Highly charged ion interactions with ultrathin dielectric films

The excitations occurring at a solid surface due to slow highly charged ion (HCI) impacts are interesting from the perspective of fundamental processes in atomic collisions and materials science. This thesis focuses on two questions: 1) How much HCI potential energy deposition is required to form permanent surface modifications?, 2) How does the presence of a thin dielectric surface film change the classical over-the-barrier picture for neutralization above a clean metal? I describe a measurement of craters in thin dielectric films formed by XeQ+ (26 ≤ Q ≤ 44) projectiles. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater σc(Q) increased by four orders of magnitude (7.9 x 10 -4 μS to 6.1 μS) as Q increased, corresponding to crater depths ranging from 2 Å to 11 Å. According to a heated spike model, the energy required to produce the craters spans from 8 keV to 25 keV over the investigated charge states. Considering energy from pre-equilibrium nuclear and electronic stopping as well as neutralization, we find that at least (27 ± 2) % of available projectile neutralization energy is deposited into the thin film during impact. Additionally, an extension of the classical over-barrier model for HCI neutralization above dielectric covered metal surfaces is presented. The model is used to obtain the critical distance for the onset of neutralization above C60/Au(111), Al2O3/ Co, and LiF/Au(111) targets. The model predicts that for thin films with low electrical permittivity and positive electron affinity, the onset of neutralization can begin with the electrons in the metal, and at further ion-surface distances than for clean metals. The model describes three distinct over-the-barrier regimes of \u27vacuum limited\u27 capture from the metal, \u27thin film\u27 limited capture from the metal, and capture from the insulator. These regimes are detailed in terms of charge state, target material parameters and film thickness

Dynamics of a dispersively coupled transmon qubit in the presence of a noise source embedded in the control line

We describe transmon qubit dynamics in the presence of noise introduced by an impedance-matched resistor (50 S2) that is embedded in the qubit control line. To obtain the time evolution, we rigorously derive the circuit Hamiltonian of the qubit, readout resonator and resistor by describing the latter as an infinite collection of bosonic modes through the Caldeira-Leggett model. Starting from this Jaynes-Cummings Hamiltonian with inductive coupling to the remote bath comprised of the resistor, we consistently obtain the Lindblad master equation for the qubit and resonator in the dispersive regime. We exploit the underlying symmetries of the master equation to transform the Liouvillian superoperator into a block diagonal matrix. The block diagonalization method reveals that the rate of exponential decoherence of the qubit is well-captured by the slowest decaying eigenmode of a single block of the Liouvillian superoperator, which can be easily computed. The model captures the often used dispersive strong limit approximation of the qubit decoherence rate being linearly proportional to the number of thermal photons in the readout resonator but predicts remarkably better decoherence rates when the dissipation rate of the resonator is increased beyond the dispersive strong regime. Our work provides a full quantitative description of the contribution to the qubit decoherence rate coming from the control line in chips that are currently employed in circuit QED laboratories and suggests different possible ways to reduce this source of the noise.Peer reviewe

Digital Computer Solution for Propagation of a Spherical Shock Wave in Aluminum

Physic

Controlling Single Microwave Photons:A New Frontier in Microwave Engineering

In microwave engineering we are accustomed to thinking of the electromagnetic energy in our circuits as transmitted by waves. Now, new technologies are being developed that deal with signals at the level of single photons where this is no longer valid. Here we describe some of the challenges and opportunities in this rapidly developing field