Ralf Schützhold

Dynamically assisted Schwinger mechanism.

We study electron-positron pair creation from the Dirac vacuum induced by a strong and slowly varying electric field (Schwinger effect) which is superimposed by a weak and rapidly changing electromagnetic field (dynamical pair creation). In the subcritical regime where both mechanisms separately are strongly suppressed, their combined impact yields a pair creation rate which is dramatically enhanced. Intuitively speaking, the strong electric field lowers the threshold for dynamical particle creation--or, alternatively, the fast electromagnetic field generates additional seeds for the Schwinger mechanism. These findings could be relevant for planned ultrahigh intensity lasers.

Gravity wave analogues of black holes

It is demonstrated that gravity waves of a flowing fluid in a shallow basin can be used to simulate phenomena around black holes in the laboratory. Since the speed of the gravity waves as well as their high-wave-number dispersion (subluminal vs superluminal) can be adjusted easily by varying the height of the fluid (and its surface tension) this scenario has certain advantages over the sonic and dielectric black hole analogs, for example, although its use in testing quantum effects is dubious. It can be used to investigate the various classical instabilities associated with black (and white) holes experimentally, including positive and negative norm mode mixing at horizons.

Small cosmological constant from the QCD trace anomaly

According to recent astrophysical observations the large scale mean pressure of our present universe is negative suggesting a positive cosmological constant like term. This article addresses the question of whether non-perturbative effects of self-interacting quantum fields in curved space-times may yield a significant contribution. Focusing on the trace anomaly of quantum chromo-dynamics (QCD), a preliminary estimate of the expected order of magnitude yields a remarkable coincidence with the empirical data, indicating the potential relevance of this effect. PACS: 04.62.+v, 12.38.Aw, 12.38.Lg, 98.80.Es.

Quantum analogues : from phase transitions to black holes and cosmology

The Analogue Between Rimfall and Black Holes.- Effective Horizons in the Laboratory.- Quantum Phase Transitions from Topology in Momentum Space.- Superfluid 3He as a Model System for Cosmology - Experimental Point of View.- Dynamical Aspects of Analogue Gravity: The Backreaction of Quantum Fluctuations in Dilute Bose-Einstein Condensates.- Analogue Space-time Based on 2-Component Bose-Einstein Condensates.- Links. Relating Different Physical Systems Through the Common QFT Algebraic Structure.- The Classical and Quantum Roots of Paulis Spin-statistics Relation.- Black Hole Lasers Revisited.- Cosmic Strings.

Universality of the Hawking effect

Addressing the question of whether the Hawking effect depends on degrees of freedom at ultrahigh (e.g., Planckian) energies/momenta, we propose three rather general conditions on these degrees of freedom under which the Hawking effect is reproduced to lowest order. As a generalization of Corleys results, we present a rather general model based on nonlinear dispersion relations satisfying these conditions together with a derivation of the Hawking effect for that model. However, we also demonstrate counter-examples, which do not appear to be unphysical or artificial, displaying strong deviations from Hawkings result. Therefore, whether real black holes emit Hawking radiation remains an open question and could give nontrivial information about Planckian physics.

Hawking radiation in an electromagnetic waveguide

It is demonstrated that the propagation of electromagnetic waves in an appropriately designed waveguide is (for large wavelengths) analogous to that within a curved space-time--such as around a black hole. As electromagnetic radiation (e.g., microwaves) can be controlled, amplified, and detected (with present-day technology) much easier than sound, for example, we propose a setup for the experimental verification of the Hawking effect. Apart from experimentally testing this striking prediction, this would facilitate the investigation of the trans-Planckian problem.

Catalysis of Schwinger vacuum pair production

We propose a new catalysis mechanism for nonperturbative vacuum electron-positron pair production, by superimposing a plane-wave x-ray probe beam with a strongly focused optical laser pulse, such as is planned at the Extreme Light Infrastructure (ELI) facility. We compute the absorption coefficient arising from vacuum polarization effects for photons below threshold in a strong electric field. This setup should facilitate the (first) observation of this nonperturbative QED effect with planned light sources such as ELI yielding an envisioned intensity of order 10{sup 26} W/cm{sup 2}.

Pattern recognition on a quantum computer

PACS: 03.67.Lx, 03.67.-a, 42.30.Sy, 89.70.+c. Introduction Pattern recognition is one of the basic problems in artificial intelligence, see, e.g., [1]. For example, generally a short look at a picture like the one in Fig. 1 suffices for the human brain to spot the region with the pattern. However, it is a rather non-trivial task to accomplish the same performance with a computer – in particular if the orientation and the structure of the pattern are not known a priori. Besides the detection and localization of pattern (for example identifying seismic waves in the outputs of seismographs) the comparison and matching of the observed pattern to a set of templates (such as face recognition) is another interesting question. Usually these problems are solved with special classifiers, such as neuronal networks or Fourier analysis, etc., cf. [1]. The specific properties of the task of pattern recognition (one may consider many combinations simultaneously and is interested in global features only) give raise to the hope that quantum algorithms may be advantageous in comparison with classical (local) computational methods (with a unique entry). During the last decade the topic of quantum computing has attracted increasing interest, see, e.g., [2] for a review. It has been shown that quantum algorithms can be enormously faster that the best (known) classical techniques: Shor’s factoring algorithm [3], which exhibits an exponential speed-up relative to the best known classical method; Grover’s search routine [4] with a quadratic speed-up; and several black-box problems [5–8], some of which also exhibit an exponential speed-up. In the following a quantum algorithm for the detection and localization of certain patterns in an otherwise random data set is presented. It turns out that this method is also exponentially faster than its classical counterpart. The idea of using quantum computers for the aforementioned task of template matching (which is different from pattern detection/localization) has been elaborated in [9]. More generally, Ref. [10] points out the advantages of a quantum memory in this respect. Note, however, that the necessity of loading the complete data set into a quantum memory may represent a drawback. In [11] an algorithm for data clustering (in pattern recognition problems) is developed, which is based on/inspired by principles of quantum mechanics – but does not involve quantum computation. Description of the Problem Let us consider a N ×M array containing P = %NM points with a homogeneous density % < 1 (for example % = 1/2). Without loss of generality (w.l.o.g.) we may assume % ≤ 1/2 – otherwise we could just consider the complementary (negative) picture %→ 1− %. A small fraction χ of these points (say χ = 1/10) forms a pattern in a region of the size χNM , cf. Fig. 1. For simplicity we restrict our consideration to linear patterns, i.e., the angles within the pattern do not change.

Trembling cavities in the canonical approach

We present a canonical formalism facilitating investigations of the dynamical Casimir effect by means of a response theory approach. We consider a massless scalar field confined inside of an arbitaray domain

Dynamical Casimir Effect at Finite Temperature

G(t)