Bryan H. Fong

Scalar and Tensor Holographic Artificial Impedance Surfaces

We have developed a method for controlling electromagnetic surface wave propagation and radiation from complex metallic shapes. The object is covered with an artificial impedance surface that is implemented as an array of sub-wavelength metallic patches on a grounded dielectric substrate. We pattern the effective impedance over the surface by varying the size of the metallic patches. Using a holographic technique, we design the surface to scatter a known input wave into a desired output wave. Furthermore, by varying the shape of the patches we can create anisotropic surfaces with tensor impedance properties that provide control over polarization. As an example, we demonstrate a tensor impedance surface that produces circularly polarized radiation from a linearly polarized source.

Holographic artificial impedance surfaces for conformal antennas

We have developed a method for generating arbitrary radiation patterns from antennas on complex objects. The object is coated with an artificial impedance surface consisting of a lattice of sub-wavelength metal patches on a grounded dielectric substrate. The effective surface impedance depends on the size of the patches, and can be varied as a function of position. Using holography, the surface impedance is designed to generate any desired radiation pattern from currents in the surface. With this technique we can create antennas with novel properties such as radiation toward angles that would otherwise be shadowed

Isotopically enhanced triple-quantum-dot qubit

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time. Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.

The mass of a solar quiescent prominence

This paper follows up on our recent paper on the role of prominence mass in the storage of magnetic energy for driving a coronal mass ejection (CME). The previous paper erroneously rejected a set of sheet-prominence solutions, the recovery of which allows for a simple theoretical estimate of the mass of a quiescent prominence. For coronal fields of 5-10 G, these hydromagnetic solutions suggest that a prominence mass of (1-26) × 1016 g is needed to hold detached magnetic fields of intensity comparable to the coronal fields in an unbounded atmosphere such that the global magnetic field is energetically able to spontaneously open up and still have enough energy to account for the kinetic and gravitational potential energies carried away in a CME. This simple result is discussed in relation to observed prominence magnetic field intensities, densities, and masses, pointing to the relevance of such observations to the question of magnetic energy storage in the solar corona.

High fidelity quantum gates via dynamical decoupling.

Realizing the theoretical promise of quantum computers will require overcoming decoherence. Here we demonstrate numerically that high fidelity quantum gates are possible within a framework of quantum dynamical decoupling. Orders of magnitude improvement in the fidelities of a universal set of quantum gates, relative to unprotected evolution, is achieved over a broad range of system-environment coupling strengths, using recursively constructed (concatenated) dynamical decoupling pulse sequences.

Explosive instabilities: from solar flares to edge localized modes in tokamaks

The mechanisms for the explosive loss of plasma confinement that occurs in solar flares, magnetospheric sub-storms, tokamak disruptions and edge localized modes remain largely unexplained. Modelling the rapid onset of such events provides a considerable challenge to theory. A possible explanation for these events, nonlinear explosive ballooning, is discussed. In this mechanism a narrow finger of plasma erupts from inside the plasma growing explosively and pushing aside other field lines—the instability spreads from a small region until it disturbs lines across a large section of plasma. The model predicts the observed features of some high β tokamak disruptions.

Near-Optimal Dynamical Decoupling of a Qubit

We present a near-optimal quantum dynamical decoupling scheme that eliminates general decoherence of a qubit to order n using O(n2) pulses, an exponential decrease in pulses over all previous decoupling methods. Numerical simulations of a qubit coupled to a spin bath demonstrate the superior performance of the new pulse sequences.

Adaptive artificial impedance surface conformal antennas

Moving platforms, like manned and unmanned aerial vehicles, require adaptable directive antennas for both communication and radar applications. The integration of these antennas into the body of the platform drives significant development time and expense, and in many cases adversely affects the overall vehicle performance by adversely affecting its aerodynamics or weight. Being able to integrate the antenna function directly in the skin of the platform will result in simpler better performing vehicles. We have developed an AIB with which we have used to demonstrate 2D electronic beam steering. These results show how AIB technology can be utilized for the realization of next generation conformal, electrically steerable directive antennas.

Quiescent Solar Prominences and Magnetic-Energy Storage

Analytical solutions are presented to describe the hydromagnetic support of quiescent solar prominences treated as cold plasma sheets in the characteristic normal and inverse configurations. The solar corona is modeled to be axisymmetric outside a unit sphere, with the prominence sheet lying in the equatorial plane extending from the sphere out to a finite radial distance subject to an inverse-square Newtonian gravity. The relationship between prominence support and the global topology of the surrounding poloidal magnetic field is discussed, with a particular interest in the role of magnetic flux ropes in the support of inverse prominences. A novel solution is also studied describing a rope of purely azimuthal magnetic flux held in equilibrium by the weight of an internal distribution of cold mass and by an external poloidal magnetic field rigidly anchored to the base of the model corona. This solution illustrates the role that prominence weight may play in storing magnetic energy for driving coronal mass ejections.

Polarization controlling holographic artificial impedance surfaces

We have demonstrated the necessary components for designing and constructing polarization controlling holographic tensor artificial impedance surfaces. Applications using the tensor impedance holographic method and tensor impedance characterization are currently being developed.