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
28
Apr
2012
Time-Reversal Symmetry and Universal Conductance Fluctuations in a Driven Two-Level System
In the presence of time-reversal symmetry, quantum interference gives strong
corrections to the electric conductivity of disordered systems. The
self-interference of an electron wavefunction
traveling time-reversed paths
leads to effects such as weak localization and universal conductance
fluctuations. Here, we investigate the effects of broken time-reversal symmetry
in a driven artificial two-level system. Using a superconducting flux qubit, we
implement scattering events as multiple Landau-Zener transitions by driving the
qubit periodically back and forth through an avoided crossing. Interference
between different qubit trajectories give rise to a speckle pattern in the
qubit transition rate, similar to the interference patterns created when
coherent light is scattered off a disordered potential. Since the scattering
events are imposed by the driving protocol, we can control the time-reversal
symmetry of the system by making the drive waveform symmetric or asymmetric in
time. We find that the fluctuations of the transition rate exhibit a sharp peak
when the drive is time-symmetric, similar to universal conductance fluctuations
in electronic transport through mesoscopic systems.
Dynamical decoupling and dephasing in interacting two-level systems
We implement dynamical decoupling techniques to mitigate noise and enhance
the lifetime of an entangled state that is formed in a superconducting flux
qubit coupled to a microscopic
two-level system. By rapidly changing the
qubit’s transition frequency relative to the two-level system, we realize a
refocusing pulse that reduces dephasing due to fluctuations in the transition
frequencies, thereby improving the coherence time of the entangled state. The
coupling coherence is further enhanced when applying multiple refocusing
pulses, in agreement with our $1/f$ noise model. The results are applicable to
any two-qubit system with transverse coupling, and they highlight the potential
of decoupling techniques for improving two-qubit gate fidelities, an essential
prerequisite for implementing fault-tolerant quantum computing.
27
Apr
2012
Characterization of addressability by simultaneous randomized benchmarking
The control and handling of errors arising from cross-talk and unwanted
interactions in multi-qubit systems is an important issue in quantum
information processing architectures. We
introduce a benchmarking protocol that
provides information about the amount of addressability present in the system
and implement it on coupled superconducting qubits. The protocol consists of
randomized benchmarking each qubit individually and then simultaneously, and
the amount of addressability is related to the difference of the average gate
fidelities of those experiments. We present the results on two similar samples
with different amounts of cross-talk and unwanted interactions, which agree
with predictions based on simple models for the amount of residual coupling.
24
Apr
2012
Parametric four-wave mixing toolbox for superconducting resonators
We study a superconducting circuit that can act as a toolbox to generate
various Bogoliubov-linear and nonlinear quantum operations on the microwave
photon modes of superconducting
resonators within one single circuit. The
quantum operations are generated by exploring dispersive four-wave mixing (FWM)
processes involving the resonator modes. Different FWM geometries can be
realized by adjusting the circuit parameters and by applying appropriate
microwave drivings. We illustrate this scheme using a circuit made of two
superconducting qubits that couple with each other. Each qubit couples with one
superconducting resonator. We also discuss main quantum errors in this scheme
and study the fidelity of the quantum operations by numerical simulation. Our
scheme provides a practical approach to realize quantum information protocols
on superconducting resonators.
Finding low-energy conformations of lattice protein models by quantum annealing
Lattice protein folding models are a cornerstone of computational biophysics.
Although these models are a coarse grained representation, they provide useful
insight into the energy
landscape of natural proteins. Finding low-energy
three-dimensional structures is an intractable problem even in the simplest
model, the Hydrophobic-Polar (HP) model. Exhaustive search of all possible
global minima is limited to sequences in the tens of amino acids. Description
of protein-like properties are more accurately described by generalized models,
such as the one proposed by Miyazawa and Jernigan (MJ), which explicitly take
into account the unique interactions among all 20 amino acids. There is
theoretical and experimental evidence of the advantage of solving classical
optimization problems using quantum annealing over its classical analogue
(simulated annealing). In this report, we present a benchmark implementation of
quantum annealing for a biophysical problem (six different experiments up to 81
superconducting quantum bits). Although the cases presented here can be solved
in a classical computer, we present the first implementation of lattice protein
folding on a quantum device under the Miyazawa-Jernigan model. This paves the
way towards studying optimization problems in biophysics and statistical
mechanics using quantum devices.
