Planar Superconducting Resonators with Internal Quality Factors above One Million

We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 10 million at high microwave powers and over 1 million at low powers, with the best low power results approaching 2 million, corresponding to ~1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators' aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit.Comment: 4 pages, 3 figures, 1 tabl

Quantum process tomography of two-qubit controlled-Z and controlled-NOT gates using superconducting phase qubits

We experimentally demonstrate quantum process tomography of controlled-Z and controlled-NOT gates using capacitively-coupled superconducting phase qubits. These gates are realized by using the $|2\rangle$ state of the phase qubit. We obtain a process fidelity of 0.70 for the controlled-phase and 0.56 for the controlled-NOT gate, with the loss of fidelity mostly due to single-qubit decoherence. The controlled-Z gate is also used to demonstrate a two-qubit Deutsch-Jozsa algorithm with a single function query.Comment: 10 pages, 8 figures, including supplementary informatio

Reduced phase error through optimized control of a superconducting qubit

Minimizing phase and other errors in experimental quantum gates allows higher fidelity quantum processing. To quantify and correct for phase errors in particular, we have developed a new experimental metrology --- amplified phase error (APE) pulses --- that amplifies and helps identify phase errors in general multi-level qubit architectures. In order to correct for both phase and amplitude errors specific to virtual transitions and leakage outside of the qubit manifold, we implement "half derivative" an experimental simplification of derivative reduction by adiabatic gate (DRAG) control theory. The phase errors are lowered by about a factor of five using this method to $\sim 1.6^{\circ}$ per gate, and can be tuned to zero. Leakage outside the qubit manifold, to the qubit $|2\rangle$ state, is also reduced to $\sim 10^{-4}$ for $20\%$ faster gates.Comment: 4 pages, 4 figures with 2 page supplementa

Excitation of superconducting qubits from hot non-equilibrium quasiparticles

Superconducting qubits probe environmental defects such as non-equilibrium quasiparticles, an important source of decoherence. We show that "hot" non-equilibrium quasiparticles, with energies above the superconducting gap, affect qubits differently from quasiparticles at the gap, implying qubits can probe the dynamic quasiparticle energy distribution. For hot quasiparticles, we predict a non-neligable increase in the qubit excited state probability P_e. By injecting hot quasiparticles into a qubit, we experimentally measure an increase of P_e in semi-quantitative agreement with the model and rule out the typically assumed thermal distribution.Comment: Main paper: 5 pages, 5 figures. Supplement: 1 page, 1 figure, 1 table. Updated to user-prepared accepted version. Key changes: Supplement added, Introduction rewritten, Figs.2,3,5 revised, Fig.4 adde