as quantum engineering. In the past two decades it has become increasingly clear that many (perhaps all) of the symptoms of classicality can be induced in quantum systems by their environments. Thus decoherence is caused by the interaction in which the environment 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 ''Schrodinger-cat states.'' The 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 wave packet.'' The 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. Effectively classical pointer states 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 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 reduced density matrices, ultimately justifying basic principles of the program of decoherence and einselection.

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Decoherence, einselection, and the quantum origins of the classical

In the last decade, ab initio simulations and especially Car-Parrinello molecular dynamics have significantly contributed to the improvement of our understanding of both the physical and chemical properties of water, ice, and hydrogen-bonded systems in general. At the heart of this family of in silico techniques lies the crucial idea of computing the many-body interactions by solving the electronic structure problem "on the fly" as the simulation proceeds, which circumvents the need for pre-parameterized potential models. In particular, the field of proton transfer in hydrogen-bonded networks greatly benefits from these technical advances. Here, several systems of seemingly quite different nature and of increasing complexity, such as Grotthuss diffusion in water, excited-state proton-transfer in solution, phase transitions in ice, and protonated water networks in the membrane protein bacteriorhodopsin, are discussed in the realms of a unifying viewpoint.

Proton transfer 200 years after von Grotthuss: insights from ab initio simulations.

Numerous studies have identified large quantum mechanical effects in the dynamics of liquid water. In this paper, we suggest that these effects may have been overestimated due to the use of rigid water models and flexible models in which the intramolecular interactions were described using simple harmonic functions. To demonstrate this, we introduce a new simple point charge model for liquid water, q-TIP4P/F, in which the O-H stretches are described by Morse-type functions. We have parametrized this model to give the correct liquid structure, diffusion coefficient, and infrared absorption frequencies in quantum (path integral-based) simulations. The model also reproduces the experimental temperature variation of the liquid density and affords reasonable agreement with the experimental melting temperature of hexagonal ice at atmospheric pressure. By comparing classical and quantum simulations of the liquid, we find that quantum mechanical fluctuations increase the rates of translational diffusion and orientational relaxation in our model by a factor of around 1.15. This effect is much smaller than that observed in all previous simulations of empirical water models, which have found a quantum effect of at least 1.4 regardless of the quantum simulation method or the water model employed. The small quantum effect in our model is a result of two competing phenomena. Intermolecular zero point energy and tunneling effects destabilize the hydrogen-bonding network, leading to a less viscous liquid with a larger diffusion coefficient. However, this is offset by intramolecular zero point motion, which changes the average water monomer geometry resulting in a larger dipole moment, stronger intermolecular interactions, and a slower diffusion. We end by suggesting, on the basis of simulations of other potential energy models, that the small quantum effect we find in the diffusion coefficient is associated with the ability of our model to produce a single broad O-H stretching band in the infrared absorption spectrum.

Competing quantum effects in the dynamics of a flexible water model

Nuclear quantum effects influence the structure and dynamics of hydrogen-bonded systems, such as water, which impacts their observed properties with widely varying magnitudes. This review highlights the recent significant developments in the experiment, theory, and simulation of nuclear quantum effects in water. Novel experimental techniques, such as deep inelastic neutron scattering, now provide a detailed view of the role of nuclear quantum effects in water's properties. These have been combined with theoretical developments such as the introduction of the principle of competing quantum effects that allows the subtle interplay of water's quantum effects and their manifestation in experimental observables to be explained. We discuss how this principle has recently been used to explain the apparent dichotomy in water's isotope effects, which can range from very large to almost nonexistent depending on the property and conditions. We then review the latest major developments in simulation algorithms and theory that have enabled the efficient inclusion of nuclear quantum effects in molecular simulations, permitting their combination with on-the-fly evaluation of the potential energy surface using electronic structure theory. Finally, we identify current challenges and future opportunities in this area of research.

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Nuclear Quantum Effects in Water and Aqueous Systems: Experiment, Theory, and Current Challenges

Nonlinear electron dynamics in metal clusters

Studies of single-particle momentum distributions in light atoms and molecules are reviewed with specific emphasis on experimental measurements using the deep inelastic neutron scattering technique at eV energies. The technique has undergone a remarkable development since the mid-1980s, when intense fluxes of epithermal neutrons were made available from pulsed neutron sources. These types of measurements provide a probe of the short-time dynamics of the recoiling atoms or molecules as well as information on the local structure of the materials. The paper introduces both the theoretical framework for the interpretation of deep inelastic neutron scattering experiments and thoroughly illustrates the physical principles underlying the impulse approximation from light atoms and molecules. The most relevant experimental studies performed on a variety of condensed matter systems in the last 20 years are reviewed. The experimental technique is critically presented in the context of a full list of published work. ...

