45 research outputs found
Lifetime and Coherence of Two-Level Defects in a Josephson Junction
We measure the lifetime () and coherence () of two-level defect
states (TLSs) in the insulating barrier of a Josephson phase qubit and compare
to the interaction strength between the two systems. We find for the average
decay times a power law dependence on the corresponding interaction strengths,
whereas for the average coherence times we find an optimum at intermediate
coupling strengths. We explain both the lifetime and the coherence results
using the standard TLS model, including dipole radiation by phonons and
anti-correlated dependence of the energy parameters on environmental
fluctuations.Comment: 4 pages, 4 figures and supplementary material (3 pages, 2 figures, 1
table
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
Transformed Dissipation in Superconducting Quantum Circuits
Superconducting quantum circuits must be designed carefully to avoid
dissipation from coupling to external control circuitry. Here we introduce the
concept of current transformation to quantify coupling to the environment. We
test this theory with an experimentally-determined impedance transformation of
and find quantitative agreement better than a factor of 2 between
this transformation and the reduced lifetime of a phase qubit coupled to a
tunable transformer. Higher-order corrections from quantum fluctuations are
also calculated with this theory, but found not to limit the qubit lifetime. We
also illustrate how this simple connection between current and impedance
transformation can be used to rule out dissipation sources in experimental
qubit systems.Comment: 4 pages, 4 figure
Banner News
https://openspace.dmacc.edu/banner_news/1092/thumbnail.jp
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 per gate, and can be tuned to zero. Leakage outside the qubit
manifold, to the qubit state, is also reduced to for
faster gates.Comment: 4 pages, 4 figures with 2 page supplementa
Generation of Three-Qubit Entangled States using Superconducting Phase Qubits
Entanglement is one of the key resources required for quantum computation, so
experimentally creating and measuring entangled states is of crucial importance
in the various physical implementations of a quantum computer. In
superconducting qubits, two-qubit entangled states have been demonstrated and
used to show violations of Bell's Inequality and to implement simple quantum
algorithms. Unlike the two-qubit case, however, where all maximally-entangled
two-qubit states are equivalent up to local changes of basis, three qubits can
be entangled in two fundamentally different ways, typified by the states
and . Here we demonstrate the operation of three coupled
superconducting phase qubits and use them to create and measure
and states. The states are fully characterized
using quantum state tomography and are shown to satisfy entanglement witnesses,
confirming that they are indeed examples of three-qubit entanglement and are
not separable into mixtures of two-qubit entanglement.Comment: 9 pages, 5 figures. Version 2: added supplementary information and
fixed image distortion in Figure 2
OpenFermion: The Electronic Structure Package for Quantum Computers
Quantum simulation of chemistry and materials is predicted to be an important
application for both near-term and fault-tolerant quantum devices. However, at
present, developing and studying algorithms for these problems can be difficult
due to the prohibitive amount of domain knowledge required in both the area of
chemistry and quantum algorithms. To help bridge this gap and open the field to
more researchers, we have developed the OpenFermion software package
(www.openfermion.org). OpenFermion is an open-source software library written
largely in Python under an Apache 2.0 license, aimed at enabling the simulation
of fermionic models and quantum chemistry problems on quantum hardware.
Beginning with an interface to common electronic structure packages, it
simplifies the translation between a molecular specification and a quantum
circuit for solving or studying the electronic structure problem on a quantum
computer, minimizing the amount of domain expertise required to enter the
field. The package is designed to be extensible and robust, maintaining high
software standards in documentation and testing. This release paper outlines
the key motivations behind design choices in OpenFermion and discusses some
basic OpenFermion functionality which we believe will aid the community in the
development of better quantum algorithms and tools for this exciting area of
research.Comment: 22 page