43 research outputs found
State tomography of capacitively shunted phase qubits with high fidelity
We introduce a new design concept for superconducting quantum bits (qubits)
in which we explicitly separate the capacitive element from the Josephson
tunnel junction for improved qubit performance. The number of two-level systems
(TLS) that couple to the qubit is thereby reduced by an order of magnitude and
the measurement fidelity improves to 90%. This improved design enables the
first demonstration of quantum state tomography with superconducting qubits
using single shot measurements.Comment: submitted to PR
Microwave Dielectric Loss at Single Photon Energies and milliKelvin Temperatures
The microwave performance of amorphous dielectric materials at very low
temperatures and very low excitation strengths displays significant excess
loss. Here, we present the loss tangents of some common amorphous and
crystalline dielectrics, measured at low temperatures (T < 100 mK) with near
single-photon excitation energies, using both coplanar waveguide (CPW) and
lumped LC resonators. The loss can be understood using a two-level state (TLS)
defect model. A circuit analysis of the half-wavelength resonators we used is
outlined, and the energy dissipation of such a resonator on a multilayered
dielectric substrate is considered theoretically.Comment: 4 pages, 3 figures, submitted to Applied Physics Letter
Improving the Coherence Time of Superconducting Coplanar Resonators
The quality factor and energy decay time of superconducting resonators have
been measured as a function of material, geometry, and magnetic field. Once the
dissipation of trapped magnetic vortices is minimized, we identify surface
two-level states (TLS) as an important decay mechanism. A wide gap between the
center conductor and the ground plane, as well as use of the superconductor Re
instead of Al, are shown to decrease loss. We also demonstrate that classical
measurements of resonator quality factor at low excitation power are consistent
with single-photon decay time measured using qubit-resonator swap experiments.Comment: 3 pages, 4 figures for the main paper; total 5 pages, 6 figures
including supplementary material. Submitted to Applied Physics Letter
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 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
Decoherence Dynamics of Complex Photon States in a Superconducting Circuit
Quantum states inevitably decay with time into a probabilistic mixture of
classical states, due to their interaction with the environment and measurement
instrumentation. We present the first measurement of the decoherence dynamics
of complex photon states in a condensed-matter system. By controllably
preparing a number of distinct, quantum-superposed photon states in a
superconducting microwave resonator, we show that the subsequent decay dynamics
can be quantitatively described by taking into account only two distinct decay
channels, energy relaxation and dephasing. Our ability to prepare specific
initial quantum states allows us to measure the evolution of specific elements
in the quantum density matrix, in a very detailed manner that can be compared
with theory.Comment: 4 pages, 4 figures, Supplementary movies can be downloaded at
http://www.physics.ucsb.edu/~martinisgroup/movies.shtm
Measurement of the decay of Fock states in a superconducting quantum circuit
We demonstrate the controlled generation of Fock states with up to 15 photons
in a microwave coplanar waveguide resonator coupled to a superconducting phase
qubit. The subsequent decay of the Fock states, due to dissipation, is then
monitored by varying the time delay between preparing the state and performing
a number-state analysis. We find that the decay dynamics can be described by a
master equation where the lifetime of the n-photon Fock state scales as 1/n, in
agreement with theory. We have also generated a coherent state in the microwave
resonator, and monitored its decay process. We demonstrate that the coherent
state maintains a Poisson distribution as it decays, with an average photon
number that decreases with the same characteristic decay time as the one-photon
Fock state.Comment: 4 pages, 5 figures, and 1 tabl
Surface loss simulations of superconducting coplanar waveguide resonators
Losses in superconducting planar resonators are presently assumed to
predominantly arise from surface-oxide dissipation, due to experimental losses
varying with choice of materials. We model and simulate the magnitude of the
loss from interface surfaces in the resonator, and investigate the dependence
on power, resonator geometry, and dimensions. Surprisingly, the dominant
surface loss is found to arise from the metal-substrate and substrate-air
interfaces. This result will be useful in guiding device optimization, even
with conventional materials.Comment: Main paper: 4 pages, 4 figures, 1 table. Supplementary material: 4
pages, 2 figures, 1 tabl
Phase qubits fabricated with trilayer junctions
We have developed a novel Josephson junction geometry with minimal volume of
lossy isolation dielectric, being suitable for higher quality trilayer
junctions implemented in qubits. The junctions are based on in-situ deposited
trilayers with thermal tunnel oxide, have micron-sized areas and a low subgap
current. In qubit spectroscopy only a few avoided level crossings are observed,
and the measured relaxation time of is in good
agreement with the usual phase qubit decay time, indicating low loss due to the
additional isolation dielectric