We generalize past work on quantum sensor networks to show that, for d input parameters, entanglement can yield a factor O(d) improvement in mean squared error when estimating an analytic function of these parameters. We show that the protocol is optimal for qubit sensors, and conjecture an optimal protocol for photons passing through interferometers. Our protocol is also applicable to continuous variable measurements, such as one quadrature of a field operator. We outline a few potential applications, including calibration of laser operations in trapped ion quantum computing.

%8 01/25/2019 %G eng %U https://arxiv.org/abs/1901.09042 %0 Journal Article %J Phys. Rev. Lett. %D 2018 %T Dark state optical lattice with sub-wavelength spatial structure %A Yang Wang %A Sarthak Subhankar %A Przemyslaw Bienias %A Mateusz Lacki %A Tsz-Chun Tsui %A Mikhail A. Baranov %A Alexey V. Gorshkov %A Peter Zoller %A James V. Porto %A Steven L. Rolston %XWe report on the experimental realization of a conservative optical lattice for cold atoms with a subwavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a one-dimensional array of ultranarrow barriers with widths less than 10 nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice and find good agreement with theoretical predictions. Even on resonance, the observed lifetimes of atoms trapped in the lattice are as long as 44 ms, nearly 105times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.

%B Phys. Rev. Lett. %V 120 %P 083601 %8 2018/02/20 %G eng %U https://link.aps.org/doi/10.1103/PhysRevLett.120.083601 %R 10.1103/PhysRevLett.120.083601 %0 Journal Article %J Phys. Rev. A %D 2018 %T Dissipation induced dipole blockade and anti-blockade in driven Rydberg systems %A Jeremy T. Young %A Thomas Boulier %A Eric Magnan %A Elizabeth A. Goldschmidt %A Ryan M. Wilson %A Steven L. Rolston %A James V. Porto %A Alexey V. Gorshkov %XWe study theoretically and experimentally the competing blockade and antiblockade effects induced by spontaneously generated contaminant Rydberg atoms in driven Rydberg systems. These contaminant atoms provide a source of strong dipole-dipole interactions and play a crucial role in the system's behavior. We study this problem theoretically using two different approaches. The first is a cumulant expansion approximation, in which we ignore third-order and higher connected correlations. Using this approach for the case of resonant drive, a many-body blockade radius picture arises, and we find qualitative agreement with previous experimental results. We further predict that as the atomic density is increased, the Rydberg population's dependence on Rabi frequency will transition from quadratic to linear dependence at lower Rabi frequencies. We study this behavior experimentally by observing this crossover at two different atomic densities. We confirm that the larger density system has a smaller crossover Rabi frequency than the smaller density system. The second theoretical approach is a set of phenomenological inhomogeneous rate equations. We compare the results of our rate-equation model to the experimental observations [E. A. Goldschmidt *et al.*, Phys. Rev. Lett. 116, 113001 (2016)] and find that these rate equations provide quantitatively good scaling behavior of the steady-state Rydberg population for both resonant and off-resonant drives.