19,405 research outputs found
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
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
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
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
Dynamical Casimir effect entangles artificial atoms
We show that the physics underlying the dynamical Casimir effect may generate
multipartite quantum correlations. To achieve it, we propose a circuit quantum
electrodynamics (cQED) scenario involving superconducting quantum interference
devices (SQUIDs), cavities, and superconducting qubits, also called artificial
atoms. Our results predict the generation of highly entangled states for two
and three superconducting qubits in different geometric configurations with
realistic parameters. This proposal paves the way for a scalable method of
multipartite entanglement generation in cavity networks through dynamical
Casimir physics.Comment: Improved version and references added. Accepted for publication in
Physical Review Letter
Optical Versus Mid-Infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies
The origin of huge infrared luminosities of ultraluminous infrared galaxies
(ULIGs) is still in question. Recently, Genzel et al. made mid-infrared (MIR)
spectroscopy of a large number of ULIGs and found that the major energy source
in them is massive stars formed in the recent starburst activity; i.e.,
70% -- 80% of the sample are predominantly powered by the starburst. However,
it is known that previous optical spectroscopic observations showed that the
majority of ULIGs are classified as Seyferts or LINERs (low-ionization nuclear
emission-line regions). In order to reconcile this difference, we compare types
of emission-line activity for a sample of ULIGs which have been observed in
both optical and MIR. We confirm the results of previous studies that the
majority of ULIGs classified as LINERs based on the optical emission-line
diagnostics turn to be starburst-dominated galaxies based on the MIR ones.
Since the MIR spectroscopy can probe more heavily-reddened, inner parts of the
ULIGs, it is quite unlikely that the inner parts are powered by the starburst
while the outer parts are powered by non-stellar ionization sources. The most
probable resolution of this dilemma is that the optical emission-line nebulae
with the LINER properties are powered predominantly by shock heating driven by
the superwind activity; i.e., a blast wave driven by a collective effect of a
large number of supernovae in the central region of galaxy mergers.Comment: 15 pages, 2 tables, and 3 eps figures. The Astrophysical Journal
(Part 1), in pres
Measuring non-Gaussian fluctuations through incoherent Cooper pair current
We study a Josephson junction (JJ) in the regime of incoherent Cooper pair
tunneling, capacitively coupled to a nonequilibrium noise source. The
current-voltage (I-V) characteristics of the JJ are sensitive to the excess
voltage fluctuations in the source, and can thus be used for wide-band noise
detection. Under weak driving, the odd part of the I-V can be related to the
second cumulant of noise, whereas the even part is due to the third cumulant.
After calibration, one can measure the Fano factors for the noise source, and
get information about the frequency dependence of the noise.Comment: 4 pages, 4 figure
Enhancing the conductance of a two-electron nanomechanical oscillator
We consider electron transport through a mobile island (i.e., a
nanomechanical oscillator) which can accommodate one or two excess electrons
and show that, in contrast to immobile islands, the Coulomb blockade peaks,
associated with the first and second electrons entering the island, have
different functional dependences on the nano-oscillator parameters when the
island coupling to its leads is asymmetric. In particular, the conductance for
the second electron (i.e., when the island is already charged) is greatly
enhanced in comparison to the conductance of the first electron in the presence
of an external electric field. We also analyze the temperature dependence of
the two conduction peaks and show that these exhibit different functional
behaviors.Comment: 16 pages, 5 figure
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