I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
16
Nov
2012
Quantum Simulations of Relativistic Quantum Physics in Circuit QED
We present a scheme for simulating relativistic quantum physics in circuit
quantum electrodynamics. By using three classical microwave drives, we show
that a superconducting qubit strongly-coupled
to a resonator field mode can be
used to simulate the dynamics of the Dirac equation and Klein paradox in all
regimes. Using the same setup we also propose the implementation of the
Foldy-Wouthuysen canonical transformation, after which the time derivative of
the position operator becomes a constant of the motion.
09
Nov
2012
Long-lived, radiation-suppressed superconducting quantum bit in a planar geometry
We present a superconducting qubit design that is fabricated in a 2D geometry
over a superconducting ground plane to enhance the lifetime. The qubit is
coupled to a microstrip resonator
for readout. The circuit is fabricated on a
silicon substrate using low loss, stoichiometric titanium nitride for capacitor
pads and small, shadow-evaporated aluminum/aluminum-oxide junctions. We observe
qubit relaxation and coherence times ($T_1$ and $T_2$) of 11.7 $pm$ 0.2 $mu$s
and 8.7 $pm$ 0.3 $mu$s, respectively. Calculations show that the proximity of
the superconducting plane suppresses the otherwise high radiation loss of the
qubit. A significant increase in $T_1$ is projected for a reduced
qubit-to-superconducting plane separation.
Improving quantum gate fidelities by using a qubit to measure microwave pulse distortions
We present a new method for determining pulse imperfections and improving the
single-gate fidelity in a superconducting qubit. By applying consecutive
positive and negative $pi$ pulses,
we amplify the qubit evolution due to
microwave pulse distortion, which causes the qubit state to rotate around an
axis perpendicular to the intended rotation axis. Measuring these rotations as
a function of pulse period allows us to reconstruct the shape of the microwave
pulse arriving at the sample. Using the extracted response to predistort the
input signal, we are able to improve the pulse shapes and to reach an average
single-qubit gate fidelity higher than 99.8%.
Observation of quantum state collapse and revival due to the single-photon Kerr effect
Photons are ideal carriers for quantum information as they can have a long
coherence time and can be transmitted over long distances. These properties are
a consequence of their weak
interactions within a nearly linear medium. To
create and manipulate nonclassical states of light, however, one requires a
strong, nonlinear interaction at the single photon level. One approach to
generate suitable interactions is to couple photons to atoms, as in the strong
coupling regime of cavity QED systems. In these systems, however, one only
indirectly controls the quantum state of the light by manipulating the atoms. A
direct photon-photon interaction occurs in so-called Kerr media, which
typically induce only weak nonlinearity at the cost of significant loss. So
far, it has not been possible to reach the single-photon Kerr regime, where the
interaction strength between individual photons exceeds the loss rate. Here,
using a 3D circuit QED architecture, we engineer an artificial Kerr medium
which enters this regime and allows the observation of new quantum effects. We
realize a Gedankenexperiment proposed by Yurke and Stoler, in which the
collapse and revival of a coherent state can be observed. This time evolution
is a consequence of the quantization of the light field in the cavity and the
nonlinear interaction between individual photons. During this evolution
non-classical superpositions of coherent states, i.e. multi-component
Schroedinger cat states, are formed. We visualize this evolution by measuring
the Husimi Q-function and confirm the non-classical properties of these
transient states by Wigner tomography. The single-photon Kerr effect could be
employed in QND measurement of photons, single photon generation, autonomous
quantum feedback schemes and quantum logic operations.
08
Nov
2012
A hybrid quantum circuit consisting of a superconducting flux qubit coupled to both a spin ensemble and a transmission-line resonator
We propose an experimentally realizable hybrid quantum circuit for achieving
a strong coupling between a spin ensemble and a transmission-line resonator via
a superconducting flux qubit
used as a data bus. The resulting coupling can be
used to transfer quantum information between the spin ensemble and the
resonator. More importantly, in contrast to the direct coupling without a data
bus, our approach requires far less spins to achieve a strong coupling between
the spin ensemble and the resonator (e.g., 3 to 4 orders of magnitude less).
