Decoherence, einselection, and the quantum origins of the classical

Abstract

Decoherence is caused by the interaction with the environment which in effect monitors certain observables of the system, destroying coherence between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselected pointer states are stable. They can retain correlations with the rest of the Universe in spite of the environment. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly nonlocal "Schrödinger cat" states. Classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit: Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation. Only the preferred pointer observable of the apparatus can store information that has predictive power. When the measured quantum system is microscopic and isolated, this restriction on the predictive utility of its correlations with the macroscopic apparatus results in the effective "collapse of the wavepacket". Existential interpretation implied by einselection regards observers as open quantum systems, distinguished only by their ability to acquire, store, and process information. Spreading of the correlations with the effectively classical pointer states throughout the environment allows one to understand 'classical reality' as a property based on the relatively objective existence of the einselected states: They can be "found out" without being re-prepared, e.g, by intercepting the information already present in the environment. The redundancy of the records of pointer states in the environment (which can be thought of as their 'fitness' in the Darwinian sense) is a measure of their classicality. A new symmetry appears in this setting: Environment -assisted invariance or envariance sheds a new light on the nature of ignorance of the state of the system due to quantum correlations with the environment, and leads to Born's rules and to the reduced density matrices, ultimately justifying basic principles of the program of decoherence and einselection.

Atomic Hong–Ou–Mandel experiment

“…In this letter, we report on the realisation, with atoms, of a HOM experiment closely following the original protocol. This opens the prospect of testing Bell's inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics 6,7 , with an eye on theories of the quantum-to-classical transition [8][9][10][11] . Our work also demonstrates a new way to produce and benchmark twin-atom pairs 12,13 that may be of interest for quantum information processing 14 and quantum simulation 15 .…”

Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems

“…U nderstanding and controlling the coherence of multispin-qubit solid-state systems is crucial for quantum information science [1][2][3] , basic research on quantum many-body dynamics 4 and quantum sensing and metrology [5][6][7][8][9] . Examples of such systems include nitrogen-vacancy (NV) colour centres in diamond 10 , phosphorous donors in silicon 11 and quantum dots 12 .…”

Single molecule charge transport: from a quantum mechanical to a classical description

“…However, it can be shown that the characteristic timescale of the state vector collapse, known as the ''decoherence time'', is much shorter than any other characteristic dynamic timescale of the system. 33 A logical conclusion is that the state vector ''collapse'' |c s i -|a n i of the system is a consequence of the systemdevice interaction. During the measurement process the system and environment wave vectors become entangled and no longer evolve independently.…”

“…During the measurement process the system and environment wave vectors become entangled and no longer evolve independently. 33 However, information transfer usually associated with measurements is a common result of almost any interaction of the system with its environment. Thus, one can imagine that the environment constantly ''measures'' any non-isolated system, and any system state |c s i that is not an eigenstate of an observable quickly collapses into one that is.…”

“…It is obvious now that, save the classical dynamics, nothing happens to pointer states, even though they are immersed in the environment. 33 A subsequent measurement of an observable already ''measured'' by the environment will produce a predefined result, which means that the system behaves as if it were classical. Thus, classical behavior is a consequence of the ''openness'' of the systems observed.…”