In the last few decades, coherent light-atom interactions have opened unprecedented possibilities for the coherent control of atomic and optical quantum systems, paved the way for the practical realization of quantum information technologies, and allowed for the creation of novel quantum-enhanced sensors. This dissertation investigates the interaction of multiple near-resonant optical fields with hot rubidium atoms under the conditions of electromagnetically induced transparency. The main goal of the presented research is to address some fundamental challenges in using such systems for practical applications. The EIT effect relies on the strong coupling of an optical probe field and a collective long-lived ensemble of atomic spins by the means of a strong classical optical control field in a Lambda configuration. While optically-thick atomic vapor is necessary to achieve such a strong coupling regime, the increasing optical depth of the atomic ensemble also leads to the effective enhancement of other nonlinear light-atom interactions, such as the four-wave mixing effect. Here we discuss the possibility to control four-wave mixing in a three-level system without deteriorating the coherent properties of EIT by introducing an additional absorber resonant exclusively with the Stokes field. The exclusive detection of a weak probe field in the presence of a strong control field is a challenging experimental task, especially at the few-photon level. Many experiments employ polarization and/or frequency filtering to compete the task. We present an alternative filtering technique based on optical vortices for cases when the traditional methods are not sufficient or restrict the experimental arrangements. Finally, we demonstrate the possibility to manipulate the group velocity of a pulsed squeezed vacuum field by using the optical dispersion modification via Zeeman spin coherence in rubidium atoms. By changing the interaction condition, we demonstrate the switch between the ``slow'' and (for the first time) ``fast'' light regime. We also show that increased optical depth simultaneously leads to the enhancement of pulse advancement and the deterioration of squeezing fidelity in the output pulses.
In the last few decades, coherent light-atom interactions have opened unprecedented possibilities for the coherent control of atomic and optical quantum systems, paved the way for the practical realization of quantum information technologies, and allowed for the creation of novel quantum-enhanced sensors. This dissertation investigates the interaction of multiple near-resonant optical fields with hot rubidium atoms under the conditions of electromagnetically induced transparency. The main goal of the presented research is to address some fundamental challenges in using such systems for practical applications. The EIT effect relies on the strong coupling of an optical probe field and a collective long-lived ensemble of atomic spins by the means of a strong classical optical control field in a Lambda configuration. While optically-thick atomic vapor is necessary to achieve such a strong coupling regime, the increasing optical depth of the atomic ensemble also leads to the effective enhancement of other nonlinear light-atom interactions, such as the four-wave mixing effect. Here we discuss the possibility to control four-wave mixing in a three-level system without deteriorating the coherent properties of EIT by introducing an additional absorber resonant exclusively with the Stokes field. The exclusive detection of a weak probe field in the presence of a strong control field is a challenging experimental task, especially at the few-photon level. Many experiments employ polarization and/or frequency filtering to compete the task. We present an alternative filtering technique based on optical vortices for cases when the traditional methods are not sufficient or restrict the experimental arrangements. Finally, we demonstrate the possibility to manipulate the group velocity of a pulsed squeezed vacuum field by using the optical dispersion modification via Zeeman spin coherence in rubidium atoms. By changing the interaction condition, we demonstrate the switch between the slow'' and (for the first time) fast'' light regime. We also show that increased optical depth simultaneously leads to the enhancement of pulse advancement and the deterioration of squeezing fidelity in the output pulses.
This dissertation reports experimental studies of nonlinear optical effects manifested by electromagnetically induced transparency (EIT) in cold Rb atoms. The cold Rb atoms are confined in a magneto-optic trap (MOT) obtained with the standard laser cooling and trapping technique. Because of the near zero Doppler shift and a high phase density, the cold Rb sample is well suited for studies of atomic coherence and interference and related applications, and the experiments can be compared quantitatively with theoretical calculations. It is shown that with EIT induced in the multi-level Rb system by laser fields, the linear absorption is suppressed and the nonlinear susceptibility is enhanced, which enables studies of nonlinear optics in the cold atoms with slow photons and at low light intensities. Three independent experiments are described and the experimental results are presented. First, an experimental method that can produce simultaneously co-propagating slow and fast light pulses is discussed and the experimental demonstration is reported. Second, it is shown that in a three-level Rb system coupled by multi-color laser fields, the multi-channel two-photon Raman transitions can be manipulated by the relative phase and frequency of a control laser field. Third, a scheme for all-optical switching near single photon levels is developed. The scheme is based on the phase-dependent multi-photon interference in a coherently coupled four-level system. The phase dependent multi-photon interference is observed and switching of a single light pulse by a control pulse containing ∼20 photons is demonstrated. These experimental studies reveal new phenomena manifested by quantum coherence and interference in cold atoms, contribute to the advancement of fundamental quantum optics and nonlinear optics at ultra-low light intensities, and may lead to the development of new techniques to control quantum states of atoms and photons, which will be useful for applications in quantum measurements and quantum photonic devices.
We investigate weak optical probe pulse propagation in a resonant Lambda and N-interaction schemes, and investigate the role of the four-wave mixing on classical and quantum properties of the probe field. In particular, we focus our attention on two configurations. In the first case we take into account the off-resonant coupling of the strong field to the signal field ground state. Such configuration is relevant for EIT-based slow light and quantum memory. In the second configuration the additional control field is derived from an independent laser, and it is tuned to a different optical resonance from the ones forming an original Lambda system. Such interaction scheme allows realization of tunable slow and fast light, and was considered with regards to enhancement of optical gyroscopes performance. We demonstrate that in both cases the four-wave mixing (FWM) has a profound effect on signal field group velocity and absorption profile, and may even lead to gain. We present both semi-classical and fully quantum treatments for propagation of both signal and newly generated Stokes fields that include accurate description of their quantum noise. In particular, we analyze the case of a quadrature-squeezed signal field, and demonstrate that vacuum fluctuations of the Stokes field couple into the signal field through the FWM process and degrades the squeezing. The severity of this degradation grows with optical depths of an atomic medium, setting an additional practical limits for the experiments.
