Einselectionis short for environment - induced superselection, a nickname coined by Wojciech H. Zurek. Wojciech H. Zurek, Decoherence, einselection, and the quantum origins of the classical, "Reviews of Modern Physics" 2003, 75, 715 or [http://arxiv.org/abs/quant-ph/0105127] ] Classicality is an emergent property induced in open quantum systems by their environments. Due to the interactionwith the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable to entangling interaction with the environment, which in effect monitors selected observables of the system. After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale Wojciech H. Zurek, 1984, Reduction of the Wavepacket: How Long Does it Take? [http://arxiv.org/abs/quant-ph/0302044v1] ] , a generic quantum state decaysinto a mixture of pointer states. In this way the environment induces effective superselection rules. Thus, einselection precludes stable existence of superpositions of pointer states. These 'pointer states' are stable despite environmental interaction, which explains the emergence of a preferred basis in quantum measurement. The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglementand superposition.
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).
Zurek has defined einselection as follows "Decoherence leads to einselection when the states of the environment corresponding to different pointer states become orthogonal:"Equation 4:19] ,
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. Understanding the nature of these states and the process of their dynamical selection is of fundamental importance. This process has been studied first in a measurement situation: When the system is an apparatus whose intrinsic dynamics can be neglected, pointer states turn out to be
eigenstatesof the interaction Hamiltonian between the apparatus and its environment Wojciech H. Zurek, 1981, Phys. Rev. D24, 1516; ibid D26, 1862] . In more general situations, when the system's dynamics is relevant, einselection is more complicated. Pointer states result from the interplay between self--evolution and environmental monitoring.
To study einselection, an operational definition of pointer states has been introduced
Wojciech H. Zurek, 1993, Progr. Theor. Phys. 89, 281–312] Zurek, W. H., Habib, S., and Paz, J. P, 1993, Phys. Rev.Lett. 70, 1187] . 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 the evolution. The predictability sieve criterion is a way to quantify this idea by using the following algorithmic procedure: For every initial pure state , one measures the entanglement generated dynamically between the system and the environment by computing the entropy::: or some other measure of predictability Wojciech H. Zurek, 1993, Progr. Theor. Phys. 89, 281–312] Zurek, W. H., Habib, S., and Paz, J. P, 1993, Phys. Rev. Lett. 70, 1187] Tegmark, M., and Shapiro, H. S., 1994, Phys. Rev. E50, 2538–2547. Gallis, M. R., 1996, Phys. Rev. A53, 655–660. J. R. Anglin and W. H. Zurek, 1996 Phys. Rev. D53 7327; Barnett, S. M., Burnett, K., and Vacarro, J. A., 1996, J. of Res. NIST 101, 593–600; H. M. Wiseman and J. A. Vaccaro or [http://arxiv.org/abs/quant-ph/9709014v1] ] from the reduced density matrixof the system (which is initially ). The entropy is a function of time and a functional of the initial state . Pointer states are obtained by minimizing over and demanding that the answer be robust when varying the time .
The nature of pointer states has been investigated using the predictability sieve criterion only for a limited number of examples
Wojciech H. Zurek, 1993, Progr. Theor. Phys. 89, 281–312] Zurek, W. H., Habib, S., and Paz, J. P, 1993, Phys. Rev. Lett. 70, 1187] Tegmark, M., and Shapiro, H. S., 1994, Phys. Rev. E50, 2538–2547. Gallis, M. R., 1996, Phys. Rev. A53, 655–660. J. R. Anglin and W. H. Zurek, 1996 Phys. Rev. D53 7327; Barnett, S. M., Burnett, K., and Vacarro, J. A., 1996, J. of Res. NIST 101, 593–600; H. M. Wiseman and J. A. Vaccaro or [http://arxiv.org/abs/quant-ph/9709014v1] ] . 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 Brownianparticle 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 particleZurek, W. H., Habib, S., and Paz, J. P, 1993, Phys. Rev. Lett. 70, 1187] . Pointer states are the result of the interplay between self--evolution and interaction with the environment and turn out to be coherent states. There is also a quantum limit of decoherence: When the spacing between energy levelsof the system is large compared to the frequenciespresent in the environment, energy eigenstates are einselected nearly independently of the nature of the system-environment couplingJuan Pablo Paz and Wojciech H. Zurek, Quantum limit of decoherence: Environment induced superselection of energy eigenstates, Phys.Rev.Lett. 82 1999, 5181-5185 or [http://arXiv:quant-ph/9811026v1] ] .
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