techniques. Deflection of cold atoms horizontally from MOT using pushing beams are discussed in close consideration for the improvements in the precision measurements.
This thesis is organized as follows. In Chapter 1, an introduction to the importance of sub-natural narrow resonance and simplified technique in the precision measurement will be discussed.
In Chapter 2, an introduction to EIT, EIA and NMOR resonance are discussed. This chapter will provide a basic theoretical background of atom-field interactions, especially for Λ-type and N -type systems and its steady state solution using density matrix analysis and experimental tools. The most important notion of laser cooling of atoms, ions or molecule i.e., exchange of momentum between light and atoms combining with the Doppler effect will be discussed.
In Chapter 3, the observation of subnatural EIT and subnatural EIA in closed and open degenerate two-level system using room temperature vapor cell filled with Rb will be presented. Physical mechanisms that contribute to EIT and EIA, and the contrast of our results from the coherent population trapping (CPT)–type resonances are discussed in detail in appendix A.2 according to our experimental results.
In Chapter 4, the narrowing of subnatural EIT and subnatural EIA linewidth in closed and open systems again in degenerate two level transition, using the “Laguerre-Gaussian” control beam instead of generally studied Gaussian beam, will be discussed in detail.
In Chapter 5, the conversion between subnatural EIT to subnatural EIA in a degenerate Λ system will be discussed. The physical mechanism created by the introduction of a coherent counter propagating control laser to the co-propagating probe and the control laser are studied. The effect of polarization and axial velocity Doppler averaging will also be studied.
In Chapter 6, we will discuss the sensitive technique for precise measurement of small magnetic fields using the NMOR, by chopping the resonant laser beam. We will study the sensitivity and the potential application of this technique in the measurement of an atomic electric-dipole moment.
In Chapter 7, we will…
Electromagnetically Induced Transparency is an effect that allows light to propagate through an otherwise opaque material. Due to the dispersion properties of the EIT system, it also slows the information carried on the sideband frequencies of a light beam. The effect can also be used to store information in an atomic medium. It was therefore proposed that EIT systems can be applied in optical cicuits, specifically with regard to quantum information processing where it is necessary to delay, or store quantum states of light. Hence it is necessary to understand how EIT systems influence the quantum state of a light beam and whether EIT systems can ever have the potential to delay and store quantum information at the quantum noise limit. ¶ This thesis examines the transfer of an optical signal at the sideband frequencies through an EIT medium. A comprehensive characterisation of an EIT system in terms of delay and signal transmission is performed. This measurement technique provides data on EIT performance over a range of signal frequencies and a range of experimental parameters. Experiments were conducted with both pure Rubidium vapour and Rubidium vapour mixed with a noble gas. ¶ Studies have been made into a new system of measurement that will allow delay, attenuation and added noise spectra to be assessed via a single-shot measurement.
In this work, numerical calculations were implemented based on the diagonalisation of a matrix representation of the Stark Hamiltonian on a subset of the basis given by the zero-field Hamiltonian. Dipole matrix elements between the involved states were calculated, which allow for the prediction of transition strengths to Stark shifted Rydberg states. The applicability of the numerical methods were confirmed experimentally by precision spectroscopy in a rubidium vapour cell with electrodes using electromagnetically induced transparency (EIT). The numerical methods were then extended to predict ionisation rates of Rydberg states in the highly Stark shifted regime beyond the classical ionisation threshold. For this purpose, a complex absorbing potential (CAP) was specifically adjusted to the Stark Hamiltonian. An experiment was conducted, in which cold atoms from a magneto-optical trap (MOT) were excited to Stark shifted Rydberg states and the arising ions were detected. The obtained ionisation spectra agree well with the results from the numerical calculations. Another extension of the numerical methods was implemented for crossed electric and magnetic fields. This was used for approximate calculations of the motional Stark effect (MSE), which acts on atoms moving through static magnetic fields and thereby experiencing a Lorentz electric field. In a precision spectroscopy, once more using EIT in a vapour cell, the MSE was observed in low magnetic fields for the first time in rubidium. The spectral shift, which is caused by the MSE, matches the prediction from the numerical calculations. In a final experiment, the practical use of the knowledge of several properties of Stark shifted Rydberg states was illustrated. The numerical methods including the CAP were used to specifically select states for their optical accessibility, permanent electric dipole moments and ionisation rates. In the experiment, atoms from a MOT were excited to these states to observe different characteristics of the dipole blockade effect. The detection scheme for this experiment was realised by controlled ionisation using only small changes in the external electric field.