Quantum information systems require an interface between photons as a physical system to transport information and atoms which are better suited to store and process it. In this thesis the construction of a novel pulsed, bright and narrow-band light source is presented. It consists of an optical parametric oscillator featuring a periodically poled KTP crystal and a mechanical chopper and produces ?s-pulses of squeezed vacuum. The generated states are resonant to the Rubidium-DI-transition, are almost transform-limited and show more than 3 dB of squeezing. The physical effect of electromagnetically induced transparency (EIT) in hot rubidium vapor can be used to generate ultra-low group velocities and allows for a reversible adiabatic conversion of optical quantum states into collective spin excitations which can be used to stop pulses of light. I report about recent progress in slowing down a pulse of squeezed vacuum by one third of its width while still preserving 0.36 dB of squeezing in the process. I analyze electromagnetically induced transparency and light storage in an ensemble of atoms with multiple excited levels (multi-A configuration) which are coupled to one of the ground states by quantized signal fields and to the other one via classical control fields. A basis transformation of atomic and optical states which reduces the analysis of the system to that of electromagnetically induced transparency in a regular three-level configuration is presented. We demonstrate the existence of dark state polaritons and propose a protocol to transfer quantum information from one optical mode to another by adiabatic control of the control fields and present a proof-of principle experiment to adiabatically transfer light between modes of different frequencies in Rb-87 vapor. To that end a digital phase lock circuit has been designed that allows to electronically phase lock an external-cavity diode laser to a reference laser over a 7 GHz frequency range with sub-Hertz precision.
Summary form only given. The study on the interaction between electromagnetic fields and multi-level atoms or molecules has discovered many interesting effects including electromagnetically induced transparency (EIT) enhanced dispersion with reduced absorption, and lasing without inversion. Recent interests include ultraslow group velocity of light, and superluminal light propagation. Atomic coherence created between different atomic levels plays an essential rule in these experiments. On the other hand, quantum features in the interaction between electromagnetic fields and three-level systems based on atomic coherence have attracted extensive theoretical interests. These include generation of amplitude-squeezed and quantum noise-correlated states of light with EIT, entanglement of atomic states with light fields, and quantum memory for light. According to these theoretical works, manipulation of quantum states of either light fields or atoms via atomic coherence is possible. We study quantum state manipulation of light via coherently driven Raman coherence in a /spl Lambda/-type three-level system of rubidium vapor under far off one-photon resonance condition both theoretically and experimentally.
The recent prospect of efficient, reliable, and secure quantum communication relies on the ability to coherently and reversibly map nonclassical states of light onto long-lived atomic states. A promising technique that accomplishes this employs Electromagnetically Induced Transparency (EIT), in which a strong classical control field modifies the optical properties of a weak signal field in such a way that a previously opaque medium becomes transparent to the signal field. The accompanying steep dispersion in the index of refraction allows for pulses of light to be decelerated, then stored as an atomic excitation, and later retrieved as a photonic mode. This dissertation presents the results of investigations into methods for optimizing the memory efficiency of this process in an ensemble of hot Rb atoms. We have experimentally demonstrated the effectiveness of two protocols for yielding the best memory efficiency possible at a given atomic density. Improving memory efficiency requires operation at higher optical depths, where undesired effects such as four-wave mixing (FWM) become enhanced and can spontaneously produce a new optical mode (Stokes field). We present the results of experimental and theoretical investigations of the FWM-EIT interaction under continuous-wave (cw), slow light, and stored light conditions. In particular, we provide evidence that indicates that while a Stokes field is generated upon retrieval of the signal field, any information originally encoded in a seeded Stokes field is not independently preserved during the storage process. We present a simple model that describes the propagation dynamics and provides an intuitive description of the EIT-FWM process.
In this thesis we study light-matter coherence phenomena related to the interaction of a coherent laser field and the so-called Λ-system, a three-level quantum system (e.g., an atom). We observe electromagnetically induced transparency (EIT), slow and stored light in hot rubidium vapor. For example, a 6 μs Gaussian pulse propagate at a velocity of ~1 km/s (to be compared with the normal velocity of 300 000 km/s). Dynamic changes of the control parameter allows us to slow down a pulse to a complete stop, store it for ~100 μs, and then release it. During the storage time, and also during the release process, some properties of the light pulse can be changed, e.g., frequency chirping of the pulse is obtained by means of Zeeman shifting the energy levels of the Λ-system. If, bichromatic continuous light fields are applied we observe overtone generation in the beating signal, and a narrow `dip' in overtone generation efficiency on two-photon resonance, narrower than the `coherent population trapping' transparency. The observed light-matter coherence phenomena are explained theoretically from first principles, using the Lindblad master equation, in conjunction with the Maxwell's equations. Furthermore, we analyze an optical delay-line based on EIT and show that there is in principle (besides decoherence) no fundamental limitation, but the usefulness today is scant. The combination of EIT and a photonic crystal cavity is inquired into, and we show that the quality value of a small resonator (area of 2.5λ×2.5λ with a missing central rod) can be enhanced by a factor of 500 due to the increased modal density close to two-photon resonance. Open system effects (decoherence effects) are thoroughly investigated using a coherence vector formalism, furthermore, a vector form of the Lindblad equation is derived. Specifically we find an open system channel that lead to slow light and gain.