1,720,966 research outputs found
Environment-induced dynamics in a dilute Bose-Einstein condensate
No full textWe directly model the quantum many particle dynamics during the transition of a gas of N indistinguishable bosons into a Bose-Einstein condensate. To this end, we develop a quantitative quantum master equation theory, which takes into account two body interaction processes, and in particular describes the particle number fluctuations characteristic for the Bose-Einstein phase transition. Within the Markovian dynamics assumption, we analytically prove and numerically verify the Boltzmann ergodicity conjecture for a dilute, weakly interacting Bose-Einstein condensate. The new physical bottom line of our theory is the direct microscopic monitoring of the Bose-Einstein distribution during condensate formation in real-time, after a sudden quench of the non-condensate atomic density above the critical density for Bose-Einstein condensation
Finite entropy fluctuations of a quantum particle in a Bose-Einstein condensate
Full text linkA quantitative quantum field approach for a nearly-ideal, dilute Bose gas
using a complex two-dimensional representation of time is presented. A
quantitative measure for the coherence time of a quantum particle in the limit
of very dilute atomic gases is derived and it is illustrated that the process
of spontaneous symmetry breaking of the quantum field for the particle with a
well-defined gauge has its origin in the coupling of symmetric and asymmetric
parts of the underlying fugacity spectrum, which induces finite single-particle
coherence times and thus breaks time reversal symmetry and the corresponding
phase gauge symmetry of the quantum field below the critical temperature. The
coupling and gauge symmetry breaking can be understood as due to spontaneous
quantum fluctuations of two-dimensional time and the corresponding finite
entropy at equilibrium, which does not rely on coherent particle interactions.
In this context, defining a gauge for the quantum field at the Boltzmann
equilibrium is a necessary and sufficient condition for maximization of entropy
and spontaneous symmetry breaking at thermal equilibrium in the parameter
regime of the present model. The concept of two-dimensional time finally
converges to the standard scheme of purely imaginary time with time reversal
symmetry in the classical limit of large temperatures.Comment: 7 pages, 9 figure
Umgebungsinduzierte Dynamik in einem Bose-Einstein Kondensat
No full textWe directly model the quantum many particle dynamics during the transition of a gas of N indistinguishable bosons into a Bose-Einstein condensate.
To this end, we develop a quantitative quantum master equation theory, which takes into account two body interaction processes, and in particular describes the particle number fluctuations characteristic for the Bose-Einstein phase transition. Within the Markovian dynamics assumption, we analytically prove and numerically verify the Boltzmann ergodicity conjecture for a dilute, weakly interacting Bose-Einstein condensate. The physical bottom line of our theory is the direct microscopic monitoring of the Bose-Einstein distribution during condensate formation in real-time, after a sudden quench of the non-condensate atomic density above the critical density for Bose-Einstein condensation.Wir beschreiben die Vielteilchen-Quantendynamik eines Gases von N ununterscheidbaren Teilchen während des Übergangs in ein Bose-Einstein Kondensat. Hierfür entwickeln wir eine quantitative Mastergleichungstheorie, welche den Phasenübergang des Gases in die kondensierte Phase realistisch beschreibt -- unter Einschluss von Zweiteilchenwechselwirkungen und unter der Berücksichtigung von Teilchenfluktuationen.
Im Rahmen unseres Ansatzes gelingt ein analytischer Beweis der Boltzmannschen Ergodizitätshypothese für schwach wechselwirkende Quantengase unter der Annahme Markovscher Dynamik, in Übereinstimmung mit numerischen Simulationsergebnissen. Das übergreifende physikalische Ergebnis unserer Theorie ist die direkte ikrokopische Echtzeitbeschreibung der Bose-Einstein Verteilungsfunktion während der Kondensation, nach einer instantanen Änderung der atomaren ichtkondensatsdichte oberhalb der kritischen Dichte für die Bose-Einstein Kondensation
Physical aspects of symmetry breaking in Bose gases at thermal equilibrium
No full textThe theory of non-interacting Bose gases is supplemented by a numerical quantum field description with a two-dimensional non-local order parameter that allows the modeling of wave-like atomic correlations and interference effects in the limit of low atomic densities. From the present model, it is possible to explain symmetry aspects of non-interacting and very weakly interacting Bose gases in the limit of fluctuating particle numbers, like the forward propagation of time and the relation to the breaking and preservation of phase gauge symmetry in solids. In the present formalism, the propagation of one-directional time arises from the pre-defined and equivalent convergence of independent quantum fields towards the Boltzmann equilibrium, and it is shown that Glauber coherent states are related to the definition of the quantized field. Coherently coupling condensate and non-condensate parts as a direct consequence of the increasing quantum coherence time between the different quantum field components in the Bose gas from cooling to below the critical temperature, the present model describes symmetry breaking, which is originally known from the definition of a specific gauge field from Elitzur’s theorem for local gauge fields, as a global physical rather than a purely formal mathematical process
Environment-induced dynamics in a dilute Bose-Einstein condensate
No full textWe directly model the quantum many particle dynamics during the transition of a gas of N indistinguishable bosons into a Bose-Einstein condensate. To this end, we develop a quantitative quantum master equation theory, which takes into account two body interaction processes, and in particular describes the particle number fluctuations characteristic for the Bose-Einstein phase transition. Within the Markovian dynamics assumption, we analytically prove and numerically verify the Boltzmann ergodicity conjecture for a dilute, weakly interacting Bose-Einstein condensate. The new physical bottom line of our theory is the direct microscopic monitoring of the Bose-Einstein distribution during condensate formation in real-time, after a sudden quench of the non-condensate atomic density above the critical density for Bose-Einstein condensation
Spontaneously broken gauge symmetry in a Bose gas with constant particle number
Full text linkThe interplay between spontaneously broken gauge symmetries and Bose-Einstein
condensation has long been controversially discussed in science, since the
equation of motions are invariant under phase transformations. Within the
present model it is illustrated that spontaneous symmetry breaking appears as a
non-local process in position space, but within disjoint subspaces of the
underlying Hilbert space. Numerical simulations show that it is the symmetry of
the relative phase distribution between condensate and non-condensate quantum
fields which is spontaneously broken when passing the critical temperature for
Bose-Einstein condensation. Since the total number of gas particles remains
constant over time, the global U(1)-gauge symmetry of the system is preserved.Comment: 4 pages, 2 figures, final versio
Josephson Oscillations of Two Weakly Coupled Bose-Einstein Condensates
Full text linkA numerical experiment based on a particle number-conserving quantum field theory is performed for two initially independent Bose–Einstein condensates that are coherently coupled at two temperatures. The present model illustrates ab initio that the initial phase of each of the two condensates does not remain random at the Boltzmann equilibrium, but is distributed around integer multiple values of 2π from the interference and thermalization of forward and backward propagating matter waves. The thermalization inside the atomic vapors can be understood as an intrinsic measurement process that defines a temperature for the two condensates and projects the quantum states to an average wave field with zero (relative) phases. Following this approach, focus is put on the original thought experiment of Anderson on whether a Josephson current between two initially separated Bose–Einstein condensates occurs in a deterministic way or not, depending on the initial phase distribution.Quanta 2024; 13: 28–37
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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