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
12
Jul
2012
Feedback control of a solid-state qubit using high-fidelity projective measurement
We demonstrate feedback control of a superconducting transmon qubit using
discrete, projective measurement and conditional coherent driving. Feedback
realizes a fast and deterministic
qubit reset to a target state with 2.4% error
averaged over input superposition states, and cooling of the transmon from 16%
spurious excitation to 3%. This closed-loop qubit control is necessary for
measurement-based protocols such as quantum error correction and teleportation.
09
Jul
2012
Nonclassical microwave radiation from the dynamical Casimir effect
We investigate quantum correlations in microwave radiation produced by the
dynamical Casimir effect in a superconducting waveguide terminated and
modulated by a superconducting quantum
interference device. We apply
nonclassicality tests and evaluate the entanglement for the predicted field
states. For realistic circuit parameters, including thermal background noise,
the results indicate that the produced radiation can be strictly nonclassical
and can have a measurable amount of intermode entanglement. If measured
experimentally, these nonclassicalilty indicators could give further evidence
of the quantum nature of the dynamical Casimir radiation in these circuits.
06
Jul
2012
Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator
Hybrid quantum systems with inherently distinct degrees of freedom play a key
role in many physical phenomena. A strong coupling can make the constituents
loose their individual character
and form entangled states. The properties of
these collective excitations, such as polaritons of light and phonons in
semiconductors, can combine the benefits of each subsystem. In the emerging
field of quantum information control, a promising direction is provided by the
combination between long-lived atomic states and the accessible electrical
degrees of freedom in superconducting cavities and qubits. Here we demonstrate
the possibility to integrate circuit cavity quantum electrodynamics with
phonons. Besides coupling to a microwave cavity, our superconducting transmon
qubit interacts with a resonant phonon mode in a micromechanical resonator,
allowing the combination of long lifetime, strong tunable coupling, and ease of
access. We measure the phonon Stark shift, as well as the splitting of the
transmon qubit spectral line into motional sidebands representing transitions
between electromechanical polaritons formed by phonons and the qubit. In the
time domain, we observe coherent sideband Rabi oscillations between the qubit
states and phonons. This advance may allow for storage of quantum information
in long-lived phonon states, and for investigations of strongly coupled quantum
systems near the classical limit.
02
Jul
2012
Decoherence of superconducting qubits caused by quasiparticle tunneling
In superconducting qubits, the interaction of the qubit degree of freedom
with quasiparticles defines a fundamental limitation for the qubit coherence.
We develop a theory of the pure
dephasing rate Gamma_{phi} caused by
quasiparticles tunneling through a Josephson junction and of the inhomogeneous
broadening due to changes in the occupations of Andreev states in the junction.
To estimate Gamma_{phi}, we derive a master equation for the qubit dynamics.
The tunneling rate of free quasiparticles is enhanced by their large density of
states at energies close to the superconducting gap. Nevertheless, we find that
Gamma_{phi} is small compared to the rates determined by extrinsic factors in
most of the current qubit designs (phase and flux qubits, transmon, fluxonium).
The split transmon, in which a single junction is replaced by a SQUID loop,
represents an exception that could make possible the measurement of
Gamma_{phi}. Fluctuations of the qubit frequency leading to inhomogeneous
broadening may be caused by the fluctuations in the occupation numbers of the
Andreev states associated with a phase-biased Josephson junction. This
mechanism may be revealed in qubits with small-area junctions, since the
smallest relative change in frequency it causes is of the order of the inverse
number of transmission channels in the junction.
Large Dispersive Shift of Cavity Resonance Induced by a Superconducting Flux Qubit in the Straddling Regime
We demonstrate enhancement of the dispersive frequency shift in a coplanar
waveguide resonator induced by a capacitively-coupled superconducting flux
qubit in the straddling regime.
The magnitude of the observed shift, 80 MHz for
the qubit-resonator detuning of 5 GHz, is quantitatively explained by the
generalized Jaynes-Cummings model which takes into account the contribution of
the qubit higher energy levels. By applying the enhanced dispersive shift to
the qubit readout, we achieved 90% contrast of the Rabi oscillations which is
mainly limited by the energy relaxation of the qubit.
