Quantum decoherence
Beginning out of attempts to extend the understanding of quantum mechanics, the theory has developed in several directions and experimental studies have confirmed some of the key issues.
The philosophical views of Werner Heisenberg and Niels Bohr have often been grouped together as the "Copenhagen interpretation", despite significant divergences between them on important points.
[10] Although he did not use the term, the concept of quantum decoherence was first introduced in 1951 by the American physicist David Bohm,[11][12] who called it the "destruction of interference in the process of measurement".
Bohm later used decoherence to handle the measurement process in the de Broglie-Bohm interpretation of quantum theory.
[13] The significance of decoherence was further highlighted in 1970 by the German physicist H. Dieter Zeh,[14] and it has been a subject of active research since the 1980s.
[16] The study of decoherence as a proper subject began in 1970, with H. Dieter Zeh's paper "On the Interpretation of Measurement in Quantum Theory".
[4][14] Zeh regarded the wavefunction as a physical entity, rather than a calculational device or a compendium of statistical information (as is typical for Copenhagen-type interpretations), and he proposed that it should evolve unitarily, in accord with the Schrödinger equation, at all times.
Partly because of a general disinterest among physicists for interpretational questions, Zeh's work remained comparatively neglected until the early 1980s, when two papers by Wojciech Zurek[18][19] invigorated the subject.
Unlike Zeh's publications, Zurek's articles were fairly agnostic about interpretation, focusing instead on specific problems of density-matrix dynamics.
The quantum nature of the system is simply entangled into the environment so that a total superposition of the wave function still exists, but exists—at least for all practical purposes—beyond the realm of measurement.
A more rigorous derivation in Dirac notation shows how decoherence destroys interference effects and the "quantum nature" of systems.
A classical phase space contains a real-valued function in 6N dimensions (each particle contributes 3 spatial coordinates and 3 momenta).
Instead the combined state vector time-evolves a path through the "larger volume", whose dimensionality is the sum of the dimensions of the two subspaces.
When a system couples to an external environment, the dimensionality of, and hence "volume" available to, the joint state vector increases enormously.
The original system's wave function can be expanded in many different ways as a sum of elements in a quantum superposition.
The environment has effectively selected out those expansions or decompositions of the original state vector that decohere (or lose phase coherence) with each other.
As a consequence, the system behaves as a classical statistical ensemble of the different elements rather than as a single coherent quantum superposition of them.
The decoherence irreversibly converts the "averaged" or "environmentally traced-over"[26] density matrix from a pure state to a reduced mixture; it is this that gives the appearance of wave-function collapse.
represent the elements of a positive semi-definite Hermitian matrix; they characterize the decohering processes and, as such, are called the noise parameters.
This is why this type of decoherence process is called collective dephasing, because the mutual phases between all qubits of the N-qubit system are destroyed.
[26][27][28] A modern basis-independent definition of the decoherence time relies on the short-time behavior of the fidelity between the initial and the time-dependent state[36] or, equivalently, the decay of the purity.
The point is, the interaction with the environment is for all practical purposes unavoidable (e.g. even a single excited atom in a vacuum would emit a photon, which would then go off).
[38] The process of a quantum superposition gradually obliterated by decoherence was quantitatively measured for the first time by Serge Haroche and his co-workers at the École Normale Supérieure in Paris in 1996.
Due to photon scattering on cavity-mirror imperfection, the cavity field loses phase coherence to the environment.
A real quantum system inevitably meets the surrounding environment, the interaction shows up as noise in physical process.
It's extremely sensitive to environmental noise—such as electromagnetic fields, temperature fluctuations, and other external perturbations—as well as measurement, lead to decoherence.
They require that the coherence of states be preserved and that decoherence be managed, in order to actually perform quantum computation.
The most basic and direct way to reduce decoherence is to prevent the quantum system from interacting with the environment by any type of isolation.
Dynamical Decoupling (DD) is another typical quantum control technique used against decoherence, especially for systems that are coupled to noisy environments.
DD involves applying an external sequence of control pulses to the quantum system at strategically timed intervals to average out environmental interactions.
