scholarly journals Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field

2006 ◽  
Vol 73 (4) ◽  
Author(s):  
F. J. Cao ◽  
M. Feito
Keyword(s):  
External Field ◽  
Thermal State ◽  
Quantum Field

Unexpected Applications of Hill's Differential Equations in Quantum Field Theory and Cosmology

2003 ◽  
Vol 18 (12) ◽  
pp. 2159-2166
Author(s):  
V. M. Mostepanenko
Keyword(s):  
Field Theory ◽  
Quantum Field Theory ◽  
External Field ◽  
Differential Equations ◽  
Parametric Resonance ◽  
Physical Theory ◽  
Pair Creation ◽  
Physical Effect ◽  
Inflationary Cosmology ◽  
Quantum Field

The effect of the exponential pair creation from vacuum by the external field periodic in time is discussed. Two prospective applications of this physical effect in quantum field theory and in inflationary cosmology are considered. Being a nontrivial example of a parametric resonance, the effect of exponential pair creation may serve as an illustration of the effectiveness of mathematics in physical theory.

scholarly journals Topological surprises in de Sitter QFT in two-dimensions

2018 ◽  
Vol 33 (34) ◽  
pp. 1845009 ◽  
Author(s):  
Henri Epstein ◽  
Ugo Moschella
Keyword(s):  
Field Theory ◽  
Quantum Field Theory ◽  
Thermal State ◽  
Two Dimensions ◽  
Quantum Field ◽  
De Sitter ◽  
Sitter Spacetime ◽  
Klein Gordon ◽  
New Type ◽  
Two Universes

Motivated by the study of soluble models of quantum field theory, we illustrate a new type of topological effect by comparing the constructions of canonical Klein–Gordon quantum fields on the two-dimensional de Sitter spacetime as opposed to its double covering. We show that while the commutators of the two fields coincide locally, the global topological differences make the theories drastically different. Many of the well-known features of de Sitter quantum field theory disappear. In particular, there is nothing like a Bunch–Davies vacuum. Correspondingly, even though the local horizon structure is the same for the two universes, there is no Hawking–Gibbons thermal state. Finally, there is no complementary series of fields.

scholarly journals Quantifying the Unitary Generation of Coherence from Thermal Quantum Systems

Entropy ◽  
2019 ◽  
Vol 21 (8) ◽  
pp. 810 ◽  
Author(s):  
Shimshon Kallush ◽  
Aviv Aroch ◽  
Ronnie Kosloff
Keyword(s):  
External Field ◽  
Quantum Control ◽  
Population Distribution ◽  
Thermal State ◽  
Optimization Procedure ◽  
Global Optimum ◽  
Quantum Systems ◽  
Final Energy ◽  
Optimization Task ◽  
Unitary Transformations

Coherence is associated with transient quantum states; in contrast, equilibrium thermal quantum systems have no coherence. We investigate the quantum control task of generating maximum coherence from an initial thermal state employing an external field. A completely controllable Hamiltonian is assumed allowing the generation of all possible unitary transformations. Optimizing the unitary control to achieve maximum coherence leads to a micro-canonical energy distribution on the diagonal energy representation. We demonstrate such a control scenario starting from a given Hamiltonian applying an external field, reaching the control target. Such an optimization task is found to be trap-less. By constraining the amount of energy invested by the control, maximum coherence leads to a canonical energy population distribution. When the optimization procedure constrains the final energy too tightly, local suboptimal traps are found. The global optimum is obtained when a small Lagrange multiplier is employed to constrain the final energy. Finally, we explore the task of generating coherences restricted to be close to the diagonal of the density matrix in the energy representation.

scholarly journals Measurement of a vacuum-induced geometric phase

2016 ◽  
Vol 2 (5) ◽  
pp. e1501732 ◽  
Author(s):  
Simone Gasparinetti ◽  
Simon Berger ◽  
Abdufarrukh A. Abdumalikov ◽  
Marek Pechal ◽  
Stefan Filipp ◽  
...  
Keyword(s):  
External Field ◽  
Geometric Phase ◽  
Quantum Information Processing ◽  
Berry Phase ◽  
Single Mode ◽  
Vacuum State ◽  
Microwave Cavity ◽  
Quantum Field ◽  
Artificial Atom ◽  
Fock State

Berry’s geometric phase naturally appears when a quantum system is driven by an external field whose parameters are slowly and cyclically changed. A variation in the coupling between the system and the external field can also give rise to a geometric phase, even when the field is in the vacuum state or any other Fock state. We demonstrate the appearance of a vacuum-induced Berry phase in an artificial atom, a superconducting transmon, interacting with a single mode of a microwave cavity. As we vary the phase of the interaction, the artificial atom acquires a geometric phase determined by the path traced out in the combined Hilbert space of the atom and the quantum field. Our ability to control this phase opens new possibilities for the geometric manipulation of atom-cavity systems also in the context of quantum information processing.

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