12
Apr
2012
Superradiant quantum phase transition in a circuit QED system: a revisit from a fully microscopic point of view
In order to examine whether or not the quantum phase transition of Dicke type
exists in realistic systems, we revisit the model setup of the superconducting
circuit QED from a microscopic
many-body perspective based on the BCS theory
with pseudo-spin presentation. By deriving the Dicke model with the correct
charging terms from the minimum coupling principle, it is shown that the
circuit QED system can exhibit superradiant quantum phase transition in the
limit Nrightarrowinfty. The critical point could be reached at easiness by
adjusting the extra parameters, the ratio of Josephson capacitance C_{J} to
gate capacitance C_{g}, as well as the conventional one, the ratio of Josephson
energy E_{J} to charging energy E_{C}.
A Near-Term Quantum Computing Approach for Hard Computational Problems in Space Exploration
In this article, we show how to map a sampling of the hardest artificial
intelligence problems in space exploration onto equivalent Ising models that
then can be attacked using quantum
annealing implemented in D-Wave machine. We
overview the existing results as well as propose new Ising model
implementations for quantum annealing. We review supervised and unsupervised
learning algorithms for classification and clustering with applications to
feature identification and anomaly detection. We introduce algorithms for data
fusion and image matching for remote sensing applications. We overview planning
problems for space exploration mission applications and algorithms for
diagnostics and recovery with applications to deep space missions. We describe
combinatorial optimization algorithms for task assignment in the context of
autonomous unmanned exploration. Finally, we discuss the ways to circumvent the
limitation of the Ising mapping using a „blackbox“ approach based on ideas from
probabilistic computing. In this article we describe the architecture of the
D-Wave One machine and report its benchmarks. Results on random ensemble of
problems in the range of up to 96 qubits show improved scaling for median core
quantum annealing time compared with classical algorithms; whether this scaling
persists for larger problem sizes is an open question. We also review previous
results of D-Wave One benchmarking studies for solving binary classification
problems with a quantum boosting algorithm which is shown to outperform
AdaBoost. We review quantum algorithms for structured learning for multi-label
classification and introduce a hybrid classical/quantum approach for learning
the weights. Results of D-Wave One benchmarking studies for learning structured
labels on four different data sets show a better performance compared with an
independent Support Vector Machine approach with linear kernel.
11
Apr
2012
Initialization by measurement of a two-qubit superconducting circuit
We demonstrate initialization by joint measurement of two transmon qubits in
3D circuit quantum electrodynamics. Homodyne detection of cavity transmission
is enhanced by Josephson parametric
amplification to discriminate the two-qubit
ground state from single-qubit excitations non-destructively and with 98.1%
fidelity. Measurement and postselection of a steady-state mixture with 4.7%
residual excitation per qubit achieve 98.8% fidelity to the ground state, thus
outperforming passive initialization.
10
Apr
2012
Josephson junction-embedded transmission-line resonators: from Kerr medium to in-line transmon
We provide a general method to find the Hamiltonian of a linear circuit in
the presence of a nonlinearity. Focussing on the case of a Josephson junction
embedded in a transmission-line
resonator, we solve for the normal modes of the
system by taking into account exactly the effect of the quadratic (i.e.
inductive) part of the Josephson potential. The nonlinearity is then found to
lead to self and cross-Kerr effect, as well as beam-splitter type interactions
between modes. By adjusting the parameters of the circuit, the Kerr coefficient
K can be made to reach values that are weak (K < kappa), strong (K > kappa)
or even very strong (K >> kappa) with respect to the photon-loss rate kappa.
In the latter case, the resonator+junction circuit corresponds to an in-line
version of the transmon. By replacing the single junction by a SQUID, the Kerr
coefficient can be tuned in-situ, allowing for example the fast generation of
Schr“odinger cat states of microwave light. Finally, we explore the maximal
strength of qubit-resonator coupling that can be reached in this setting.
Hybrid quantum circuits: Superconducting circuits interacting with other quantum systems
Hybrid quantum circuits combine two or more physical systems, with the goal
of harnessing the advantages and strengths of the different systems in order to
better explore new phenomena
and potentially bring about novel quantum
technologies. This article presents a brief overview of the progress achieved
so far in the field of hybrid circuits involving atoms, spins and solid-state
devices (including superconducting and nanomechanical systems). We discuss how
these circuits combine elements from atomic physics, quantum optics, condensed
matter physics, and nanoscience, and we present different possible approaches
for integrating various systems into a single circuit. In particular, hybrid
quantum circuits can be fabricated on a chip, facilitating their future
scalability, which is crucial for building future quantum technologies,
including quantum detectors, simulators and computers.