UMD Theses and Dissertations
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Item Quantum Circuit Studies with Two-Level Defects of Aluminum Oxide in a Polycrystalline Phase, Amorphous Phase, and at a Metal Surface(2022) Hung, Chih-Chiao; Osborn, Kevin D; Lobb, Christopher J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis reports on recent achievements toward understanding the nanoscale two-level systems (TLS) within aluminum oxide layers. I will discuss novel experimental and theoretical methods using superconducting resonator data to characterize the TLSs, which are deleterious to qubit coherence. This includes (1) a traditional power dependent loss, which provides the information of collective TLS effects, (2) spectroscopy of individual TLSs by DC-tuning, and (3) two-tone spectroscopy of ensemble TLSs by a second saturation tone. We find that the behaviors of TLSs in different structural phases have distinguishing features. Utilizing the DC-tuning feature of our sensor, we further extract dipole moments from individual TLSs and provide the moment histograms of the two aluminum oxide film types. We observe polycrystalline oxide has an average dipole moment = 2.6 Debye and a single-peak histogram consistent with a single TLS origin. On the other hand, TLSs in amorphous oxide have a wide spread of dipole moment values probably due to oxygen deficiency. Saturation slopes of TLSs in bulk films (polycrystalline and amorphous phases) show a square root dependence of power indicating an ignorable TLS-TLS interaction. Moreover, TLSs in the polycrystalline phase are more stable in the time domain than TLSs in the amorphous phase. Unlike the previous two bulk TLSs, TLSs at the metal-air interface require an explanation from the model assuming TLS frequencies are under stochastic fluctuations originating from TLS-TLS interaction since we find a weak power dependence. We also demonstrate the first published transmon qubits which are solely made from optical lithography. They have a comparable relaxation time and junction resistance to those made from e-beam lithography.Item LOSS IN SUPERCONDUCTING QUANTUM DEVICES FROM NON-EQUILIBRIUM QUASIPARTICLES AND INHOMOGENEITY IN ENERGY GAP(2020) Zhang, Rui; Wellstood, Frederick C.; Palmer, Benjamin S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes energy dissipation and microwave loss due to non-equilibrium quasiparticles in superconducting transmon qubits and titanium nitride coplanar waveguide resonators. During the measurements of transmon T1 relaxation time and resonator quality factor QI, I observed reduced microwave loss as the temperature increased from 20 mK to approximately Tc/10 at which the loss takes on a minimum value. I argue that this effect is due to non-equilibrium quasiparticles. I measured the temperature dependence of the relaxation time T1 of the excited state of an Al/AlOx/Al transmon and found that, in some cases, T1 increased by almost a factor of two as the temperature increased from 30 mK to 100 mK with a best T1 of 0.2 ms. I present an argument showing this unexpected temperature dependence occurs due to the behavior of non-equilibrium quasiparticles in devices in which one electrode in the tunnel junction has a smaller volume, and slightly smaller superconducting energy gap, than the other electrode. At sufficiently low temperatures, non-equilibrium quasiparticles accumulate in the electrode with the smaller gap, leading to a relatively high density of quasiparticles at the junction and a short T1. Increasing the temperature gives the quasiparticles enough thermal energy to occupy the higher gap electrode, reducing the density at the junction and increasing T1. I present a model of this effect, extract the density of quasiparticles and the two superconducting energy gaps, and discuss implications for increasing the relaxation time of transmons. I also observed a similar phenomenon in low temperature microwave studies of titanium nitride coplanar waveguide resonators. I report on loss in a resonator at temperatures from 20 mK up to 1.1 K and with the application of infrared pair breaking radiation (λ=1.55 μm). With no applied IR light, the internal quality factor increased from QI = 800,000 at T < 70 mK up to QI=(2×10^6 ) at 600 mK. The resonant frequency f0 increased by 2 parts per million over the same temperature range. Above 600 mK both QI and f0 decreased rapidly, consistent with the increase in the density of thermally generated quasiparticles. With the application of IR light and for intensities below 1 aW μm^(-2) and T < 400 mK, QI increased in a similar way to increasing the temperature before beginning to decrease with larger intensities. I show that a model involving non-equilibrium quasiparticles and two regions of different superconducting gaps can explain this unexpected behavior.Item Quantum Coherent Dynamics in a dc SQUID Phase Qubit Using an LC Filter(2010) Kwon, Hyeokshin; Wellstood, Frederick C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A dc SQUID phase qubit consists of two Josephson junctions in a loop. One junction acts as a qubit with two lowest energy levels forming the |0> and |1> status. The second junction and the loop