184 research outputs found
Reduction of the Wavepacket: How Long Does it Take?
We show that the ``reduction of the wavepacket'' caused by the interaction
with the environment occurs on a timescale which is typically many orders of
magnitude shorter than the relaxation timescale . In particular, we show
that in a system interacting with a ``canonical'' heat bath of harmonic
oscillators decorrelation timescale of two pieces of the wave-packet separated
by thermal de Broglie wavelengths is approximately . Therefore,
in the classical limit dynamical reversibility is compatible with ``instantaneous'' coherence loss.Comment: 7 pages. This paper introduced what is now known as "decoherence
timescale" and gave a now broadly used estimate, Eq.(1), for quantum Brownian
motio
Quantum Discord and Maxwell's Demons
Quantum discord was proposed as an information theoretic measure of the
``quantumness'' of correlations. I show that discord determines the difference
between the efficiency of quantum and classical Maxwell's demons in extracting
work from collections of correlated quantum systems
Relative States and the Environment: Einselection, Envariance, Quantum Darwinism, and the Existential Interpretation
Starting with basic axioms of quantum theory I revisit "Relative State
Interpretation'' set out 50 years ago by Hugh Everett
Entanglement Symmetry, Amplitudes, and Probabilities: Inverting Born's Rule
Symmetry of entangled states under a swap of outcomes ("envariance") implies
their equiprobability, and leads to Born's rule. Here I show that the amplitude
of a state given by a superposition of sequences of events that share same
total count (e.g., n detections of 0 and m of 1 in a spin 1/2 measurement) is
proportional to the square root of the fraction - square root of the relative
frequency - of all the equiprobable sequences of 0's and 1's with that n and m.Comment: Submitted to Physical Review Letter
Decoherence, chaos, quantum-classical correspondence, and the algorithmic arrow of time
The environment -- external or internal degrees of freedom coupled to the
system -- can, in effect, monitor some of its observables. As a result, the
eigenstates of these observables decohere and behave like classical states:
Continuous destruction of superpositions leads to environment-induced
superselection (einselection). Here I investigate it in the context of quantum
chaos (i. e., quantum dynamics of systems which are classically chaotic).Comment: 26 pages in Tex, 3 figure
Maxwell's Demon, Szilard's Engine and Quantum Measurements
We propose and analyze a quantum version of Szilard's ``one-molecule
engine.'' In particular, we recover, in the quantum context, Szilard's
conclusion concerning the free energy ``cost'' of measurements: per bit of information.Comment: 9 pages, 1 figur
Quantum Theory of the Classical: Quantum Jumps, Born's Rule, and Objective Classical Reality via Quantum Darwinism
Emergence of the classical world from the quantum substrate of our Universe
is a long-standing conundrum. I describe three insights into the transition
from quantum to classical that are based on the recognition of the role of the
environment. I begin with derivation of preferred sets of states that help
define what exists - our everyday classical reality. They emerge as a result of
breaking of the unitary symmetry of the Hilbert space which happens when the
unitarity of quantum evolutions encounters nonlinearities inherent in the
process of amplification - of replicating information. This derivation is
accomplished without the usual tools of decoherence, and accounts for the
appearance of quantum jumps and emergence of preferred pointer states
consistent with those obtained via environment-induced superselection, or
einselection. Pointer states obtained this way determine what can happen -
define events - without appealing to Born's rule for probabilities. Therefore,
Born's rule can be now deduced from the entanglement-assisted invariance, or
envariance - a symmetry of entangled quantum states. With probabilities at hand
one also gains new insights into foundations of quantum statistical physics.
Moreover, one can now analyze information flows responsible for decoherence.
