Christoph Bruder

Bio: Christoph Bruder is an academic researcher from University of Basel. The author has contributed to research in topics: Josephson effect & Qubit. The author has an hindex of 43, co-authored 203 publications receiving 8543 citations. Previous affiliations of Christoph Bruder include Karlsruhe Institute of Technology & ETH Zurich.

Cold Bosonic Atoms in Optical Lattices

Abstract: The dynamics of an ultracold dilute gas of bosonic atoms in an optical lattice can be described by a Bose-Hubbard model where the system parameters are controlled by laser light We study the continuous (zero temperature) quantum phase transition from the superfluid to the Mott insulator phase induced by varying the depth of the optical potential, where the Mott insulator phase corresponds to a commensurate filling of the lattice (``optical crystal'') Examples for formation of Mott structures in optical lattices with a superimposed harmonic trap and in optical superlattices are presented

Andreev scattering in anisotropic superconductors

Abstract: Three new classes of superconductors have been discovered in the past decade: the organic superconductors, the heavy-fermion superconductors, and the oxide superconductors. All of them show characteristic anomalies that point to the possibility that they are anisotropic superconductors with a directionally dependent (k-dependent) gap function that vanishes in points or lines on the Fermi surface. The problem to identify the symmetry type of an anisotropic superconductor has not found a satisfactory solution yet. Although a number of experiments have been proposed that allow one in principle to distinguish between different symmetry types, most of them are ambiguous because they do not couple to the order parameter directly. Here we propose a new experiment: Andreev scattering, i.e., scattering of low-energy normal quasiparticles off the spatially varying order parameter when the quasiparticles approach a normal-metal--superconductor interface from the normal side.The idea is investigated in detail for anisotropic even-parity superconductors. To describe the quasiparticle dynamics, the Bogoliubov--de Gennes equations for anisotropic superconductors are introduced and approximated by the Andreev equations. The nonideality of the interface is taken into account by an interface potential parametrized by a reflection coefficient. This leads to a boundary condition for the Andreev equations at the interface. The pair potential \ensuremath{\Delta}(k^,r), i.e., the directionally and space-dependent order parameter that occurs as a scattering potential in the Andreev equations, is determined self-consistently for various nonideal interfaces to d-wave superconductors. This is equivalent to solving the proximity effect for interfaces with a finite reflection coefficient, and it is done using the quasiclassical formalism. Once \ensuremath{\Delta}(k^,r) has been obtained, the Andreev equations are integrated numerically, and the k-dependent Andreev reflection and transmission coefficients as well as the corresponding conductivities are computed.The theory predicts a directional dependence of the conductivities from which the k dependence of the order parameter can be reconstructed. For this effect to be a useful tool, new experiments will have to be devised. A double-point-contact experiment is proposed for an experimental realization of the idea.

Quantum Synchronization of a Driven Self-Sustained Oscillator

Abstract: Synchronization is a universal phenomenon that is important both in fundamental studies and in technical applications. Here we investigate synchronization in the simplest quantum-mechanical scenario possible, i.e., a quantum-mechanical self-sustained oscillator coupled to an external harmonic drive. Using the power spectrum we analyze synchronization in terms of frequency entrainment and frequency locking in close analogy to the classical case. We show that there is a steplike crossover to a synchronized state as a function of the driving strength. In contrast to the classical case, there is a finite threshold value in driving. Quantum noise reduces the synchronized region and leads to a deviation from strict frequency locking.

Local density of states in a dirty normal metal connected to a superconductor

Abstract: A superconductor in contact with a normal metal not only induces superconducting correlations, known as the proximity effect, but also modifies the density of states at some distance from the interface. These modifications can be resolved experimentally in microstructured systems. We therefore study the local density of states N(E,x) of a superconductor--normal-metal heterostructure. We find a suppression of N(E,x) at small energies, which persists to large distances. If the normal metal forms a thin layer of thickness ${\mathit{L}}_{\mathit{n}}$, a minigap in the density of states appears which is of the order of the Thouless energy \ensuremath{\sim}\ensuremath{\Elzxh}D/${\mathit{L}}_{\mathit{n}}^{2}$. A magnetic field suppresses the features. We find good agreement with recent experiments of Gu\'eron et al. \textcopyright{} 1996 The American Physical Society.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Many-Body Physics with Ultracold Gases

Abstract: This paper reviews recent experimental and theoretical progress concerning many-body phenomena in dilute, ultracold gases. It focuses on effects beyond standard weak-coupling descriptions, such as the Mott-Hubbard transition in optical lattices, strongly interacting gases in one and two dimensions, or lowest-Landau-level physics in quasi-two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near-Feshbach resonances in the BCS-BEC crossover.

Quantum Phase Transition From a Superfluid to a Mott Insulator in a Gas of Ultracold Atoms

Abstract: For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.

Cavity Optomechanics

Abstract: We review the field of cavity optomechanics, which explores the interaction between electromagnetic radiation and nano- or micromechanical motion This review covers the basics of optical cavities and mechanical resonators, their mutual optomechanical interaction mediated by the radiation pressure force, the large variety of experimental systems which exhibit this interaction, optical measurements of mechanical motion, dynamical backaction amplification and cooling, nonlinear dynamics, multimode optomechanics, and proposals for future cavity quantum optomechanics experiments In addition, we describe the perspectives for fundamental quantum physics and for possible applications of optomechanical devices

Stochastic Processes in Physics and Chemistry

Abstract: N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.