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Journal of Modern Optics

Effect of quantum interference on a three-level atom driven by two laser fields

Uzma Akram, Z. Ficek, S. Swain

Published May 1, 2001• 17 citations
Publisher

Abstract

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling configurations of the lasers to the atom are analysed: (a) both lasers coupled to the same atomic transition, (b) each laser coupled to different atomic transition and (c) each laser coupled to both atomic transitions. Dressed states for the three coupling configurations are identified, and the population distribution and absorptive properties of the weaker field are interpreted in terms of transition dipole moments and transition frequencies among these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the population can be trapped in a superposition of the dressed states induced by quantum interference and the stronger field. We show, that the trapping of the population, which results from the cancellation of transition dipole moments, does not prevent the weaker field to be coupled to the cancelled (dark) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic transitions the population can be trapped in a linear superposition of the excited bare atomic states leaving the ground state unpopulated in the steady state. Moreover, we find that the absorption rate of the weaker field depends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong field is resonant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned halfway between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.

DOI
10.1080/09500340010020222

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We consider the effect of quantum interference on population distribution and photon statistics of a cavity field interacting with dressed states of a strongly driven three-level atom. We analyse three coupling configurations of the cavity field to the driven atom, with the cavity frequency tuned to the outer Rabi sideband, the inner Rabi sideband and the central frequency of the 'singly dressed' three-level atom. The quantum doubly dressed states for each configuration are identified and the population distribution and photon statistics are interpreted in terms of transitions among these dressed states and their populations. We find that the population distribution depends strongly on quantum interference and the cavity damping. For the cavity field tuned to the outer or inner Rabi sidebands the cavity damping induces transitions between the dressed states which are forbidden for the ordinary spontaneous emission. Moreover, we find that in the case of the cavity field coupled to the inner Rabi sideband the population distribution is almost Poissonian with a large average number of photons that can be controlled by quantum interference. This system can be considered as a one-atom dressed-state laser with controlled intensity.
We examine the absorption of a weak probe beam by a laser driven V-type atom with a pair of closely lying excited levels, where both the driving and probe lasers interact simultaneously with the two transitions. The effects of quantum interference among decay channels on the absorption spectra are also investigated. We introduce dipole moments in the dressed-state representation and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. In the degenerate case, features similar to that of a driven two-level atomic system are found due to some dark transitions in the spontaneous emission and the fact that the probe beam only detects certain transitions. In the nondegenerate case, the absorption spectrum is strongly influenced by the degree of quantum interference, resulting in different line shapes for emission peaks, absorption peaks, and dispersionlike profiles. The effect of probe polarization on the absorption spectrum is also investigated.
The steady-state populations and the absorption spectrum of a V-type three-level atom driven by a strong coherent field and a weak stochastic one simultaneously are investigated. The population inversion in bare-state and dressed-state bases can happen significantly due to the effect of the stochastic field. A relation between the level gap of both atomic excited states and the Rabi frequency is found to decide in which excited state the population is larger than in another one for the same frequency detuning δ between the coherent and the stochastic fields. The line profiles of the absorption spectrum are strongly dependent on the frequency detuning δ. Different lineshapes such as a sharp emission peak (gain), an absorption peak or a dispersion-like peak can be exhibited at the line centre for different δ. Asymmetrical spectral features and line narrowing can be reached by adjusting δ and the Rabi frequency. Probe gain in sidebands can occur due to the unbalanced population distribution in dressed states.
This thesis examines the effect of quantum interference occuring in a three-level atom in the V-configuration, on a variety of optical effects such as controlled spontaneous emission, population trapping, lasing without population inversion, controlled field intensity and photon statistics. Calculations are carried out numerically via the master equation approach and analytically using dressed atom analysis. Various atom-laser coupling configurations are investigated and dressed states identified in each case. The system is shown to exhibit trapping states for a variety of atom-laser coupling configurations. Moreover, the thesis focuses on the effect of quantum interference on a cavity QED system. Three different cavity detunings with respect to the entangled atom-laser system are considered. It is shown in each case how the singly dressed entangled system interacts with the cavity field to form entangled doubly dressed structures. Again, the system is shown to exhibit population trapping states with dependence on the cavity damping rate in one of the detunings considered. In addition, photon statistics of the cavity field are calculated and shown to be sensitive to the degree of quantum interference. Interestingly, controlled field intensity and sub-Poissonian statistics are obtained for certain parameters of the two off-resonance cases considered
We consider a three-level V-type atomic system with the ground state coupled by a laser field to only one of the excited states, and with the two excited states coupled together by a dc field. Although the dipole moments of the two dipole-allowed transitions are assumed perpendicular, we demonstrate that this system emulates to a large degree a three-level system with parallel dipole moments---the latter being a system that exhibits quantum interference and displays a number of interesting features. As examples, we show that the system can produce extremely large values for the intensity-intensity correlation function, and that its resonance fluorescence spectrum can display ultranarrow lines. The dressed states for this system are identified, and the spectral features are interpreted in terms of transitions among these dressed states. We also show that this system is capable of exhibiting considerable squeezing.
We investigate quantum interference and classical-interference effects when three-level atoms in the ``$\ensuremath{\Lambda}$'' or ``V'' configuration interact with both a cavity-field mode and an external driving field mode. We describe electromagnetically induced transparency ($\ensuremath{\Lambda}$ configuration), as well as a ``field-induced transparency'' (V configuration), and we go beyond the weak cavity-field studies of previous papers by utilizing a Heisenberg-picture density matrix approach. We find that under certain circumstances the cavity field evolves to be equal in magnitude to, but $180\ifmmode^\circ\else\textdegree\fi{}$ out of phase with, the external pump field when the pump-field frequency equals the cavity frequency, a surprising effect found in two-level atoms in a cavity excited by an external pump field that persists in three-level atoms. The better the cavity, the quicker this buildup occurs. When the cavity field reaches this ``out-of-phase'' condition, the resonance fluorescence from the atom in the cavity goes to zero. This is a purely classical-interference effect between the two out-of-phase fields, with the resonance fluorescence going to zero at the same time as the two excited state populations go to zero. This is quite different from the quantum interference that occurs under the right circumstances when the state populations are coherently driven into a linear combination that is decoupled from any applied field and population is trapped in the excited states (``coherent population trapping''). When the cavity frequency is tuned to the frequency at which quantum interference would occur, the two interference effects compete with each other. The cavity field begins to evolve toward the $180\ifmmode^\circ\else\textdegree\fi{}$ out-of-phase condition, thereby approaching a classical destructive interference condition with the pump field. Then at some point before 100% classical interference occurs, depending on the cavity damping rate and atom/cavity coupling strength, the atom builds up its coherence between state populations to produce the quantum interference effect. At that point, the atom becomes transparent and resonance fluorescence disappears, including fluorescence into the cavity, thereby causing the cavity field to die away.