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In a three-level closed Λ system with incoherent pumping, both the dynamically induced coherence (DIC) and the spontaneously generated coherence (SGC) play a significant role in inducing different nonlinear processes like amplification without population inversion (AWI), electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA). These two coherences can interfere destructively and constructively giving rise to different nonlinear processes (EIT, EIA, AWI) when the strength of these two coherences is comparable. Therefore the system can switch over from one nonlinear process to the other and the desired response of the system can be achieved by controlling the system parameters. It has been shown that this switching can occur by changing the relative strength of Rabi frequencies for the coherent and probe fields and the mode of incoherent pumping (unidirectional and bidirectional) for a chosen SGC parameter. Further control over the response can be achieved by changing the phase between two radiation fields. But the dependence on phase is different when both the coherences contribute than that obtained only from SGC. Exact analytical expressions for the coherences and populations in the steady state limit have been derived (keeping all orders of system parameters) to generalize the analysis and any restrictions over the system parameters (e.g. spontaneous decay widths on probe and coherent transitions are equal) have been avoided to make it applicable to various atomic and molecular systems. However the approximate expressions can be derived from these exact values. Numerical values obtained by solving the density matrix equations agreed well with these exact analytical values.

The light propagation of a probe field in a three-level Λ system with incoherent pumping has been studied when both dynamically induced coherence (DIC) and spontaneously generated coherence (SGC) play a significant role. We have investigated the group velocity of probe field and hence the group index of a three-level Λ system with incoherent pumping when both DIC and SGC play a significant role. We have shown that by varying the probe field Rabi frequency one can control the interference between these two coherences which leads to different nonlinear response (amplification without inversion, electromagnetically induced transparency and electromagnetically induced absorption) leading to different (positive and negative) dispersion. Hence control over switching of group velocity from subluminal to superluminal and vice versa can be achieved. We have also shown that when the contributions from both the coherences are comparable, the dependence of group velocity of probe field in a three-level Λ system with incoherent pumping on phase difference between probe and coherent fields is different from that obtained under the weak probe field condition. Going beyond the weak probe field approximation we have derived analytical expressions for group velocity and hence the group index in the steady state limit (keeping all orders of system parameters) to generalize the analysis, and these expressions can be used for any set of system parameters without any restriction. The numerical values obtained by solving the density matrix equations agree well with these exact analytical values at a large time limit. We have proposed a scheme for experimental realization of EIT and hence subluminal light propagation in molecules by invoking spontaneously generated coherence.

A recent experiment with squeezed light has shown that two-photon absorption by an atom can occur with a linear intensity dependence. We point out that this result verifies a prediction made by us a decade back from an analysis of a non-local model of Quantum Electrodynamics. This model had earlier been proposed by us in an ad hoc manner to interpret certain features of multiphoton double ionisation and above-threshold ionisation in an atom placed in a strong laser field ; in this paper we show that the model can be obtained field-theoretically by demanding covariance of the field Lagrangian under a nonlocal U(1) gauge transformation. We also show that the model makes direct contact with squeezed light, and thus allows us to describe these two completely different scenarios from a unified point of view. We obtain a fundamentally new result from our non-local model of QED, namely that only the past, but not the future, can influence the present -thus establishing a non-thermodynamic arrow of time. We also show that correlations within a quantum system should necessarily be of the Einstein-Podolsky-Rosen (EPR)-type, a result that agrees with Bell's theorem. These results follow from the simple requirement of energy conservation in matter-radiation interaction. Furthermore, we also predict new and experimentally verifiable results on the basis of our model QED.

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