Is emission/absorption of a photon lossy? I recall vaguely that energy is absorbed/radiated in packets called quanta. Quanta were what are now known as photons. 
What I'm curious about - Is absorption/radiation vis-a-vis photon lossy? Do the total number of photons exactly match the energy acquired/released?
 A: It is not a lossy process.  For there to be a loss at one place would require there to be a gain somewhere else.  When the atom releases the photon there would be recoil of the atom that would contain some energy, but 100% of the energy lost by the atom would be gained by the photon.
A: Absorption is often lossy.  A photons energy is inversely proportional to wavelength, if that energy is larger than the bandgap of the material absorbing the photon then the excess energy goes into heat.  In Silicon, as an example, if a green photon is absorbed and if the electron hole pair is allowed to recombine, there is a photon emitted that has a wavelength that corresponds to the bandgap of Si of 1.12 eV.
Emission is not lossy.
A: The emission and absorption can be lossy, but it does not have to be. 
While in the overall picture energy is conserved other processes can either increase or decrease the photon energy. In practice this is used for cooling atoms with a laser beam. This doppler cooling uses photons to reduce the speed of gas atoms, the incoming photon energy is split between the kinetic energy of the atom to slow it down, the remaining energy is emitted as another photon with less energy. 
As a side note: in the future we might be able to cool larger objects with this method, as the photons can be used to 'destroy' phonons and therefore cool a solid transparent material. 
A: Photon number does not have to be conserved in light-matter interaction processes. For example, take the light storage experiments in hot/cold atomic vapor, where a pulse of light is converted to atomic coherence (definite phase relationship between the ground states of the atomic system). Dissipation in the form of phonons is also possible.
Ultra-Slow Light and Enhanced Nonlinear Optical Effects in a Coherently Driven Hot Atomic Gas
