27,179 research outputs found
Optomechanical-like coupling between superconducting resonators
We propose and analyze a circuit that implements a nonlinear coupling between
two superconducting microwave resonators. The resonators are coupled through a
superconducting quantum interference device (SQUID) that terminates one of the
resonators. This produces a nonlinear interaction on the standard
optomechanical form, where the quadrature of one resonator couples to the
photon number of the other resonator. The circuit therefore allows for
all-electrical realizations of analogs to optomechanical systems, with coupling
that can be both strong and tunable. We estimate the coupling strengths that
should be attainable with the proposed device, and we find that the device is a
promising candidate for realizing the single-photon strong-coupling regime. As
a potential application, we discuss implementations of networks of
nonlinearly-coupled microwave resonators, which could be used in
microwave-photon based quantum simulation.Comment: 10 pages, 7 figure
Dispersive Charge and Flux Qubit Readout as a Quantum Measurement Process
We analyze the dispersive readout of superconducting charge and flux qubits
as a quantum measurement process. The measurement oscillator frequency is
considered much lower than the qubit frequency. This regime is interesting
because large detuning allows for strong coupling between the measurement
oscillator and the signal transmission line, thus allowing for fast readout.
Due to the large detuning we may not use the rotating wave approximation in the
oscillator-qubit coupling. Instead we start from an approximation where the
qubit follows the oscillator adiabatically, and show that non-adiabatic
corrections are small. We find analytic expressions for the measurement time,
as well as for the back-action, both while measuring and in the off-state. The
quantum efficiency is found to be unity within our approximation, both for
charge and flux qubit readout.Comment: 26 pages, 3 figures, To be published in Journal of Low Temperature
Physic
The dynamical Casimir effect in superconducting microwave circuits
We theoretically investigate the dynamical Casimir effect in electrical
circuits based on superconducting microfabricated waveguides with tunable
boundary conditions. We propose to implement a rapid modulation of the boundary
conditions by tuning the applied magnetic flux through superconducting quantum
interference devices (SQUIDs) that are embedded in the waveguide circuits. We
consider two circuits: (i) An open waveguide circuit that corresponds to a
single mirror in free space, and (ii) a resonator coupled to a microfabricated
waveguide, which corresponds to a single-sided cavity in free space. We analyze
the properties of the dynamical Casimir effect in these two setups by
calculating the generated photon-flux density, output-field correlation
functions, and the quadrature squeezing spectra. We show that these properties
of the output field exhibit signatures unique to the radiation due to the
dynamical Casimir effect, and could therefore be used for distinguishing the
dynamical Casimir effect from other types of radiation in these circuits. We
also discuss the similarities and differences between the dynamical Casimir
effect, in the resonator setup, and downconversion of pump photons in
parametric oscillators.Comment: 18 pages, 14 figure
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.Comment: 5 pages, 3 figure
Self Interference of Single Electrodynamic Particle in Double Slit
It is by the long established fact in experiment and theory that
electromagnetic waves, here as one component of an IED particle, passing a
double slit will undergo self inference each, producing at a detector plane
fringed intensities. The wave generating point charge of a zero rest mass, as
the other component of the particle, is maintained a constant energy and speed
by a repeated radiation reabsorption/reemission scheme, and in turn steered in
direction in its linear motion by the reflected radiation field, and will
thereby travel to the detector along (one of) the optical path(s) of the waves
leading to a bright interference fringe. We elucidate the process formally
based on first principles solutions for the IED particle and known principles
for wave-matter interaction.Comment: Presentation at The 6th Int. Symp. Quantum Theory and Symmetries,
Univ. Kent, 2009
Readout methods and devices for Josephson-junction-based solid-state qubits
We discuss the current situation concerning measurement and readout of
Josephson-junction based qubits. In particular we focus attention of dispersive
low-dissipation techniques involving reflection of radiation from an oscillator
circuit coupled to a qubit, allowing single-shot determination of the state of
the qubit. In particular we develop a formalism describing a charge qubit read
out by measuring its effective (quantum) capacitance. To exemplify, we also
give explicit formulas for the readout time.Comment: 20 pages, 7 figures. To be published in J. Phys.: Condensed Matter,
18 (2006) Special issue: Quantum computin
Coherent multiple Andreev reflections and current resonances in SNS junctions
We study coherent multiple Andreev reflections in quantum SNS junctions of
finite length and arbitrary transparency. The presence of superconducting bound
states in these junctions gives rise to great enhancement of the subgap
current. The effect is most pronounced in low-transparency junctions, ,
and in the interval of applied voltage , where the
amplitude of the current structures is proportional to the first power of the
junction transparency . The resonant current structures consist of steps and
oscillations of the two-particle current and also of multiparticle resonance
peaks. The positions of the two-particle current structures have pronounced
temperature dependence which scales with , while the positions of
the multiparticle resonances have weak temperature dependence, being mostly
determined by the junction geometry. Despite the large resonant two-particle
current, the excess current at large voltage is small and proportional to
. Pacs: 74.50.+r, 74.80.Fp, 74.20.Fg, 73.23.AdComment: 23 pages, 16 figure
Undoing measurement-induced dephasing in circuit QED
We analyze the backaction of homodyne detection and photodetection on
superconducting qubits in circuit quantum electrodynamics. Although both
measurement schemes give rise to backaction in the form of stochastic phase
rotations, which leads to dephasing, we show that this can be perfectly undone
provided that the measurement signal is fully accounted for. This result
improves upon that of Phys. Rev. A, 82, 012329 (2010), showing that the method
suggested can be made to realize a perfect two-qubit parity measurement. We
propose a benchmarking experiment on a single qubit to demonstrate the method
using homodyne detection. By analyzing the limited measurement efficiency of
the detector and bandwidth of the amplifier, we show that the parameter values
necessary to see the effect are within the limits of existing technology
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