Measurement of momentum distribution of lightatoms and molecules in condensed matter systems using inelastic neutron scattering

The Polaris instrument at the ISIS spallation neutron source operates as a medium resolution powder diffractometer. The high incident neutron flux enables datasets to be collected with comparatively short counting times or from extremely small sample volumes. Examples of recent experiments performed on Polaris, which exploit the high count rate and the particular advantages offered by fixed geometry diffraction measurements performed on a pulsed neutron source, are presented.

The Polaris powder diffractometer at ISIS

A deep-inelastic (Compton) neutron scattering experiment on ${\mathrm{H}}_{2}\mathrm{O}\ensuremath{-}{\mathrm{D}}_{2}\mathrm{O}$ mixtures at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}293\mathrm{K}$ has been carried out. Well-resolved H and D neutron recoil peaks in the time-of-flight spectra permit the direct determination of the ratio $Q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\sigma}}_{\mathrm{H}}/{\ensuremath{\sigma}}_{\mathrm{D}}$ of the H and D total cross sections. In contrast to every conventional expectation, $Q$ has been observed to vary strongly with the H-D composition. This striking effect provides, for the first time, direct evidence for short-lived entanglement of adjacent protons (deuterons) in condensed matter at room temperature.

Anomalous Deep Inelastic Neutron Scattering from Liquid H 2 O- D 2 O: Evidence of Nuclear Quantum Entanglement

Neutron Compton scattering measurements presented here of the momentum distribution of hydrogen in KH 2 PO 4 just above and well below the ferroelectric transition temperature are sufficiently sensitive to show clearly that the proton is coherent over both sites in the high temperature phase, a result that invalidates the commonly accepted order-disorder picture of the transition. The Born-Oppenheimer potential for the hydrogen, extracted directly from data for the first time, is consistent with neutron diffraction data, and the vibrational spectrum is in substantial agreement with infrared absorption measurements. The measurements are sensitive enough to detect the effect of surrounding ligands on the hydrogen bond, and can be used to study the systematic effect of the variation of these ligands in other hydrogen bonded systems.

Direct observation of tunneling in KDP using Neutron Compton scattering.

In the impulse approximation (IA), which is used to interpret deep inelastic neutron scattering measurements, the predicted scattering is identical to that which would be obtained from a gas of noninteracting atoms, with the momentum distribution of atoms in the target system. The validity of the IA rest on the approximation that the struck particle recoils freely after the collision with the neutron. Departures from the IA are usually attributed to final-state effects (FSE), which are caused by the breakdown of this approximation. A second implicit assumption of the IA, which has received little attention in the literature, is that the atoms in the target system have a distribution of energies in the initial state, i.e., before the collision. This is not true in a quantum system at zero temperature, where, although there is a distribution of atomic momenta, the initial state has a unique energy. It is shown that ``initial-state effects'' (ISE), which are caused by the breakdown of the latter assumption at low temperatures, largely account for observed asymmetries and peak shifts from the IA prediction for S(q?,\ensuremath{\omega}). It is shown that if FSE's are negligible, ISE's are negligible when q\ensuremath{\gg}${p}_{i}$, where q is the momentum transfer and ${p}_{i}$ is the root-mean-square atomic momentum. Finally, it is shown that FSE's are negligible when q\ensuremath{\gg}${p}_{i}$, in systems other than quantum fluids and that, since in this regime ISE's are also negligible, the IA is reached for such systems when q\ensuremath{\gg}${p}_{i}$. .AE

J. Mayers

Papers

Measurement of momentum distribution of lightatoms and molecules in condensed matter systems using inelastic neutron scattering

The Polaris powder diffractometer at ISIS

Anomalous Deep Inelastic Neutron Scattering from Liquid H 2 O- D 2 O: Evidence of Nuclear Quantum Entanglement

Direct observation of tunneling in KDP using Neutron Compton scattering.

Quantum effects in deep inelastic neutron scattering.