Quantum molecular decoherence

E. Bittner and P. Rossky (1997) Decoherent histories and non adiabatic quantum molecular dinamics simulations. J. Chem. Phys. 107, p. 8611... 

The concerted discussion of the topics outlined above should help us advance the new paradigm that addresses our abilities to diagnose and manipulate the entangled states of complex quantum objects and their robustness against decoherence. These abilities are required for quantum information (QI) applications or matter-wave interferometry in molecular, semiconducting or superconducting systems. On the fundamental level, this book may help establish the notion of dynamical information exchange between quantum systems and chart in detail the route from unitarity to classicality. 

Abstract We present a review of recent experiments on molecular coherence and decoherence with fullerene molecules. Nearly perfect quantum interference with high fringe contrast can be observed in far-field diffraction as well as in near-field interferometry, when the molecules are sufficiently well isolated from their environment. This is true for ambient pressures below 10-7 mbar and internal temperatures below 1000 K. The fringe contrast decreases gradually as the interaction with the environment is smoothly turned on by either increasing the ambient pressure or by heating the molecules. 

Keywords Molecular rotor, Motive power, Decoherence, Classical motion, Quantum control, Nano-thermodynamics... 

In a separate contribution [11], we have analysed within the present framework an assessment of the various arrows of time and the possible symmetry violations instigated by gravitation including the fundamental problem of molecular chirality [12]. Other related developments involve Penrose s concept of objective reduction (OR), i.e. gravity s role in quantum state reduction and decoherence as a fundamental concept that relates micro-macro domains including theories of human consciousness [13], see also Ref. [3] for more details. Note also efforts to derive quantum mechanics from general relativity [14]. 

Whereas coherence can persist up to the nanosecond range for atomic and molecular systems exposed to dilute gaseous environments, the situation is radically different in liquids and solids. Interactions with neighbouring atoms, with phonons in crystalline materials and with conduction electrons in metals, shift the coherence times down by several orders of magnitude, and local quantum superpositions are usually not observable. Intermediate cases are the electronic states used as qubits in the form of superconducting islands introduced by Y. Nakamura et al. [4]. The latest reports [5] show coherence times up to 10 s for these objects, which would allow time for operations of a quantum computer. The decoherence mechanisms in such circuits have been discussed theoretically by Burkhard et al. [6],... 

During my student days (pre-university, university, PhD), we learned quantum mechanics from the books authored by L. D. Landau and E. M. Lifshitz, A. S. Davydov, D. Bohm, Feynman s course of Lectures on Physics, and from P. A. M. Dirac s Principles . We were exeited with the theories of hidden variables, EPR paradox, decoherence, entanglement, and concerned for a life of immortal Schrodinger s cat - they were in the air at that time Did I understand it Yes - because, due to a conventional wisdom, I used it more than 24 hours a day and every day. I however doubt - doubt together with Feynman who once remarked that Nobody understands it - that I ve actually understood it. I touched and used it throughout the molecular world, which is nowadays inhabited by 21 million molecules, and which I studied as a quantum chemist - in fact, by education, I am a theoretical physicist. 

In a similar way, chemically induced dimmer configuration prepared on the silicon Si(l 0 0) surface is essentially untitled and differs, both electronically and structurally, from the dynamically tilting dimers normally found on this surface [71]. The dimer units that compose the bare Si(l 0 0) surface tilt back and forth in a low-frequency ( 5 THz) seesaw mode. In contrast, dimers that have reacted with H2 have their Si—Si dimer bonds elongated and locked in the horizontal plane of the surface. They are more reactive than normal dimers. For molecular hydrogen (H2) adsorption, the enhancement is even 10 at room temperature. In a similar way, boundaries between crystaUites and amorphous regions seem to be active sites of chain adsorption on CB surface. CB nanoparticles can be understood as open quantum systems, and the uncompensated forces can be analyzed in terms of quantum decoherence effects [70]. The dynamic approach to reinforcement proposed in this chapter becomes an additional support in epistemology of it, and with data from sub-nanolevel. 

Then the recent notion of nonadiabaticity in electron d3mamics is introduced. To be consistent with the wavepacket bifurcation, we introduce the method of electron wavepacket d3mamics that undergoes bifurcation while being carried along the so-called non-Born-Oppenheimer paths, which also branch due to nonadiabatic interactions. We will further proceed to the discussion about the interaction of molecular nonadiabatic states with intense laser fields. In this way, we penetrate on one hand into unknown domains of molecular properties such as (1) electron-nuclear quantmn entanglement due to nonadiabatic transitions and its experimental observation, (2) coherence and decoherence of electron and nuclear wavepackets, which qualitatively dominate the quantmn mechanical probabiUties of quantum transition dynamics, (3) characteristic phenomena arising from the time-dependent fluctuation of molecular electronic states, (4) the physics of interference between the nonadiabatic djmamics and external fields, and so on. 

The first part of the chapter focuses on the derivation of a mixed quantum-classical theory for rationalizing chemical reactions involving two electronic states. The central piece of this mixed quantum-classical rate constant is the appearance of a characteristic decoherence time. The second part of the chapter deals with numerical approaches at the atomic level that can be carried out to decipher the molecular mechanisms governing decoherence in real systems of biological interest. 

As a first applicative example we consider the case of a spin crossing reaction between a singlet and a triplet electronic state within a copper dioxygen complex. This process is relevant to the question of the activation of molecular oxygen by coordination to a metallic inorganic complex. We first provide a general biochemical background of this type of reaction and we then report our analysis of quantum decoherence. 

In this chapter we have focused on the application of a mixed quantum-classical approach for rationalizing the kinetics of chemical reactions involving more than one electronic state. While previous theoretical frameworks like those of Marcus or Lorquet considered a complete decoupling between the quantum and classical phases of evolution of the molecular system, we have proposed an original path where the quantum-to-classical transition operates in a smooth fashion. As a result we have ended up with a new expression for estimating the probability for the system to hop from one step to the other when decoherence occurs. In the second part of this chapter we have shown how the characteristic decoherence times could be evaluated by atomistic simulations on large molecular systems (from 30 to 40 000 atoms in the... 

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