Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Back-Action Evading Measurements of Nanomechanical Motion Approaching Quantum Limits(2009) Hertzberg, Jared Barney; Schwab, Keith C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The application of quantum mechanics to macroscopic motion suggests many counterintuitive phenomena. While the quantum nature of the motion of individual atoms and molecules has long been successfully studied, an equivalent demonstration of the motion of a near-macroscopic structure remains a challenge in experimental physics. A nanomechanical resonator is an excellent system for such a study. It typically contains > 1010 atoms, and it may be modeled in terms of macroscopic parameters such as bulk density and elasticity. Yet it behaves like a simple harmonic oscillator, with mass low enough and resonant frequency high enough for its quantum zero-point motion and single energy quanta to be experimentally accessible. In pursuit of quantum phenomena in a mechanical oscillator, two important goals are to prepare the oscillator in its quantum ground state, and to measure its position with a precision limited by the Heisenberg uncertainty principle. In this work we have demonstrated techniques that advance towards both of these goals. Our system comprises a 30 micron × 170 nm, 2.2 pg, 5.57 MHz nanomechanical resonator capacitively coupled to a 5 GHz superconducting microwave resonator. The microwave resonator and nanomechanical resonator are fabricated together onto a single silicon chip and measured in a dilution refrigerator at temperatures below 150 mK. At these temperatures the coupling of the motion to the thermal environment is very small, resulting in a very high mechanical Q, approaching ∼ 106. By driving with a microwave pump signal, we observed sidebands generated by the mechanical motion and used these to measure the thermal motion of the resonator. Applying a pump tone red-detuned from the microwave resonance, we used the microwave field to damp the mechanical resonator, extracting energy and "cooling" the motion in a manner similar to optical cooling of trapped atoms. Starting from a mode temperature of ∼ 150 mK, we reached ∼ 40 mK by this "backaction cooling" technique, corresponding to an occupation factor only ∼ 150 times above the ground state of motion. We also determined the precision of our device in measurement of position. Quantum mechanics dictates that, in a continuous position measurement, the precision may be no better than the zero-point motion of the resonator. Increasing the coupling of the resonator to detector will eventually result in back-action driving of the motion, adding imprecision and enforcing this limit. We demonstrated that our system is capable of precisions approaching this limit, and identified the primary experimental factors preventing us from reaching it: noise added to the measurement by our amplifier, and excess dissipation appearing in our microwave resonator at high pump powers. Furthermore, by applying both red- and blue-detuned phase-coherent microwave pump signals, we demonstrated back-action evading (BAE) measurement sensitive to only a single quadrature of the motion. By avoiding the back-action driving in the measured quadrature, such a technique has the potential for precisions surpassing the limit of the zero-point motion. With this method, we achieved a measurement precision of ∼ 100 fm, or 4 times the quantum zero-point motion of the mechanical resonator. We found that the measured quadrature is insensitive to back-action driving by at least a factor of 82 relative to the unmeasured quadrature. We also identified a mechanical parametric amplification effect which arises during the BAE measurement. This effect sets limits on the BAE performance but also mechanically preamplifies the motion, resulting in a position resolution 1.3 times the zero-point motion. We discuss how to overcome the experimental limits set by amplifier noise, pump power and parametric amplification. These results serve to define the path forward for demonstrating truly quantum-limited measurement and non-classical states of motion in a nearly-macroscopic object.Item dc SQUID Phase Qubit(2008-08-06) palomaki, tauno; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis examines the behavior of dc SQUID phase qubits in terms of their proposed use in a quantum computer. In a phase qubit, the two lowest energy states (n=0 and n=1) of a current-biased Josephson junction form the qubit states, with the gauge invariant phase difference across the junction being relatively well defined. In a dc SQUID phase qubit, the Josephson junction is isolated from the environment using an inductive isolation network and Josephson junction, which are connected across the phase qubit junction to form a dc SQUID. Five dc SQUID phase qubits were examined at temperatures down to 25 mK. Three of the devices had qubit junctions that were Nb/AlOx/Nb junctions with critical currents of roughly 30 microamps. The other two had Al/AlOx/Al junctions with critical currents of roughly 1.3 microamps. The device that had the best performance was an Al/AlOx/Al device with a relaxation