01
Jul
2012
Fast quantum information transfer with superconducting flux qubits coupled to a cavity
We present a way to realize quantum information transfer with superconducting
flux qubits coupled to a cavity. Because only resonant qubit-cavity interaction
and resonant qubit-pulse
interaction are applied, the information transfer can
be performed much faster, when compared with the previous proposals. This
proposal does not require adjustment of the qubit level spacings during the
operation. Moreover, neither uniformity in the device parameters nor exact
placement of qubits in the cavity is needed by this proposal.
26
Jun
2012
Control of microwave signals using circuit nano-electromechanics
and circuit quantum electrodynamics (cQED) [2]. Coupled to
artificial atoms in the form of superconducting"]qubits [3, 4], they now provide
a technologically promising and scalable platform for quantum information
processing tasks [2, 5-8]. Coupling these circuits, in situ, to other quantum
systems, such as molecules [9, 10], spin ensembles [11, 12], quantum dots [13]
or mechanical oscillators [14, 15] has been explored to realize hybrid systems
with extended functionality. Here, we couple a superconducting coplanar
waveguide resonator to a nano-coshmechanical oscillator, and demonstrate
all-microwave field controlled slowing, advancing and switching of microwave
signals. This is enabled by utilizing electromechanically induced transparency
[16-18], an effect analogous to electromagnetically induced transparency (EIT)
in atomic physics [19]. The exquisite temporal control gained over this
phenomenon provides a route towards realizing advanced protocols for storage of
both classical and quantum microwave signals [20-22], extending the toolbox of
control techniques of the microwave field.
25
Jun
2012
State Transfer Between a Mechanical Oscillator and Microwave Fields in the Quantum Regime
or a combination of
refrigeration and laser-like cooling"][2, 3]. This exciting result has
encouraged notions that mechanical oscillators may perform useful functions in
the processing of quantum information with superconducting circuits [1, 4-7],
either by serving as a quantum memory for the ephemeral state of a microwave
field or by providing a quantum interface between otherwise incompatible
systems [8, 9]. As yet, the transfer of an itinerant state or propagating mode
of a microwave field to and from a mechanical oscillator has not been
demonstrated owing to the inability to agilely turn on and off the interaction
between microwave electricity and mechanical motion. Here we demonstrate that
the state of an itinerant microwave field can be coherently transferred into,
stored in, and retrieved from a mechanical oscillator with amplitudes at the
single quanta level. Crucially, the time to capture and to retrieve the
microwave state is shorter than the quantum state lifetime of the mechanical
oscillator. In this quantum regime, the mechanical oscillator can both store
and transduce quantum information.
21
Jun
2012
Weak Qubit Measurement with a Nonlinear Cavity: Beyond Perturbation Theory
We analyze the use of a driven nonlinear cavity to make a weak continuous
measurement of a dispersively-coupled qubit. We calculate the backaction
dephasing rate and measurement rate
beyond leading-order perturbation theory
using a phase-space approach which accounts for cavity noise squeezing.
Surprisingly, we find that increasing the coupling strength beyond the regime
describable by leading-order perturbation theory (i.e. linear response) allows
one to come significantly closer to the quantum limit on the measurement
efficiency. We interpret this behaviour in terms of the non-Gaussian photon
number fluctuations of the nonlinear cavity. Our results are relevant to recent
experiments using superconducting microwave circuits to study quantum
measurement.
20
Jun
2012
Gain, directionality and noise in microwave SQUID amplifiers: Input-output approach
We present a new theoretical framework to analyze microwave amplifiers based
on the dc SQUID. Our analysis applies input-output theory generalized for
Josephson junction devices biased
in the running state. Using this approach we
express the high frequency dynamics of the SQUID as a scattering between the
participating modes. This enables us to elucidate the inherently nonreciprocal
nature of gain as a function of bias current and input frequency. This method
can, in principle, accommodate an arbitrary number of Josephson harmonics
generated in the running state of the junction. We report detailed calculations
taking into account the first few harmonics that provide simple
semi-quantitative results showing a degradation of gain, directionality and
noise of the device as a function of increasing signal frequency. We also
discuss the fundamental limits on device performance and applications of this
formalism to real devices.