24 research outputs found
Josephson tunnel junctions with ferromagnetic interlayer
Superconductivity and ferromagnetism are well-known physical properties of solid states that have been widely studied and long thought about as antagonistic phenomena due to difference in spin ordering. It turns out that the combination of both superconductor and ferromagnet leads to a very rich and interesting physics. One particular example, the phase oscillations of the superconducting order parameter inside the ferromagnet, will play a major role for the devices discussed in this work. In this thesis, I present Josephson junctions with a thin Al2O3 tunnel barrier and a ferromagnetic interlayer, i.e. superconductor-insulator-ferromagnet-superconductor (SIFS) stacks. The fabrication of junctions was optimized regarding the insulation of electrodes and the homogeneity of the current transport. The junctions were either in the 0 or pi coupled ground state, depending on the thickness of the ferromagnetic layer and on temperature. The influence of ferromagnetic layer thickness on the transport properties and the coupling (0, pi) of SIFS tunnel junctions was studied. Furthermore, using a stepped ferromagnetic layer with well-chosen thicknesses, I obtained the so-called 0-pi Josephson junction. At a certain temperature this 0-pi junction can be made perfectly symmetric. In this case the ground state corresponds to a vortex of supercurrent creating a magnetic flux which is a fraction of the magnetic flux quantum Phi_0. Such structures allow to study the physics of fractional vortices and to build various electronic circuits based on them. The SIFS junctions presented here have an exponentially vanishing damping at T--> 0. The SIFS technology developed within the framework of this work may be used to construct classical and quantum devices such as oscillators, memory cells and qubits
Identifying capacitive and inductive loss in lumped element superconducting hybrid titanium nitride/aluminum resonators
We present a method to systematically locate and extract capacitive and
inductive losses in superconducting resonators at microwave frequencies by use
of mixed-material, lumped element devices. In these devices, ultra-low loss
titanium nitride was progressively replaced with aluminum in the
inter-digitated capacitor and meandered inductor elements. By measuring the
power dependent loss at 50 mK as the Al-TiN fraction in each element is
increased, we find that at low electric field, i.e. in the single photon limit,
the loss is two level system in nature and is correlated with the amount of Al
capacitance rather than the Al inductance. In the high electric field limit,
the remaining loss is linearly related to the product of the Al area times its
inductance and is likely due to quasiparticles generated by stray radiation. At
elevated temperature, additional loss is correlated with the amount of Al in
the inductance, with a power independent TiN-Al interface loss term that
exponentially decreases as the temperature is reduced. The TiN-Al interface
loss is vanishingly small at the 50 mK base temperature.Comment: 10 pages, 5 figure
Characterization and In-situ Monitoring of Sub-stoichiometric Adjustable Tc Titanium Nitride Growth
The structural and electrical properties of Ti-N films deposited by reactive
sputtering depend on their growth parameters, in particular the Ar:N2 gas
ratio. We show that the nitrogen percentage changes the crystallographic phase
of the film progressively from pure \alpha-Ti, through an \alpha-Ti phase with
interstitial nitrogen, to stoichiometric Ti2N, and through a substoichiometric
TiNX to stoichiometric TiN. These changes also affect the superconducting
transition temperature, Tc, allowing, the superconducting properties to be
tailored for specific applications. After decreasing from a Tc of 0.4 K for
pure Ti down to below 50 mK at the Ti2N point, the Tc then increases rapidly up
to nearly 5 K over a narrow range of nitrogen incorporation. This very sharp
increase of Tc makes it difficult to control the properties of the film from
wafer-to-wafer as well as across a given wafer to within acceptable margins for
device fabrication. Here we show that the nitrogen composition and hence the
superconductive properties are related to, and can be determined by,
spectroscopic ellipsometry. Therefore, this technique may be used for process
control and wafer screening prior to investing time in processing devices
Coherence in a transmon qubit with epitaxial tunnel junctions
We developed transmon qubits based on epitaxial tunnel junctions and
interdigitated capacitors. This multileveled qubit, patterned by use of
all-optical lithography, is a step towards scalable qubits with a high
integration density. The relaxation time T1 is .72-.86mu sec and the ensemble
