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.

/pdf/nuclear-quantum-effects-in-water-and-aqueous-systems-1wgd2g27no.pdf

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

Nonlinear electron dynamics in metal clusters

The neutron scattering cross sections for hydrogen (H) and deuterium (D) in two metal hydrides have been studied in the Compton (or deep inelastic) scattering regime. Strongly reduced H cross sections were observed in both hydrides. This anomaly was found to depend on the characteristic scattering time, which is a function of the scattering angle and varies from about 0.1 to 1.5 fs in the present experiments. For times longer than 0.6 fs the H cross sections approached the normal, tabulated value. A smaller anomaly was observed for the D cross sections. Our experiments indicate that these, and similar anomalies earlier reported for H/D cross section ratios in water and organic compounds like polymers, amphiphiles, and organic liquids, are caused by very short-lived protonic (deuteronic) quantum correlations\char22{}which may also involve electronic degrees of freedom\char22{}of neighboring hydrogen atoms in condensed media. The neutron wavelengths are much smaller than the interparticle distances in these experiments. Still, with the support from recent calculations of neutron cross sections for quantum entangled pairs we propose that the anomalies can be explained by the fact that adjacent protons and/or deuterons in the hydrides cannot be considered as individual scattering objects. Entanglement will then modify the cross sections, for scattering times not significantly longer than the decoherence time. Possible mechanisms behind this entanglement and its decoherence are shortly described.

Anomalous neutron Compton scattering cross sections in niobium and palladium hydrides

We show that deep inelastic neutron scattering from hydrogen(or other light nuclei) can be used to measure a spectrum of anharmonic contributions to the target atom momentum distribution with high and known accuracy . The method is applied here to determine the momentum distribution of the hydrogen in the hydrogen bonded system KHC2O4(potassium binoxalate), where 13 anharmonic coefficients are obtained at the 2-sigma to 3-sigma level. The momentum distribution is obtained to an accuracy of better than few percent at all significant values of momentum.

Momentum Distribution Spectroscopy Using Deep Inelastic Neutron Scattering

The authors give a simple derivation of the impulse approximation for scattering of neutrons with large momentum and energy transfers from a single particle in a potential well. It does not rely on any definition of 'interaction time'. It clearly shows how the apparently free-particle nature of the final state allows the deduction of the ground-state momentum distribution from the scattered intensity and gives an unambiguous criterion for the validity of the impulse approximation.

https://iopscience.iop.org/article/10.1088/0022-3719/19/36/001/pdf

A new approach to impulsive neutron scattering

In recoil scattering with neutrons the impulse limit [1,2] is only reached by utilising large incident neutron energies generally in excess of 1eV, which are now readily available on spallation neutron sources. Since the property of direct interest is the atomic momentum distribution, n(p), (where p represents the atomic momentum), it is important to present and compare the recoil scattering data in terms of this quantity and not in terms of the measured scattering function S(q,ω) [3]. This transformation is relatively easy to undertake and, moreover, offers several distinct advantages over a comparison in time-of-flight or energy transfer.

Resolution in deep inelastic neutron scattering using pulsed neutron sources

The single atom kinetic energy $\mathbf{\ensuremath{\kappa}}$ of high purity solid hcp $^{4}\mathrm{He}$ has been measured by neutron Compton scattering, at temperatures between 0.07 and 0.4 K and a pressure of 40 bar. Within statistical error of $\ensuremath{\sim}2%$ no change in $\mathbf{\ensuremath{\kappa}}$ was observed. The values of $\mathbf{\ensuremath{\kappa}}$ at $\ensuremath{\sim}0.07\text{ }\text{ }\mathrm{K}$ were the same in a single crystal and a polycrystalline sample and were also unaffected (within statistical error) by the addition of 10 ppm of $^{3}\mathrm{He}$. The lattice constant was also found to be independent of temperature to within 1 part in 2000. These results suggest that the supersolid transition in $^{4}\mathrm{He}$ has a different microscopic origin to the superfluid transition in the liquid.

J. Mayers

Papers

Anomalous neutron Compton scattering cross sections in niobium and palladium hydrides

Momentum Distribution Spectroscopy Using Deep Inelastic Neutron Scattering

A new approach to impulsive neutron scattering

Resolution in deep inelastic neutron scattering using pulsed neutron sources

Measurement of the kinetic energy and lattice constant in hcp solid helium at temperatures 0.07-0.4 K.