We report on hybrid circuit QED experiments with focused ion beam implanted Er3+ ions in Y2SiO5 coupled to an array of superconducting lumped element microwave resonators. The Y2SiO5crystal is divided into several areas with distinct erbium doping concentrations, each coupled to a separate resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic properties of the implanted spin ensemble by evaluating the temperature dependence of the coupling. The efficiency of the implantation process is analyzed and the results are compared to a bulk doped Er:Y2SiO5 sample. We demonstrate the successful integration of these engineered erbium spin ensembles with superconducting circuits.
We have investigated dielectric losses in amorphous SiO thin films under operating conditions of superconducting qubits (mK temperatures and low microwave powers). For this purpose,we have developed a broadband measurement setup employing multiplexed lumped element resonators using a broadband power combiner and a low-noise amplifier. The measured temperature and power dependences of the dielectric losses are in good agreement with those predicted for atomic two-level tunneling systems (TLS). By measuring the losses at different frequencies, we found that the TLS density of states is energy dependent. This had not been seen previously in loss measurements. These results contribute to a better understanding of decoherence effects in superconducting qubits and suggest a possibility to minimize TLS-related decoherence by reducing the qubit operation frequency.
Interfacing photonic and solid-state qubits within a hybrid quantum
architecture offers a promising route towards large scale distributed quantum
computing. Ideal candidates for coherentqubit interconversion are optically
active spins magnetically coupled to a superconducting resonator. We report on
a cavity QED experiment with magnetically anisotropic Er3+:Y2SiO5 crystals and
demonstrate strong coupling of rare-earth spins to a lumped element resonator.
In addition, the electron spin resonance and relaxation dynamics of the erbium
spins are detected via direct microwave absorption, without aid of a cavity.
For high-performance superconducting quantum devices based on Josephson
junctions (JJs) decreasing lateral sizes is of great importance. Fabrication of
sub-mu m JJs is challenging dueto non-flat surfaces with step heights of up
to several 100 nm generated during the fabrication process. We have refined a
fabrication process with significantly decreased film thicknesses, resulting in
almost flat surfaces at intermediate steps during the JJ definition. In
combination with a mix-&-match process, combining electron-beam lithography
(EBL) and conventional photolithography, we can fabricate JJs with lateral
dimensions down to 0.023 mu m^2. We propose this refined process as an
alternative to the commonly used chemical-mechanical polishing (CMP) procedure.
We present transport measurements of JJs at 4.2 K that yield critical-current
densities in the range from 50 to 10^4 A/cm^2. Our JJ process yields excellent
quality parameters, Rsg/Rn up to ~50 and Vgap up to 2.81 mV, and also allows
the fabrication of high-quality sub-mu m wide long JJs (LJJs) for the study of
Josephson vortex behavior. The developed technique can also be used for similar
multilayer processes and is very promising for fabricating sub-mu m JJs for
quantum devices such as SQUIDs, qubits and SIS mixers.