We report results from a search for stable particles with charge ≳10^{-5}e in bulk matter using levitated dielectric microspheres in high vacuum. No evidence for such particles was found in a total sample of 1.4 ng, providing an upper limit on the abundance per nucleon of 2.5×10^{-14} at the 95% confidence level for the material tested. These results provide the first direct search for single particles with charge ≲0.1e bound in macroscopic quantities of matter and demonstrate the ability to perform sensitive force measurements using optically levitated microspheres in vacuum.

We present the results of a search for unknown interactions that couple to mass between an optically levitated microsphere and a gold-coated silicon cantilever. The scale and geometry of the apparatus enables a search for new forces that appear at distances below 100 µm and which would have evaded previous searches due to screening mechanisms. The data are consistent with electrostatic backgrounds and place upper limits on the strength of new interactions at < 0.1 fN in the geometry tested. For the specific example of a chameleon interaction with an inverse power law potential, these results exclude matter couplings β > 5.6 × 10 4 in the region of parameter space where the self-coupling Λ 5 meV and the microspheres are not fully screened.Observations indicate that the universe is expanding at an accelerating rate [1][2][3]. This acceleration can be explained by the presence of 'dark energy' throughout the universe [4]. Although the nature of dark energy is unknown, one possibility is that it consists of a scalar field that couples to mass [5,6]. Astrophysical measurements of the dark energy density imply an energy scale of Λ = 2.4 meV, corresponding to a length scale of c/Λ ∼ 80 µm.It might be possible to detect the presence of a scalar field constituting dark energy by searching for new interactions between objects separated by distances below the dark energy length scale [5][6][7][8]. In many cases, the resulting forces can be substantially larger than Newtonian gravity at short distances [6,9]. The most sensitive previous searches for violations of Newtonian gravity at or below the dark energy length scale employed macroscopic test masses or a conductive shield between the probe and test masses to minimize electromagnetic backgrounds [8,[10][11][12][13]. Although these experiments place stringent constraints on deviations from Newtonian gravity, it is possible to construct theories of dark energy involving new forces that could have avoided detection due to the geometry and scale of previous experiments [6,9,14,15]. For these screened interactions, recent searches using microscopic test masses such as atoms [16,17] or neutrons [18][19][20] often provide the strongest constraints.Several screening mechanisms have been proposed to evade existing experimental constraints on scalar interactions in the laboratory and solar system [6]. A specific example is the chameleon mechanism [21,22], in which the effective mass of the chameleon particle (corresponding to the inverse length scale of the interaction) depends on the local matter density. At cosmological distances where the matter density is low, the chameleon field would mediate a long range interaction that explains the accelerating expansion of the universe [23]. However, most laboratory experiments are carried out in regions of high matter density, where the forces arising from the chameleon interaction are suppressed. This work presents a search for screened interactions below the dark energy length scale using optically levitated µm-size dielectric spheres ...

A technique to levitate and measure the three-dimensional position of micrometer-sized dielectric spheres with heterodyne detection is presented. The two radial degrees of freedom are measured by interfering light transmitted through the microsphere with a reference wavefront, while the axial degree of freedom is measured from the phase of the light reflected from the surface of the microsphere. This method pairs the simplicity and accessibility of single-beam optical traps to a measurement of displacement that is intrinsically calibrated by the wavelength of the trapping light and has exceptional immunity to stray light. A theoretical shot noise limit of 1.3 × 10 −13 m/ √ Hz for the radial degrees of freedom, and 3.0 × 10 −15 m/ √ Hz for the axial degree of freedom can be obtained in the system described. The measured acceleration noise in the radial direction is 7.5 × 10 −5 (m/s 2 )/ √ Hz.

We report on the electrically driven rotation of 2.4-µm-radius, optically levitated dielectric microspheres. Electric fields are used to apply torques to a microsphere's permanent electric dipole moment, while angular displacement is measured by detecting the change in polarization state of light transmitted through the microsphere (MS). This technique enables greater control than previously achieved with purely optical means because the direction and magnitude of the electric torque can be set arbitrarily. We measure the spin-down of a microsphere released from a rotating electric field, the harmonic motion of the dipole relative to the instantaneous direction of the field, and the phase lag between the driving electric field and the dipole moment of the MS due to drag from residual gas. We also observe the gyroscopic precession of the MS when the axis of rotation of the driving field and the angular momentum of the microsphere are orthogonal. These observations are in quantitative agreement with the equation of motion. The control offered by the electrical drive enables precise measurements of microsphere properties and torque as well as a method for addressing the direction of angular momentum for an optically levitated particle. arXiv:1812.09625v3 [physics.optics]

We report on the use of 4.7-µm-diameter, optically levitated, charged microspheres to image the three-dimensional force field produced by charge distributions on an Au-coated, microfabricated Si beam in vacuum. An upward-propagating, single-beam optical trap, combined with an interferometric imaging technique, provides optimal access to the microspheres for microscopy. In this demonstration, the Au-coated surface of the Si beam can be brought as close as ∼10 µm from the center of the microsphere while forces are simultaneously measured along all three orthogonal axes, fully mapping the vector force field over a total volume of ∼10 6 µm 3 . We report a force sensitivity of (2.5 ± 1.0) × 10 −17 N/ √ Hz, in each of the three degrees of freedom, with a linear response to up to ∼10 −13 N. While we discuss the case of mapping static electric fields using charged microspheres, it is expected that the technique can be extended to other force fields, using microspheres with different properties.