Fundamental constraints on particle tracking with optical tweezers

A general quantum limit to the sensitivity of particle position measurements is derived following the simple principle of the Heisenberg microscope. The value of this limit is calculated for particles in the Rayleigh and Mie scattering regimes, and with parameters which are relevant to optical tweezers experiments. The minimum power required to observe the zero-point motion of a levitating bead is also calculated, with the optimal particle diameter always smaller than the wavelength. We show that recent optical tweezers experiments are within two orders of magnitude of quantum limited sensitivity, suggesting that quantum optical resources may soon play an important role in high sensitivity tracking applications

Controlling a mesoscopic spin environment by quantum bit manipulation

We present a unified description of cooling and manipulation of a mesoscopic bath of nuclear spins via coupling to a single quantum system of electronic spin (quantum bit). We show that a bath cooled by the quantum bit rapidly saturates. Although the resulting saturated states of the spin bath (``dark states'') generally have low degrees of polarization and purity, their symmetry properties make them a valuable resource for the coherent manipulation of quantum bits. Specifically, we demonstrate that the dark states of nuclear ensembles can be used to coherently control the system-bath interaction and to provide a robust, long-lived quantum memory for qubit states.Comment: 4 pages, 3 figure

Measuring the correlation length of intergalactic magnetic fields from observations of gamma-ray induced cascades

Context. The imaging and timing properties of {\gamma}-ray emission from electromagnetic cascades initiated by very-high-energy (VHE) {\gamma}-rays in the intergalactic medium depend on the strength B and correlation length {\lambda}B of intergalactic magnetic fields (IGMF). Aims. We study the possibility of measuring both B and {\lambda}B via observations of the cascade emission with {\gamma}-ray telescopes. Methods. For each measurement method, we find two characteristics of the cascade signal, which are sensitive to the IGMF B and {\lambda}B values in different combinations. For the case of IGMF measurement using the observation of extended emission around extragalactic VHE {\gamma}-ray sources, the two characteristics are the slope of the surface brightness profile and the overall size of the cascade source. For the case of IGMF measurement from the time delayed emission, these two characteristics are the initial slope of the cascade emission light curve and the overall duration of the cascade signal. Results. We show that measurement of the slope of the cascade induced extended emission and/or light curve can both potentially provide measure of the IGMF correlation length, provided it lies within the range 10 kpc< {\lambda}B <1 Mpc. For correlation lengths outside this range, gamma-ray observations can provide upper or lower bound on {\lambda}B. The latter of the two methods holds great promise in the near future for providing a measurement/constraint using measurements from present/next-generation {\gamma}-ray-telescopes. Conclusions. Measurement of the IGMF correlation length will provide an important constraint on its origin. In particular, it will enable to distinguish between an IGMF of galactic wind origin from an IGMF of cosmological origin.Comment: 5 pages, 3 figure