105 research outputs found
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
Energy decay and frequency shift of a superconducting qubit from non-equilibrium quasiparticles
Quasiparticles are an important decoherence mechanism in superconducting
qubits, and can be described with a complex admittance that is a generalization
of the Mattis-Bardeen theory. By injecting non-equilibrium quasiparticles with
a tunnel junction, we verify qualitatively the expected change of the decay
rate and frequency in a phase qubit. With their relative change in agreement to
within 4% of prediction, the theory can be reliably used to infer quasiparticle
density. We describe how settling of the decay rate may allow determination of
whether qubit energy relaxation is limited by non-equilibrium quasiparticles.Comment: Main paper: 4 pages, 3 figures, 1 table. Supplementary material: 8
pages, 3 figure
Deterministic entanglement of photons in two superconducting microwave resonators
Quantum entanglement, one of the defining features of quantum mechanics, has
been demonstrated in a variety of nonlinear spin-like systems. Quantum
entanglement in linear systems has proven significantly more challenging, as
the intrinsic energy level degeneracy associated with linearity makes quantum
control more difficult. Here we demonstrate the quantum entanglement of photon
states in two independent linear microwave resonators, creating N-photon NOON
states as a benchmark demonstration. We use a superconducting quantum circuit
that includes Josephson qubits to control and measure the two resonators, and
we completely characterize the entangled states with bipartite Wigner
tomography. These results demonstrate a significant advance in the quantum
control of linear resonators in superconducting circuits.Comment: 11 pages, 11 figures, and 3 tables including supplementary materia
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
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
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
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
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
Dynamic quantum Kerr effect in circuit quantum electrodynamics
A superconducting qubit coupled to a microwave resonator provides a
controllable system that enables fundamental studies of light-matter
interactions. In the dispersive regime, photons in the resonator exhibit
induced frequency and phase shifts which are revealed in the resonator
transmission spectrum measured with fixed qubit-resonator detuning. In this
static detuning scheme, the phase shift is measured in the far-detuned, linear
dispersion regime to avoid measurement-induced demolition of the qubit quantum
state. Here we explore the qubit-resonator dispersive interaction over a much
broader range of detunings, by using a dynamic procedure where the qubit
transition is driven adiabatically. We use resonator Wigner tomography to
monitor the interaction, revealing exotic non-linear effects on different
photon states, e.g., Fock states, coherent states, and Schrodinger cat states,
thereby demonstrating a quantum Kerr effect in the dynamic framework.Comment: 7 pages, 4 figure
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