time of 30 ns and a coherence time of 24 ns. The devices were characterized using microwave spectroscopy, Rabi oscillations, relaxation and Ramsey fringe measurements. I was also able to see coupling between two Nb/AlOx/Nb dc SQUID phase qubits and perform Rabi oscillations with them. The Nb/AlOx/Nb devices had a relaxation time and coherence time that were half that of the Al/AlOx/Al device. One of the goals of this work was to understand the nature of parasitic quantum systems (TLSs) that interact with the qubit. Coupling between a TLS and a qubit causes an avoided level crossing in the transition spectrum of the qubit. In the Al/AlOx/Al devices unintentional avoided level crossings were visible with sizes up to 240 MHz, although most visible splittings were of order ~20 MHz. The measured spectra were compared to a model of the avoided level crossing based on the TLSs coupling to the junction, through either the critical current or the voltage across the junction.Item Critical behavior of superconductors and electrical transport properties of carbon nanotube thin films(2007-08-28) Xu, Hua; Anlage, Steven M; Lobb, Christopher J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With AC microwave measurements from 10 MHz up to 50 GHz and DC nano-volt level measurements we have investigated the superconducting phase transition of YBa 2 Cu 3 O 7-δ films in zero magnetic field and electrical transport properties of single walled carbon nanotube networks. We studied the microwave conductivity of YBa 2 Cu 3 O 7-δ thin films around Tc for different incident microwave power and observed that the microwave fluctuation conductivity deviates from scaling theory at low frequency around Tc. We systematically investigated the length scales involved in AC measurements and found the probed length scale depends on both frequency and current. At low current density J but high frequency ω, we observed critical behavior without hindrance from finite-size effects. However, at low current density J and low frequency ω, the experimentally probed length scale LAC may approach the thickness d of the sample, and then the critical behavior will be destroyed by finite-size effects. In this regime, we can not observe the phase transition. With very small applied microwave power, specifically -46dBm, and high frequency data, we have investigated the critical fluctuations of YBa2 Cu 3 O 7-δ thin films around Tc. It is shown that the determination of Tc is crucial for obtaining critical exponents. Improved temperature stability and conductivity calibration allow us to take high quality data at small temperature intervals (50mK). This improves the conventional data analysis method and allows a new method of extracting exponents to be developed. With these two methods, consistent values of Tc and the critical exponent were precisely determined. Experiments on 6 samples have been done and the results give a dynamical scaling exponent z=1.55±0.15. The scaling behavior of the fluctuation conductivity is also established. We have also investigated fluctuation effects of YBa2 Cu 3 O 7-δ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. The dc measurement verified that the applied microwave power -46dBm in our ac measurement is small enough for the correct determination of Tc and critical exponents. For both dc and ac experiments the scaling behavior of the data was investigated. We found that the dc measurement could be affected by disorder. For high quality YBCO films and crystal, the critical exponent z is also around 1.5, which is consistent with ac measurement. Finally, using our broadband experimental technique and DC current-voltage characteristic measurement system, we measured the transport properties of single-walled carbon nanotube films. Based on the real and imaginary parts of the microwave conductivity, we calculated the shielding effectiveness for various film thickness. Shielding effectiveness of 43 dB at 10 MHz and 28 dB at 10 GHz is found for films with 90% optical transmittance, which suggests that single walled carbon nanotube(SWCNT) films are promising as a type of transparent microwave shielding material. We also investigated the frequency and electric field dependent conductivity of single walled carbon nanotube networks of various densities. We found the ac conductivity as a function of frequency follows the extended pair approximation model and increases with frequency above an onset frequency ω0 which varies over seven decades with a range of film thickness from sub-monolayer to 200 nm. The nonlinear electric field-dependent conductivity shows strong dependence on film thickness as well. Measurement of the electric field dependence of the resistance allows for the determination of the localization length scale L of localized states, which is found to systematically decrease with increasing film thickness. The onset frequency ω0 of enhanced ac conductivity and the localization length scale L of SWCNT networks are found to be correlated, and an empirical formula relating them has been proposed. Such studies will help the understanding of transport properties and broaden the applications of this novel material system.