inductance act to isolate the qubit junction from noise. In this thesis, I report on the improvement of the relaxation time and the coherence time in a dc SQUID phase qubit that used an LC filter. I also report the measurement of anomalous switching curves. In order to improve the relaxation and coherence times, I used two isolation networks, an LC isolation network and an inductive isolation network, to decouple the device from the current bias lines. This produced a very large total effective resistance of the input leads that increases the relaxation time of the qubit. In addition, I connected a low-loss SiNx shunting capacitor across the qubit junction to reduce dielectric losses. I measured two dc SQUID phase qubits. Device DS6 had a 4 (μm)2 Al/AlOx/Al qubit junction with a critical current of 0.5 μA and a 1 pF shunting capacitor. It used an LC filter made from a 10 nH inductor and a 145 pF capacitor. The capacitors contained N-H rich SiNx which produced a loss tangent of about 7×10-4. Device DS8 had a 2 (μm)2 Al/AlOx/Al qubit junction with a critical current of 77 nA and an LC filter similar to the first one. The shunting capacitor contained Si-H rich SiNx. Using a pulse readout technique, I measured the characteristics of the qubits, including the transition spectrum, Rabi oscillations, relaxation, Ramsey fringes and state tomography. The best relaxation time T1 for device DS6 was 32 ns and 280 ns for device DS8. The best Rabi decay time T' for DS6 was 42 ns while for device DS8 it was 120 ns. From these and other data I obtained estimates for the best coherence time T2 in device DS6 of 61 ns and 76 ns in device DS8. In DS8, I observed anomalous switching curves; i.e. switching curves which were qualitatively different from conventional switching curves. In the conventional case, the switching curve for the superposition state is the weighted sum of the |0> and |1> curves, but it was not in device DS8. Instead, the switching curve shifted along the current axis as the exited state probability increased. I present a model for understanding the behavior and use this model to extract the probability to be in the excited state.Item Coherence in dc SQUID phase qubits(2007-09-17) Paik, Hanhee; Lobb, Christopher J; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I report measurements of energy relaxation and quantum coherence times in an aluminum dc SQUID phase qubit and a niobium dc SQUID phase qubit at 80 mK. In a dc SQUID phase qubit, the energy levels of one Josephson junction are used as qubit states and the rest of the SQUID forms an inductive network to isolate the qubit junction. Noise current from the SQUID's current bias leads is filtered by the network, with the amount of filtering depending on the ratio of the loop inductance to the Josephson inductance of the isolation junction. The isolation unction inductance can be tuned by adjusting the current, and this allows the isolation to be varied in situ. I quantify the isolation by the isolation factor rI which is the ratio of the current noise power in the qubit junction to the total noise current power on its bias leads. I measured the energy relaxation time T1, the spectroscopic coherence time T2* and the decay time constant T' of Rabi oscillations in the Al dc SQUID phase qubit AL1 and the Nb dc SQUID phase qubit NBG, which had a gradiometer loop. In particular, I investigated the dependence of T1 on the isolation rI . T1 from the relaxation measurements did not reveal any dependance on the isolation factor rI. For comparison, I found T1 by fitting to the thermally induced background escape rate and found that it depended on rI . However, further investigation suggests that this apparent dependence may be due to a small-noise induced population in j2i so I cannot draw any firrm conclusion. I also measured the spectroscopic coherence time T2* , Rabi oscillations and the decay constant T' at significantly different isolation factors. Again, I did not observe any dependence of T2* and T' on rI , suggesting that the main decoherence source in the qubit AL1 was not the noise from the bias current. Similar results were found previously in our group's Nb devices. I compared T1, T2* and T0 for the qubit AL1 with those for NBG and a niobium dc SQUID phase qubit NB1 and found significant differences in T2* and T' among the devices but similar T1 values. If flux noise was dominant, NBG which has a gradiometer loop would have the longest Rabi decay time T'. However, T' for NBG was similar to NB1, a Nb dc SQUID phase qubit without a gradiometer. I found that T' = 28 ns for AL1, the Al dc SQUID phase qubit, and this was more than twice as long as in NBG (T' ~ 15 ns) or NB1 (T' ~ 15 ns). This suggests that materials played an important role in determining the coherence times of the different devices. Finally, I discuss the possibility of using a Cooper pair box to produce variable coupling between phase qubits. I calculated the effective capacitance of a Cooper pair box as a function of gate voltage. I also calculated the energy levels of a Josephson phase qubit coupled to a Cooper pair box and showed that the energy levels of the phase qubit can be tuned with the coupled Cooper pair box.