Session W29: Focus Session: Superconducting Qubits: Resonators and Loss Mechanisms

Microwave Loss in Josephson Junction Embedded Superconducting Resonators

We report on progress to identify and mitigate sources of microwave frequency loss in Josephson junction resonant circuits at low temperature and power--the operating regime of superconducting qubits. Large critical current junctions ($>$500nA) were embedded in lumped element $4-8$ GHz superconducting resonators based on single crystal silicon dielectric overlap capacitors. Small critical current junctions (25-100nA) were characterized by measuring the $T_{1}$ relaxation time of transmon qubits. For both sets of measurements, the critical current density was varied from 20-800 $A/cm^{2}$. We present the dependence of junction loss on junction area and environmental factors such as shielding and filtering.   

Superconducting Microwave Resonators

High quality factor superconducting microwave resonators play a key role in applications to quantum computation and single photon detection schemes. We have optimized our aluminum quarter wavelength coplanar waveguide resonators in an effort to improve energy decay times. As the characteristic decay times in our samples begin to approach the requirements set out by fault tolerant error correction algorithms, reproducibility becomes a growing focus. Consistent reproduction of high quality factor resonators requires reliable determination of device parameters independent of experimental imperfections and environmental influences. These measurements permit an improved understanding of the variations between nominally identical resonators as well as variations in an individual sample over time. Recent experimental results will be discussed.   

Improving the Quality Factor of Superconducting Resonators

Superconducting resonators hold great promise for quantum information storage in quantum computing. Improving the coherence lifetimes is therefore of central interest. We have focused on improving the interface between the resonator's underlying dielectric and superconducting metallization, as simulations have shown this interface to be a major source of loss, possibly associated with two-level states. After mitigating for stray light and magnetic fields, we have achieved low power intrinsic quality factors in excess of one million at single photon energies, with high power Q's in excess of ten million. Attaining such high quality factors is dependent on substrate preparation before depositing the superconductor, as well as the deposition method. We will describe the fabrication method and characterization of resonators that consistently achieve quality factors above one million.   

Investigation of superconducting resonator designs for measuring the microwave response of vortices

The microwave response of superconductors can be influenced by the presence of vortices and the dynamics they exhibit at high frequencies. We present measurements of vortices trapped in superconducting resonators fabricated from thin aluminum films, a common material for superconducting qubit circuits. In particular, we are studying the dependence of the threshold magnetic field for trapping vortices on the resonator geometry. We perform field-cooled measurements of various configurations of coplanar waveguide resonators to study the magnetic field, frequency, and temperature dependence of the microwave vortex response. The addition of vortices results in a downward shift in resonant frequency and a reduction in the resonator quality factor. We discuss the optimization of the resonator layout for detecting the response from only a few vortices trapped in the superconducting film.   

Anomalous magnetic field effects in high Q superconducting resonators

Superconducting coplanar wave guide resonators are an important tool in quantum computing for use as memory elements. Recent process improvements have allowed for quality factors in excess of 1.5 million at single photon excitations. While allowing for more sensitive experiments, the most recent group of resonators exhibit very high sensitivity to magnetic fields. Ordinarily Abrikosov vortex physics is expected to govern the magnetic response of the resonators. During field cooling, vortices begin to form at a threshold field, $B_{th}$, that depends quadratically on the width of the resonator. However these resonators show an observed $B_{th}$ two orders of magnitude lower than predicted by theory and without any scaling with resonator width. We explore increased sensitivity to frequency fluctuations at nonzero field as a possible explanation for reduced quality factor long before vortices are expected to form.   

Quality Factor Measurements with Improved Superconducting Stripline Resonators

Superconducting microwave resonators can be coupled to a superconducting qubit to manipulate, read out and protect the qubit from the environment. Improvements in resonator quality factors, Q, offer a variety of benefits. One benefit of higher Qs is the possibility for longer coherence times. Alternatively, higher Q resonators could be implemented as a quantum memory. We fabricated and tested superconducting stripline resonators which are designed to minimize sensitivity to surface dielectric losses. These devices allow the possibility of sensitive testing of superconducting film quality and/or losses in the bulk substrate dielectric. By measuring these losses we can better understand possible mechanisms of decoherence in superconducting qubits. Preliminary quality factor results, taken at milliKelvin temperatures, will be given.   

