In quantum mechanics, einselections, short for "environment-induced superselection", is a name coined by Wojciech H. Zurek[1] for a process which is claimed to explain the appearance of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions.
In this approach, classicality is described as an emergent property induced in open quantum systems by their environments.
Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable due to entangling interaction with the environment, which in effect monitors selected observables of the system.
The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglement and superposition.
Advocates of this approach argue that since only quasi-local, essentially classical states survive the decoherence process, einselection can in many ways explain the emergence of a (seemingly) classical reality in a fundamentally quantum universe (at least to local observers).
[3] So the question of whether the 'einselection' account can really explain the phenomenon of wave function collapse remains unsettled.
",[1] Einselected pointer states are distinguished by their ability to persist in spite of the environmental monitoring and therefore are the ones in which quantum open systems are observed.
[5][6] This is the "predictability sieve" criterion, based on an intuitive idea: Pointer states can be defined as the ones which become minimally entangled with the environment in the course of their evolution.
The predictability sieve criterion is a way to quantify this idea by using the following algorithmic procedure: For every initial pure state
The nature of pointer states has been investigated using the predictability sieve criterion only for a limited number of examples.
[5][6][7] Apart from the already mentioned case of the measurement situation (where pointer states are simply eigenstates of the interaction Hamiltonian) the most notable example is that of a quantum Brownian particle coupled through its position with a bath of independent harmonic oscillators.
In such case pointer states are localized in phase space, even though the interaction Hamiltonian involves the position of the particle.
[8] There has been significant work on correctly identifying the pointer states in the case of a massive particle decohered by collisions with a fluid environment, often known as collisional decoherence.
In particular, Busse and Hornberger have identified certain solitonic wavepackets as being unusually stable in the presence of such decoherence.