Physics
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Item Reducing Decoherence in dc SQUID Phase Qubits(2010) Przybysz, Anthony Joseph; Wellstood, Frederick C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis examines sources of dissipation and dephasing in a dc SQUID phase qubit. Coupling of the qubit to the bias lines and lossy dielectrics causes the qubit to lose quantum information through a process known generally as decoherence. Using knowledge of the possible sources of decoherence, a dc SQUID phase qubit is designed with parameters that should have made it resistant to dissipation and dephasing from those sources. Device PB9 was a dc SQUID with one small area 0.23 (μm)2 Josephson junction with a critical current of 130 nA, which was meant to be the qubit junction, and a larger area 5 (μm)2 junction with a critical current of 8.6 μA, which acted as part of an inductive isolation network. The qubit junction was shunted by a 1.5 pF low-loss interdigitated capacitor. The dc current bias line had an on-chip LC filter with a cutoff frequency of 180 MHz. The other control lines were also designed to minimize coupling of dissipative elements to the qubit. According to a theoretical model of the dissipation and dephasing, the qubit was expected to have an energy relaxation T1 ≤ 8.4 μs and dephasing time Tphi ~ 1 μs. Because of the relatively high Josephson inductance of the qubit junction, the device did not act perform like a conventional isolated single-junction phase qubit. Instead, the resonant modes that I observed were the normal modes of the entire SQUID. At 20 mK and a frequency of 4.047 GHz, the maximum energy relaxation time of the device was found to be 350 ± 70 ns, despite the optimized design. Through a study of T1 versus applied flux, T1 was found to depend on the strength of the coupling of the microwave drive line to the qubit. When the line was more coupled, T1 was shorter. This was evidence that the microwave line was overcoupled to the qubit, and was limiting the lifetime of the excited state T1. Through a study of the spectroscopic coherence time T2*, which measured the effects of low-frequency inhomogeneous broadening and higher frequency dephasing from noise, I discovered that device PB9 has several sweet spots. In particular, the presence of a sweet spot with respect to critical current fluctuations allowed me to identify critical current noise as a major source of broadening and dephasing in the qubit. From the spectroscopy I estimated the 1/f critical current noise power density at 1 Hz was and the 1/f flux noise power spectral density at 1 Hz was . Both of these values were quite high, possibly due to switching of the device between measurements.Item Scanning Tunneling Microscopy at milliKelvin Temperatures: Design and Construction(2010) Gubrud, Mark Avrum; Anderson, James R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation reports on work toward the realization of a state-of-the-art scanning tunneling microscopy and spectroscopy facility operating at milliKelvin temperatures in a dilution refrigerator. Difficulties that have been experienced in prior efforts in this area are identified. Relevant issues in heat transport and in the thermalization and electrical filtering of wiring are examined, and results are applied to the design of the system. The design, installation and characterization of the pumps, plumbing and mechanical vibration isolation, and the design and installation of wiring and fabrication and characterization of electrical filters are described.Item Cryogenic test of gravitational inverse square law below 100-micrometer length scales(2010) Yethadka Venkateswara, Krishna Raj; Paik, Ho Jung; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The inverse-square law is a hallmark of theories of gravity, impressively demonstrated from astronomical scales to sub-millimeter scales, yet we do not have a complete quantized theory of gravity applicable at the shortest distance scale. Problems within modern physics such as the hierarchy problem, the cosmological constant problem, and the strong CP problem in the Standard Model motivate a search for new physics. Theories such as large extra dimensions, ‘fat gravitons,’ and the axion, proposed to solve these problems, can result in a deviation from the gravitational inverse-square law below 100 μm and are thus testable in the laboratory. We have conducted a sub-millimeter test of the inverse-square law at 4.2 K. To minimize Newtonian errors, the experiment employed a near-null source, a disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, were positioned on the two sides of the source mass at a nominal distance of 280 μm. As the source was driven sinusoidally, the response of the test masses was sensed through a superconducting differential accelerometer. Any deviations from the inverse-square law would appear as a violation signal at the second harmonic of the source frequency, due to symmetry. We improved the design of the experiment significantly over an earlier version, by separating the source mass suspension from the detector housing and making the detector a true differential accelerometer. We identified the residual gas pressure as an error source, and developed ways to overcome the problem. During the experiment we further identified the two dominant sources of error - magnetic cross-talk and electrostatic coupling. Using cross-talk cancellation and residual balance, these were reduced to the level of the limiting random noise. No deviations from the inverse-square law were found within the experimental error (2σ) down to a length scale λ = 100 μm at the level of coupling constant |α|≤2. Extra dimensions were searched down to a length scale of 78 μm (|α|≤4). We have also proposed modifications to the current experimental design in the form of new tantalum source mass and installing additional accelerometers, to achieve an amplifier noise limited sensitivity.Item Design of a Large Bandwidth Scanning SQUID Microscope using a Cryocooled Hysteretic dc SQUID(2006-01-25) Kwon, Soun Pil; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present the design and analysis of a large bandwidth scanning Superconducting Quantum Interference Device (SQUID) microscope. Currently available SQUID microscopes are limited to detecting magnetic fields with frequencies less than 1 MHz. However, for observing nanosecond time scale phenomena such as logic operations in today's computer chips, SQUID microscopes with 1 GHz bandwidth and larger are required. The major limitation in SQUID microscope bandwidth is not the SQUID itself but the electronics and readout technique. To increase bandwidth, the fast transition of a hysteretic dc SQUID from the zero voltage state to the resistive state can be used as the detection element in a new SQUID readout technique, referred to as pulsed SQUID sampling. The technique involves pulsing the bias current to the dc SQUID while monitoring the voltage across it. As the pulse length shortens, the SQUID measures the applied external magnetic flux with shorter sampling time, which increases the bandwidth. Experimental tests of the technique have demonstrated the possibility of following signals with frequencies up to 1 GHz using a dc SQUID with Nb-AlOx-Nb Josephson junctions at around 4 K. Ringing in the pulse profile permitted the effective bandwidth of the sampling technique to be much greater than the nominal value suggested by the pulse length setting on the generator. I identify additional means of increasing bandwidth: redesigning the dc SQUID, implementing transmission line wiring, adding high speed superconducting circuits, etc. which should allow bandwidths to reach 40 GHz and higher. Towards creating a large bandwidth SQUID microscope, I also assembled and tested with collaborators a fully functional 4 K scanning SQUID microscope. With the microscope, which used a nonhysteretic niobium dc SQUID with conventional flux-locked-loop SQUID electronics, I was able to obtain the magnetic field image of a current carrying circuit.