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  Two-level
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Linear Absorption & Beer-Lambert Law
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Weak probe pulses and continuous-wave fields in a two-level absorber.
Demonstrates Beer-Lambert attenuation, the role of spontaneous decay, and
how increasing optical depth reshapes the transmitted pulse.

.. toctree::
   :maxdepth: 1

   mbs-two-weak-pulse-few-atoms
   mbs-two-weak-pulse
   mbs-two-weak-pulse-more-atoms
   mbs-two-weak-pulse-decay
   mbs-two-weak-pulse-more-atoms-decay
   mbs-two-weak-cw-decay
   mbs-two-weak-cw-more-atoms-decay
   mbs-two-weak-square-decay

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Self-Induced Transparency & Optical Solitons
***************************************************

Strong resonant pulses whose area exceeds π can form area-conserving solitons
via self-induced transparency (McCall–Hahn). 2π solitons propagate
without loss; larger areas break up into multiple solitons.

.. toctree::
   :maxdepth: 1

   mbs-two-gaussian-0.8pi
   mbs-two-gaussian-1.8pi
   mbs-two-sech-2pi
   mbs-two-sech-4pi
   mbs-two-sech-6pi
   mbs-two-sech-2pi-collision

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Area Theorem: Odd-π Instability
*************************************

Odd multiples of π are unstable fixed points of the area theorem — a pulse
entering at 1π, 3π, or 5π reshapes as it propagates.

.. toctree::
   :maxdepth: 1

   mbs-two-sech-odd-pi

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Photon Echo & Coherence Time ($T_2$) Measurement
***************************************************

A π/2 pulse followed by a π pulse creates a photon echo at $t = +\tau$ in an
inhomogeneously broadened (Doppler-broadened) medium. The echo amplitude decays
as $e^{-2\tau/T_2}$, allowing the **homogeneous** coherence time $T_2$ to be
measured even when the lineshape is dominated by inhomogeneous broadening.

.. toctree::
   :maxdepth: 1

   mbs-two-photon-echo