3,659 research outputs found
A superconducting qubit with Purcell protection and tunable coupling
We present a superconducting qubit for the circuit quantum electrodynamics
architecture that has a tunable coupling strength g. We show that this coupling
strength can be tuned from zero to values that are comparable with other
superconducting qubits. At g = 0 the qubit is in a decoherence free subspace
with respect to spontaneous emission induced by the Purcell effect. Furthermore
we show that in the decoherence free subspace the state of the qubit can still
be measured by either a dispersive shift on the resonance frequency of the
resonator or by a cycling-type measurement.Comment: 4 pages, 3 figure
Apple of Gold and Picture of Silver: How Abraham Lincoln Would Analyze the Fourteenth Amendment\u27s Equal Protection Clause
RUMILLY, (Robert), de l’Académie canadienne-française, Histoire de la province de Québec, tome XX : Philippe Landry, Montréal 1948, 211 p.
Tunable joint measurements in the dispersive regime of cavity QED
Joint measurements of multiple qubits have been shown to open new
possibilities for quantum information processing. Here, we present an approach
based on homodyne detection to realize such measurements in the dispersive
regime of cavity/circuit QED. By changing details of the measurement, the
readout can be tuned from extracting only single-qubit to only multi-qubit
properties. We obtain a reduced stochastic master equation describing this
measurement and its effect on the qubits. As an example, we present results
showing parity measurements of two qubits. In this situation, measurement of an
initially unentangled state can yield with near unit probability a state of
significant concurrence.Comment: 4 pages, 4 figure
Alien Registration- Blais, Marie J. (Lewiston, Androscoggin County)
https://digitalmaine.com/alien_docs/30724/thumbnail.jp
Signatures of Hong-Ou-Mandel Interference at Microwave Frequencies
Two-photon quantum interference at a beam splitter, commonly known as
Hong-Ou-Mandel interference, was recently demonstrated with
\emph{microwave-frequency} photons by Lang \emph{et
al.}\,\cite{lang:microwaveHOM}. This experiment employed circuit QED systems as
sources of microwave photons, and was based on the measurement of second-order
cross-correlation and auto-correlation functions of the microwave fields at the
outputs of the beam splitter. Here we present the calculation of these
correlation functions for the cases of inputs corresponding to: (i) trains of
\emph{pulsed} Gaussian or Lorentzian single microwave photons, and (ii)
resonant fluorescent microwave fields from \emph{continuously-driven} circuit
QED systems. The calculations include the effects of the finite bandwidth of
the detection scheme. In both cases, the signature of two-photon quantum
interference is a suppression of the second-order cross-correlation function
for small delays. The experiment described in Ref.
\onlinecite{lang:microwaveHOM} was performed with trains of \emph{Lorentzian}
single photons, and very good agreement between the calculations and the
experimental data was obtained.Comment: 11 pages, 3 figure
Improved Superconducting Qubit Readout by Qubit-Induced Nonlinearities
In dispersive readout schemes, qubit-induced nonlinearity typically limits
the measurement fidelity by reducing the signal-to-noise ratio (SNR) when the
measurement power is increased. Contrary to seeing the nonlinearity as a
problem, here we propose to use it to our advantage in a regime where it can
increase the SNR. We show analytically that such a regime exists if the qubit
has a many-level structure. We also show how this physics can account for the
high-fidelity avalanchelike measurement recently reported by Reed {\it et al.}
[arXiv:1004.4323v1].Comment: 4 pages, 5 figure
Optimal Modeling and Filtering of Stochastic Time Series for Geoscience Applications
Sequences of observations or measurements are often modeled as realizations of stochastic processes with some stationary properties in the first and second moments. However in practice, the noise biases and variances are likely to be different for different epochs in time or regions in space, and hence such stationarity assumptions are often questionable. In the case of strict stationarity with equally spaced data, the Wiener-Hopf equations can readily be solved with fast Fourier transforms (FFTs) with optimal computational efficiency. In more general contexts, covariance matrices can also be diagonalized using the Karhunen-Loève transforms (KLTs), or more generally using empirical orthogonal and biorthogonal expansions, which are unfortunately much more demanding in terms of computational efforts. In cases with increment stationarity, the mathematical modeling can be modified and generalized covariances can be used with some computational advantages. The general nonlinear solution methodology is also briefly overviewed with the practical limitations. These different formulations are discussed with special emphasis on the spectral properties of covariance matrices and illustrated with some numerical examples. General recommendations are included for practical geoscience applications
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