Carsten Henkel

Van der Waals versus optical interaction between metal nanoparticles.

We derive closed expressions for the Casimir-Polder potential between metal nanoparticles as well as for the light-induced interaction owing to the gradient force. Within the validity of the dipole approximation, the maximum interaction energy turns out to be proportional to the plasma frequency, and it is comparable to the thermal energy at T=300 K. On the other hand, the light-induced interaction depends linearly on the light intensity, and only for strongly focused laser beams (10-100 mW/microm(2)) does it become comparable in strength to the Casimir-Polder interaction.

Specular versus diffuse reflection of atoms from an evanescent-wave mirror

We tested the specularity of the ref lection of slow atoms from an evanescent-wave mirror at normal incidence. In two of the three prisms that we tested the atoms were ref lected diffusely. This nonspecular ref lection appears to be correlated with the rms roughness of the surface supporting the evanescent wave. Only the highest quality surface (rms roughness of the order of 0.1 nm) leads to specular ref lection. This discovery imposes stringent limits on the use of these mirrors in atomic-optics experiments.

Casimir energy and entropy between dissipative mirrors

We discuss the Casimir effect between two identical, parall el s abs, emphasizing the role of dissipation and temperature. Starting from quit e general assumptions, we analyze the behavior of the Casimir entropy in the limit T → 0 and link it to the behavior of the slab’s reflection coefficients at low frequencies. We also de rive a formula in terms of a sum over modes, valid for dissipative slabs that can be interpre ted in terms of a damped quantum oscillator.We discuss the Casimir effect between two identical, parallel slabs, emphasizing the role of dissipation and temperature. Starting from quite general assumptions, we analyze the behavior of the Casimir entropy in the limit T → 0 and link it to the behavior of the slabs reflection coefficients at low frequencies. We also derive a formula in terms of a sum over modes, valid for dissipative slabs that can be interpreted in terms of a damped quantum oscillator.

Magnetic interactions of cold atoms with anisotropic conductors

Abstract.We analyze atom-surface magnetic interactions on atom chips where the magnetic trapping potentials are produced by current carrying wires made of electrically anisotropic materials. We discuss a theory for time dependent fluctuations of the magnetic potential, arising from thermal noise originating from the surface. It is shown that using materials with a large electrical anisotropy results in a considerable reduction of heating and decoherence rates of ultra-cold atoms trapped near the surface, of up to several orders of magnitude. The trap loss rate due to spin flips is expected to be significantly reduced upon cooling the surface to low temperatures. In addition, the electrical anisotropy significantly suppresses the amplitude of static spatial potential corrugations due to current scattering within imperfect wires. Also the shape of the corrugation pattern depends on the electrical anisotropy: the preferred angle of the scattered current wave fronts can be varied over a wide range. Materials, fabrication, and experimental issues are discussed, and specific candidate materials are suggested.

Magnetostatic field noise near metallic surfaces

Abstract.We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterised by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leads to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials.

Noise from metallic surfaces : Effects of charge diffusion

Non-local electrodynamic models are developed for describing metallic surfaces for a diffusive metal. The electric field noise at a distance z_0 from the surface is evaluated and compared with data from ion chips that show anomalous heating with a noise power decaying as z_0^{-4}. We find that high surface diffusion can account for the latter result.

Wiring up single electron traps to perform quantum gates

Two electrons in planar Penning traps can be coupled by connecting the trap electrodes with a metallic wire. The wire provides a capacitive and resistive link between the image charges that the electron oscillation (axial motion) is inducing in the electrodes. This can be operated as a quantum network to transfer excitation quanta between the traps and to entangle the two electrons. We give a detailed analysis of this system in the dispersive limit and assess the impact of resistive thermal noise. The latter is found to be remarkably small at temperatures below 4 K in miniaturized planar traps (sizes below 1 mm). The master equation for the two-electron system in the presence of resistive noise is solved exactly. We compute fidelities for basic quantum gates needed in quantum information processing.

Quantum reflection: atomic matter-wave optics in an attractive exponential potential

We show that an attractive potential created by an evanescent wave quantum mechanically reflects an atom with nonzero probability if the atom’s incident de Broglie wavelength is larger than the decay length of the evanescent wave. The amplitude reflection coefficient is calculated by use of an analytical solution of the corresponding Schrodinger equation. We also discuss electromagnetic wave analog of the effect, the partial reflection of light at an exponentially increasing index. We interpret the quantum reflection in terms of a virtual turning point, by means of a complex stationary phase approximation. If the potential remains exponential until it becomes as deep as several hundred recoil energies, the reflection coefficient for a low-energy de Broglie wave is independent of the details of the potential far from the virtual turning point. This means that, if the evanescent wave is strong enough, interactions between the atom and the glass surface at which the evanescent wave is created can be neglected.

Recoil and momentum diffusion of an atom close to a vacuum-dielectric interface

We derive the quantum-mechanical master equation (generalized optical Bloch equation) for an atom in the vicinity of a flat dielectric surface. This equation gives access to the semiclassical radiation pressure force and the atomic momentum diffusion tensor, that are expressed in terms of the vacuum field correlation function (electromagnetic field susceptibility). It is demonstrated that the atomic center-of-mass motion provides a nonlocal probe of the electromagnetic vacuum fluctuations. We show in particular that in a circularly polarized evanescent wave, the radiation pressure force experienced by the atoms is not colinear with the evanescent waves propagation vector. In a linearly polarized evanescent wave, the recoil per fluorescence cycle leads to a net magnetization for a Jg=1/2 ground state atom.

Quantum fluctuations in the image of a Bose gas

We analyze the information content of density profiles for an ultracold Bose gas of atoms and extract resolution limits for observables contained in these images. Our starting point is density correlations that we compute within the Bogoliubov approximation, taking into account quantum and thermal fluctuations beyond mean-field theory. This provides an approximate way to construct the joint counting statistics of atoms in an array of pixels covering the gas. We derive the Fisher information of an image and the associated Cramer-Rao sensitivity bound for measuring observables contained in the image. We elaborate on our recent study on position measurements of a dark soliton [Negretti et al., Phys. Rev. A 77, 043606 (2008)] where a sensitivity scaling with the atomic density as n^{-3/4} was found. We discuss here a wider class of soliton solutions and present a detailed analysis of the Bogoliubov excitations and the gapless (Goldstone) excitation modes. These fluctuations around the mean field contribute to the noise in the image, and we show how they can actually improve the ability to locate the position of the soliton.