Linear absorption and dispersion properties, as well as Kerr nonlinear index, of three-level atomic systems can be greatly modified under the condition of electromagnetically induced transparency (EIT). By placing such EIT atoms in a vapor cell inside an optical ring cavity, the cavity transmission spectrum can be altered and controlled. We show that the cavity transmission linewidth can be narrowed substantially comparing to the empty cavity linewidth due to the sharp normal (linear) dispersion associated with the EIT resonance (which can be considered as photons traveling with a slower speed inside the optical cavity). On the other hand, the Kerr nonlinear dispersion has the opposite slope comparing to the linear dispersion near the EIT resonance, which can be used to balance the linear dispersion and lead to total anomalous dispersion for the intracavity atomic medium (with ``superluminal photon speed'' inside the optical cavity). Such anomalous dispersion in the intracavity medium makes the cavity transmission linewidth broader than the empty cavity linewidth. Under certain parameters, the so called ``white-light cavity'' condition can be satisfied, which makes the cavity transmission linewidth very broad and, at the same time, have high cavity transmission. Such modified and controlled cavity transmission linewidths can have many interesting applications in frequency locking, cavity ring-down spectroscopy, nonlinear optical spectroscopy, cavity-QED, and even recycling cavity of the gravitational-wave detector.

Atomic coherence and interference manifested by electromagentically induced transparency (EIT) and coherent population trapping (CPT) plays an important role in the current studies of atom-photon interactions and has found numerous applications in optical physics. EIT is created in a three-level atomic system by a coupling field and results in destructive interference between two excitation paths of a weak probe laser interacting with the atomic medium. This leads to suppressed linear absorption and rapidly varying atomic dispersion for the probe laser near the atomic resonance, which provides the platform for a variety of applications such as nonlinear optics at low light levels, slow light manipulation, and quantum state engineering for photons and atoms.

Here we extend the simple three-level EIT system to more complicated and versatile configurations in a multi-level atomic system coupled by multiple laser fields. We show that with multiple excitation paths provided by different laser fields, phase-dependent quantum interference is induced: either constructive or destructive interference can be realized by varying the relative phases among the laser fields. Two specific examples are discussed. One is a three-level system coupled by bichromatic coupling and probe fields, in which the phase dependent interference between the resonant two-photon Raman transitions can be initiated and controlled. Another is a four-level system coupled by two coupling fields and two probe fields, in which a double-EIT configuration is created by the phase-dependent interference between three-photon and one-photon excitation processes. We analyze the coherently coupled multi-level atomic system and discuss the control parameters for the onset of constructive or destructive quantum interference. We describe two experiments performed with cold Rb atoms that can be approximately treated as the coherently coupled three-level and four-level atomic systems respectively. The experimental results show the phase-dependent quantum coherence and interference in the multi-level Rb atomic system, and agree with the theoretical calculations based on the coherently coupled three-level or four-level model system.

In this chapter, we give a brief review of our recent research work related to atomic localization via the effects of atomic coherence and quantum interferences, as well as its application in atom nano-lithograph, which includes atomic localization based on double-dark resonance effects, sub-half-wavelength localization via two standing-wave fields and an atom nano-lithograph scheme via two orthogonal standing-wave fields, etc..

We present a collection of our recent schemes that have been proposed for continuous variable entanglement. Included in this collection are two types of optical systems. One is the four-wave mixing in resonantly or near-resonantly driven atomic systems. The numerical results and physical analyses are presented by using dressed atomic states and squeeze transformed cavity modes. Two dissipation channels are identified, through which the dressed atoms simultaneously absorb in the excitations from the pair of squeeze transformed modes. It is in the presence of such two channels that the entanglement is greatly enhanced and the best achievable state is the original Einstein-Podolsky-Rosen (EPR) entangled state. This scheme is applicable in the optical regime where atomic spontaneous emission has to be taken into account, unlike the two-step atomic reservoir engineering scheme, which is limited to the microwave regime. The other kind of systems is the quantum-beat laser with incoherent pump or coherent driving. Such a laser, when it operates well above threshold, produces entangled light. The numerical results and physical analyses are presented by using the collective modes of the lasing fields. The relative mode is decoupled from the active medium and thus remains in its vacuum state, while the sum mode operates well above threshold and displays sub-shot noise. The quantum-beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of respective beams.

In this chapter, we give a brief review of our recent research works on photonic bandgaps induced by standing-wave (SW) coupling fields in the regime of electromagnetically induced transparency (EIT). EIT refers to the absorption suppression or elimination of a weak probe going through an atomic ensemble at the presence of a strong coupling, which is a result of laser induced destructive quantum interference. On the other hand, a light signal cannot freely propagate in the media with periodic refractive indices (the so-called photonic crystals) if its carrier frequencies fall inside the photonic bandgaps. Utilizing a SW coupling to attain the periodically modulated refractive index with little absorption, one can establish an induced photonic bandgap with its width and position dynamically tunable. The potential media may be either cold atomic ensembles or solid materials exhibiting defect states, such as Pr3+: Y2SiO5 and diamond containing N-V color centers. We first consider the steady optical responses of the atomic and solid media with dynamically induced bandgaps to a time- independent probe by focusing on the dispersion curves of Bloch wave vectors and the spectra of reflection and transmission. Then, we examine the propagation dynamics of a probe pulse through a cold atomic sample in two different situations where a dynamically induced bandgap exists or not. We find that the SW coupling field can be easily modulated to mold the traveling-light flow and to control the stationary-light generation with quite high flexibility, and thus may have applications in the fields of classical and quantum information processing of optical signals. In performing theoretical simulations, we also demonstrated several different but almost equivalent mathematical methods, i.e. the two-mode approximation method, the transfer-matrix method, and the Maxwell-Liouville equation method, to deal with the problems of SW-EIT. Comparison of these methods is quite helpful to understand the underlying physics of the formation of dynamically induced photonic bandgaps.

Atomic coherence is the interaction process between light and atoms, that is one or more coherent fields couple the different atomic states and cause the quantum interference between the different transition channels. Atomic coherence has led to many interesting and unexpected consequences, such as the Hanle effects, electromagnetically induced transparency (EIT), coherence population trapping (CPT), stimulated Raman adiabatic passage (STIRAP), spontaneous emission control, resonant enhancement of optical nonlinearity, slow and superluminal light propagation, quantum light storage, etc. In which, light storage based on atomic coherence plays an important role in the coherent control of light pulse information. In this chapter, we present some previous works on atomic coherence and optical storage. Firstly, we present the storage and recovery of light pulse based on F-STIRAP, which is fundamentally different from the conventional EIT-based process. Secondly, we present the applications of light storage based on EIT in a Pr3+:Y2SiO5 crystal, which includes the erasure of stored optical information, all-optical routing by light storage, and the coherent control of double light pulses. At last, we propose theoretically and demonstrate experimentally a method to control the atomic coherence by a STIRAP or F-STIRAP process.

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