We propose a method to study and characterize the spatial and temporal properties of degenerate photon pairs emitted in SPDC, using a filtering system combined with temperature variation of the nonlinear crystal. The photons can be distinguished. We relate these to the measured Hong-Ou-Mandel interference dip of the photons, measured in a parallel experiment. The theoretical plots match very well with the experimental results.
In a Λ-type atomic system with electromagnetically-induced transparency, we probe the fidelity of the storage
and retrieval of an electromagnetic signal, as the control laser field is varied with time. We study numerically the
adiabatic transfer problem for an isolated atom, and show that for a weak signal, even the slowest variations of
the control field take the system out of the dark state, which is a coherent superposition of the two lower levels
of the Λ system. Following this, we incorporate the effect of dissipation on system dynamics by allowing for
spontaneous decay of the system to the lower levels in a wavefunction approach. We conclude that dissipation
definitely aids the retrieval process but not so much the storage. Further, for storing the signal, the control field
should be switched off as slowly as possible, but while retrieving it, the faster we switch on the control field, the
better the signal is retrieved. Also, for a given system there is an optimal control power for the best retrieval.
Our results find partial support in the reported experimental observations.
We present a realistic theoretical treatment of a three-level Λ system in a hot atomic vapor interacting with a
coupling and a probe field of arbitrary strengths, leading to electromagnetically-induced transparency and slow
light under the two-photon resonance condition. We take into account all the relevant decoherence processes
including collisions. Velocity-changing collisions (VCCs) are modeled in the strong collision limit effectively,
which helps in achieving optical pumping by the coupling beam across the entire Doppler profile. We take into
account a dynamic rate of influx of atoms in the two lower levels of the Λ, and an outflux from all the three
levels. The steady-state expressions for the atomic density-matrix elements are numerically evaluated to yield the
experimentally measured response characteristics. Our predictions are in excellent agreement with the reported
experimental results for <sup>4</sup>He*. The role played by the VCC parameter is seen to be distinct from that by the
transit time or Raman coherence decay rate.