How does decoherence take place ?

This is a very fundamental and tantamount question. I remember, in a letter to Pauli, Einstein questioned the superposition principle of quantum mechanics, asking why a bullet is always there instead of everywhere. This is an old example of the question posed as the title. Although, it is now accepted by many authors that, the answer should be closely related to the concept of decoherence, it is not clear how this happens and if it is the case with every system.

I want to mention some other examples:

• In statistical mechanics, it is assumed that, the average of any observable should be taken over all thermodynamically accessible energy eigenstates with corresponding Boltzmann weights. This implies that, all the interferences that might occur during unitary evolution have been set aside. Usually, the physical system under interest is bulk and immersed in a heat bath, this assumption should work well. Nonetheless, violations may arise as long as the interference time becomes discernible. This situation is comparable to what is happening to light interference. For natural light, the polarization has no significant effect in interference experiments, which, however, is not so with a laser. It is quite evident that, such decoherence should be ascribed to interaction with heat bath, which represents a stochastic source. A general assertion regarding relations between system size, temperature and coherence time is lacking.
• The measurement theory has been debated since the discovery of quantum mechanics. How does a measurement lead to wave function collapse ? Does a measurement necessarily involve a classical object ? Or does a measurement actually involve decoherence ?
  
• The third is usually named 'Hund Paradox', which
  

  concerns how to explain from first principles why molecules often appear as enantiomers, i.e., either in a left-handed configuration or as in the right-handed image

  That is, the mixing of these two configurations disappears, contrary to one's expectation based on parity symmetry.

The last question was recently carefully addressed in Ref.[1], where the authors made use of molecular scattering theory and master equation to investigate the simplest molecule D2S2. They concluded that, the dominant collisional decoherence is due to a parity sensitive higher-order dispersive interaction term that is usually dropped in dealing with thermodynamic properties. They also made predictions on the conditions for experimental stabilization of enantiomers.

[1]PRL, 103:023202(2009)