Dielectric loss analysis using linear resonators with different impedances

It is known that amorphous dielectrics are a major source of decoherence in superconducting qubits due to energy absorption by two-level systems coupled to the electric fields. Linear resonators have been applied extensively to study loss in different dielectrics used in qubit circuits due to their versatility and relative simplicity in design, fabrication and measurement. We have designed linear resonators with multi-turn inductors and parallel-plate capacitors with resonance frequencies of 4.8-6.4 GHz. We achieve substantially different L/C values and capacitor volumes by varying the number of inductance turns and the area of the capacitors. We will present results of continuous wave measurements with SiNx capacitors and show how loss tangent and phase noise are related to impedance and capacitor volume.   

Reduction of Microwave Loss in Titanium Nitride CPW Resonators

Titanium Nitride (TiN) thin films, when optimally grown and processed, exhibit very low microwave loss at high and low power. We investigate reducing the loss by systematically removing Si substrate material from the gap region in TiN coplanar waveguides (CPWs) fabricated on intrinsic Si substrates. By exploiting the radial dependence of the etch rate in a parallel plate reactive ion etcher, otherwise identical CPWs with only the gaps etched to varying depth, i.e. trenched, were created in a single TiN film within a single processing step. The high power loss is similar for all resonators, $<$ $2\times10^{-7}$. However, when comparing the loss from all trench depths in the single photon regime at 50 mK we find that loss was reduced for the deeper trenches with the deepest reduced by a factor of 2. Predictions from finite-element analysis, with a reduced participation of lossy surface oxides in the deeper trenched CPW gaps, fit well to the measured reduction.   

Cooper Pair Transistor Embedded in a dc-Biased High-Q Microwave Cavity

A high-Q microwave cavity design based on the circuit quantum electrodynamics architecture has been developed to introduce a dc bias to the center conductor of the cavity without significantly degrading the Q at high frequencies [1]. Here we directly couple Cooper pair transistors (CPTs) to such a cavity. In the subgap region of the CPT, the dc bias generates a tunable oscillating current through the CPT via the ac Josephson effect. Evidence of such self-oscillations has been observed as current peaks in our dc measurements, which are in good agreement with calculated cavity modes, and indicate the strong coupling between the CPT and the cavity. Recent experimental results will be discussed.\\[4pt] [1] F. Chen, A. J. Sirois, R. W. Simmonds and A. J. Rimberg, Appl. Phys. Lett., 98, 132509 (2011).   

Photon Emission from a Self-oscillating Cavity-Embedded-Cooper Pair Transistor

A strongly non-linear superconducting device consisting of a Cooper pair transistor embedded in a dc voltage biased microwave cavity is investigated. The cavity-embedded-Cooper pair transistor (CECPT) is driven via the ac Josephson effect by an applied dc bias and exhibits self-oscillation without an external ac drive. Tunneling Cooper pairs can both emit photons into and absorb photons from microwave cavity modes. Photons emitted into the cavity are directly probed and are in good agreement with dc measurements. Photon emission arising from both sequential tunneling and cotunnelling processes has been observed. The CECPT offers an interesting system for studying nonlinear quantum dynamics and the quantum-to-classical transition. Recent experimental results will be discussed.   

Superconducting microstrip resonators for circuit QED experiments

Superconducting microstrip resonators have several advantages when designing scalable circuit QED systems. Their simple geometry facilitates the implementation of additional circuit elements and control lines, and, most importantly, their spectrum tends to exhibit nearly no parasitic modes up to 20 GHz even for more complicated geometries. However, due to their specific field configuration they are not expected to yield high Q-factors at very low temperatures. We analyzed such resonators at Millikelvin temperatures and find experimentally useful quality factors of approximately 1500 even in the low temperature low power limit. Our analysis indicates that even ten times higher quality factors can be achieved straightforwardly by choosing substrates with better dielectric properties. Supported by the DFG via SFB 631 and by the German Excellence Initiative via NIM   

Superconducting Resonators with Parasitic Electromagnetic Environments

Microwave losses in niobium superconducting resonators are investigated at milli-Kelvin temperatures and with low drive power. In addition to the well-known suppression of Q-factor that arises from coupling between the resonator and two-level defects in the dielectric substrate [1-4], we report strong dependence of the loaded Q-factor and resonance line-shape on the electromagnetic environment. Methods to suppress parasitic coupling between the resonator and its environment are demonstrated.\\[4pt] [1] Day, P.K. et al., Nature 425, 817-821 (2003).\\[0pt] [2] Wallraff, A. et. al., Nature 451, 162-167 (2004).\\[0pt] [3] Macha, P. et. al., Appl. Phys. Lett., 96, 062503 (2010).\\[0pt] [4] O'Connell, A.D. et. al., Appl. Phys. Lett., 92, 112903 (2008).