These information flows explain how perception of objective classical reality
arises from the quantum substrate: Effective amplification they represent
accounts for the objective existence of the einselected states of macroscopic
quantum systems through the redundancy of pointer state records in their
environment - through quantum Darwinism
Preferred Observables, Predictability, Classicality, and the Environment-Induced Decoherence
Selection of the preferred classical set of states in the process of
decoherence -- so important for cosmological considerations -- is discussed
with an emphasis on the role of information loss and entropy. {\it Persistence
of correlations} between the observables of two systems (for instance, a record
and a state of a system evolved from the initial conditions described by that
record) in the presence of the environment is used to define classical
behavior. From the viewpoint of an observer (or any system capable of
maintaining records) {\it predictability} is a measure of such persistence.
{\it Predictability sieve} -- a procedure which employs both the statistical
and algorithmic entropies to systematically explore all of the Hilbert space of
an open system in order to eliminate the majority of the unpredictable and
non-classical states and to locate the islands of predictability including the
preferred {\it pointer basis} is proposed. Predictably evolving states of
decohering systems along with the time-ordered sequences of records of their
evolution define the effectively classical branches of the universal
wavefunction in the context of the ``Many Worlds Interpretation". The relation
between the consistent histories approach and the preferred basis is
considered. It is demonstrated that histories of sequences of events
corresponding to projections onto the states of the pointer basis are
consistent.Comment: to appear in ``The Physical Origins of Time Asymmetry'' ed by J.J.
Halliwell et al., Cambridge Univ. Press. 38 Pages, Preprint LA-UR-92-2051.
Content-Length: 12308
Quantum Reversibility Is Relative, Or Do Quantum Measurements Reset Initial Conditions?
I compare the role of the information in the classical and quantum dynamics
by examining the relation between information flows in measurements and the
ability of observers to reverse evolutions. I show that in the Newtonian
dynamics reversibility is unaffected by the observer's retention of the
information about the measurement outcome. By contrast -- even though quantum
dynamics is unitary, hence, reversible -- reversing quantum evolution that led
to a measurement becomes in principle impossible for an observer who keeps the
record of its outcome. Thus, quantum irreversibility can result from the
information gain rather than just its loss -- rather than just an increase of
the (von Neumann) entropy. Recording of the outcome of the measurement resets,
in effect, initial conditions within the observer's (branch of) the Universe.
Nevertheless, I also show that observer's friend -- an agent who knows what
measurement was successfully carried out and can confirm that the observer
knows the outcome but resists his curiosity and does not find out the result --
can, in principle, undo the measurement. This relativity of quantum
reversibility sheds new light on the origin of the arrow of time and elucidates
the role of information in classical and quantum physics. Quantum discord
appears as a natural measure of the extent to which dissemination of
information about the outcome affects the ability to reverse the measurement
Wave-packet collapse and the core quantum postulates: Discreteness of quantum jumps from unitarity, repeatability, and actionable information
An unknown quantum state of a single system cannot be discovered, as a
measured system is reprepare: it jumps into an eigenstate of the measured
observable. This impossibility of finding the quantum state and other symptoms
usually blamed on wave-packet collapse follow (as was recently demonstrated for
pure states of measured systems) from unitarity (which does not, however, allow
for a literal collapse) and from the repeatability of measurements: Continuous
unitary evolution and repeatability suffice to establish the discreteness that
underlies quantum jumps. Here we consider mixed states of a macroscopic, open
system (such as an apparatus), and we allow its microscopic state to change
when, e.g., measured by an observer, provided that its salient features remain
unchanged and that observers regard macroscopic state of the pointer as
representing the same record. We conclude that repeatably accessible states of
macroscopic systems (such as the states of the apparatus pointer) must
correspond to orthogonal subspaces in the Hilbert space. The symmetry breaking
we exhibit defies the egalitarian quantum superposition principle and unitary
symmetry of the Hilbert space, as it singles out preferred subspaces. We
conclude that the resulting discreteness (which underlies quantum jumps)
emerges from the continuity of the core quantum postulates plus repeatability
also in macroscopic and open, but ultimately quantum systems such as measuring
devices accessed by observers, where (in contrast with pure states of
microsystems) repeatability is paramount.Comment: Changes in the presentation, figure added, et
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