This drastic reduction of the number of spins in the ensemble can greatly
improve the quantum coherence of the spin ensemble. This proposed hybrid
quantum circuit could enable a long-time quantum memory when storing
information in the spin ensemble.
Controlling a mechanical oscillator with a tunable coherent feedback network
We demonstrate a fully cryogenic microwave feedback network composed of
modular superconducting devices interconnected by transmission lines and
designed to control a mechanical oscillator
coupled to one of the devices. The
network is partitioned into an electromechanical device and a dynamically
tunable controller that coherently receives, processes and feeds back
continuous microwave signals that modify the dynamics and readout of the
mechanical state. While previous electromechanical systems represent some
compromise between efficient control and efficient readout of the mechanical
state, as set by the electromagnetic decay rate, this flexible controller
yields a closed-loop network that can be dynamically and continuously tuned
between both extremes much faster than the mechanical response time. We
demonstrate that the microwave decay rate may be modulated by at least a factor
of 10 at a rate greater than $10^4$ times the mechanical response rate.
02
Nov
2012
A Quantum Single Photon Transistor in Circuit Quantum Electrodynamics
We introduce a circuit quantum electrodynamical setup for a quantum single
photon transistor. In our approach single photons propagate in two open
transmission lines that are coupled
via two interacting transmon qubits. The
interaction is such that photons are not exchanged between the two transmission
lines but a photon in one line can completely block respectively enable the
propagation of photons in the other line. High on-off ratios can be achieved
for feasible experimental parameters. Our approach is inherently scalable as
all photon pulses can have the same pulse shape and carrier frequency such that
output signals of one transistor can be input signals for a consecutive
transistor.
Demonstrating a Driven Reset Protocol of a Superconducting Qubit
Qubit reset is crucial at the start of and during quantum information
algorithms. We present the experimental demonstration of a practical method to
force qubits into their ground state,
based on driving certain qubit and cavity
transitions. Our protocol, nicknamed DDROP (Double Drive Reset of Population)
is tested on a superconducting transmon qubit in a 3D cavity. Using a new
method for measuring population, we show that we can prepare the ground state
with a fidelity of at least 99.5 % in less than 3 microseconds; faster times
and higher fidelity are predicted upon parameter optimization.
01
Nov
2012
Self-Consistent Quantum Process Tomography
Quantum process tomography is a necessary tool for verifying quantum gates
and diagnosing faults in architectures and gate design. We show that the
standard approach of process tomography
is grossly inaccurate in the case where
the states and measurement operators used to interrogate the system are
generated by gates that have some systematic error, a situation all but
unavoidable in any practical setting. These errors in tomography can not be
fully corrected through oversampling or by performing a larger set of
experiments. We present an alternative method for tomography to reconstruct an
entire library of gates in a self-consistent manner. The essential ingredient
is to define a likelihood function that assumes nothing about the gates used
for preparation and measurement. In order to make the resulting optimization
tractable we linearize about the target, a reasonable approximation when
benchmarking a quantum computer as opposed to probing a black-box function.
25
Okt
2012
Process verification of two-qubit quantum gates by randomized benchmarking
We implement a complete randomized benchmarking protocol on a system of two
superconducting qubits. The protocol consists of randomizing over gates in the
Clifford group, which experimentally
are generated via an improved two-qubit
cross-resonance gate implementation and single-qubit unitaries. From this we
extract an optimal average error per Clifford of 0.0936. We also perform an
interleaved experiment, alternating our optimal two-qubit gate with random
two-qubit Clifford gates, to obtain a two-qubit gate error of 0.0653. We
compare these values with a two-qubit gate error of ~0.12 obtained from quantum
process tomography, which is likely limited by state preparation and
measurement errors.