Michael Brown-Hayes

Anomalies in electrostatic calibrations for the measurement of the Casimir force in a sphere-plane geometry

Dipartimento di Fisica “Galileo Galilei”, Universit`a di Padova, Via Marzolo 8, Padova 35131, Italy(Dated: December 1, 2008)We have performed precision electrostatic calibrations in the sphere-plane geometry, and observedanomalous behavior. Namely, the scaling exponent of the electrostatic signal with distance was foundto be smaller than expected on the basis of the pure Coulombian contribution, and the residualpotential found to be distance dependent. We argue that these findings affect the accuracy of theelectrostatic calibrations and invite reanalysis of previous determinations of the Casimir force.

Reply to "Comment on 'Anomalies in electrostatic calibrations for the measurement of the Casimir force in a sphere-plane geometry' "

In a recent Comment, Decca et al. [Phys. Rev. A 79, 026101 (2009)] discussed the origin of the anomalies recently reported by us in Phys. Rev. A 78, 036102(R) (2008). Here we restate our view corroborated by their considerations that quantitative geometrical and electrostatic characterizations of the conducting surfaces (a topic not discussed explicitly in the literature until very recently) are critical for the assessment of precision and accuracy of the demonstration of the Casimir force and for deriving meaningful limits on the existence of Yukawian components possibly superimposed to the Newtonian gravitational interaction.

Towards a precision measurement of the Casimir force in a cylinder-plane geometry

We report on a proposal aimed at measuring the Casimir force in the cylinder-plane configuration. The Casimir force is evaluated including corrections due to finite parallelism, conductivity, and temperature. The range of validity of the proximity force approximation is also discussed. An apparatus to test the feasibility of a precision measurement in this configuration has been developed, and we describe both a procedure to control the parallelism and the results of the electrostatic calibration. Finally we discuss the possibility of measuring the thermal contribution to the Casimir force and deviations from the proximity force approximation, both of which are expected at relatively large distances.

On electrostatic and Casimir force measurements between conducting surfaces in a sphere-plane configuration

We report on measurements of forces acting between two conducting surfaces in a spherical-plane configuration in the 35 nm-1 μm separation range. The measurements are obtained by performing electrostatic calibrations followed by a residuals analysis after subtracting the electrostatic-dependent component. We find in all runs optimal fitting of the calibrations for exponents smaller than the one predicted by electrostatics for an ideal sphere-plane geometry. We also find that the external bias potential necessary to minimize the electrostatic contribution depends on the sphere-plane distance. In spite of these anomalies, by implementing a parametrization-dependent subtraction of the electrostatic contribution we have found evidence for short-distance attractive forces of magnitude comparable to the expected Casimir-Lifshitz force. We finally discuss the relevance of our findings in the more general context of Casimir-Lifshitz force measurements, with particular regard to the critical issues of the electrical and geometrical characterization of the involved surfaces.

Optimal cooling strategies for magnetically trapped atomic Fermi-Bose mixtures

We discuss cooling efficiency for different-species Fermi-Bose mixtures in magnetic traps. A better heat capacity matching between the two atomic species is achieved by a proper choice of the Bose cooler and the magnetically trappable hyperfine states of the mixture. When a partial spatial overlap between the two species is also taken into account, the deepest Fermi degeneracy is obtained for an optimal value of the trapping frequency ratio. This can be achieved by assisting the magnetic trap with a deconfining light beam, as shown in the case of fermionic 6Li mixed with 23Na, 87Rb, and 133Cs, with optimal conditions found for the not yet explored 6Li-87Rb mixture.

Thermodynamical approaches to efficient sympathetic cooling in ultracold Fermi-Bose atomic mixtures

modynamical approach. The dynamics of evaporative cooling trajectories is analyzed in the specific case of bichromatic optical dipole traps also taking into account the effect of partial spatial overlap between the Fermi gas and the thermal component of the Bose gas. We show that large trapping frequency ratios between the Fermi and the Bose species allow for the achievement of a deeper Fermi degeneracy, consolidating in a thermodynamic setting earlier arguments based on more restrictive assumptions. In particular, we confirm that the minimum temperature of the mixture is obtained at the crossover between boson and fermion heat capacities, and that below such a temperature sympathetic cooling vanishes. When the effect of partial overlap is taken into account, optimal sympathetic cooling of the Fermi species may be achieved by properly tuning the relative trapping strength of the two species in a time-dependent fashion. Alternatively, the dimensionality of the trap in the final stage of cooling can be changed by increasing the confinement strength, which also results in a crossover of the heat capacities at deeper Fermi degeneracies. This technique may be extended to FermiBose degenerate mixtures in optical lattices.

Development of an apparatus for cooling 6Li-87Rb fermi-bose mixtures in a light-assisted magnetic trap

We describe an experimental setup designed to produce ultracold trapped gas clouds of fermionic 6Li and bosonic 87Rb. This combination of alkali metals has the potential to reach deeper Fermi degeneracy with respect to other mixtures since it allows for improved heat capacity matching which optimizes sympathetic cooling efficiency. Atomic beams of the two species are independently produced and then decelerated by Zeeman slowers. The slowed atoms are collected into a magneto-optical trap and then transferred into a quadrupole magnetic trap. An ultracold Fermi gas with temperature in the 10−3TF range should be attainable through selective confinement of the two species via a properly detuned laser beam focused in the center of the magnetic trap.