dephasing time T2 is slightly larger than T1. The dephasing time T2 (1.36mu
sec) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker
level splitting than observed in qubits with amorphous barriers in
equivalent-size junctions. The qubit's inferred microwave loss closely matches
the weighted losses of the individual elements (junction, wiring dielectric,
and interdigitated capacitor), determined by independent resonator
measurements
Etch Induced Microwave Losses in Titanium Nitride Superconducting Resonators
We have investigated the correlation between the microwave loss and
patterning method for coplanar waveguide titanium nitride resonators fabricated
on Si wafers. Three different methods were investigated: fluorine- and
chlorine-based reactive ion etches and an argon-ion mill. At high microwave
probe powers the reactive etched resonators showed low internal loss, whereas
the ion-milled samples showed dramatically higher loss. At single-photon powers
we found that the fluorine-etched resonators exhibited substantially lower loss
than the chlorine-etched ones. We interpret the results by use of numerically
calculated filling factors and find that the silicon surface exhibits a higher
loss when chlorine-etched than when fluorine-etched. We also find from
microscopy that re-deposition of silicon onto the photoresist and side walls is
the probable cause for the high loss observed for the ion-milled resonator
Correlating decoherence in transmon qubits: Low frequency noise by single fluctuators
We report on long-term measurements of a highly coherent, non-tunable
superconducting transmon qubit, revealing low-frequency burst noise in
coherence times and qubit transition frequency. We achieve this through a
simultaneous measurement of the qubit's relaxation and dephasing rate as well
as its resonance frequency. The analysis of correlations between these
parameters yields information about the microscopic origin of the intrinsic
decoherence mechanisms in Josephson qubits. Our results are consistent with a
small number of microscopic two-level systems located at the edges of the
superconducting film, which is further confirmed by a spectral noise analysis.Comment: 10 Pages, 6 figure
Rabi oscillations in a superconducting nanowire circuit
We investigate the circuit quantum electrodynamics of anharmonic superconducting nanowire oscillators. The sample circuit consists of a capacitively shunted nanowire with a width of about 20 nm and a varying length up to 350 nm, capacitively coupled to an on-chip resonator. By applying microwave pulses we observe Rabi oscillations, measure coherence times and the anharmonicity of the circuit. Despite the very compact design, simple top-down fabrication and high degree of disorder in the oxidized (granular) aluminum material used, we observe lifetimes in the microsecond range
Sub-micrometer epitaxial Josephson junctions for quantum circuits
We present a fabrication scheme and testing results for epitaxial
sub-micrometer Josephson junctions. The junctions are made using a
high-temperature (1170 K) "via process" yielding junctions as small as 0.8 mu m
in diameter by use of optical lithography. Sapphire (Al2O3) tunnel-barriers are
grown on an epitaxial Re/Ti multilayer base-electrode. We have fabricated
devices with both Re and Al top electrodes. While room-temperature (295 K)
resistance versus area data are favorable for both types of top electrodes, the
low-temperature (50 mK) data show that junctions with the Al top electrode have
a much higher subgap resistance. The microwave loss properties of the junctions
have been measured by use of superconducting Josephson junction qubits. The
results show that high subgap resistance correlates to improved qubit
performance
Concentric transmon qubit featuring fast tunability and an anisotropic magnetic dipole moment
We present a planar qubit design based on a superconducting circuit that we call concentric transmon. While employing a straightforward fabrication process using Al evaporation and lift-off lithography, we observe qubit lifetimes and coherence times in the order of 10 μs10 μs. We systematically characterize loss channels such as incoherent dielectric loss, Purcell decay and radiative losses. The implementation of a gradiometric SQUID loop allows for a fast tuning of the qubit transition frequency and therefore for full tomographic control of the quantum circuit. Due to the large loop size, the presented qubit architecture features a strongly increased magnetic dipole moment as compared to conventional transmon designs. This renders the concentric transmon a promising candidate to establish a site-selective passive direct ẐẐ coupling between neighboring qubits, being a pending quest in the field of quantum simulation