2,472 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
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 qubit bifurcation readout in the straddling regime of circuit QED
We study bifurcation measurement of a multi-level superconducting qubit using
a nonlinear resonator biased in the straddling regime, where the resonator
frequency sits between two qubit transition frequencies. We find that
high-fidelity bifurcation measurements are possible because of the enhanced
qubit-state-dependent pull of the resonator frequency, the behavior of
qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit
that can be realized in this regime. Numerical simulations find up to a
threefold improvement in qubit readout fidelity when operating in, rather than
outside of, the straddling regime. High-fidelity measurements can be obtained
at much smaller qubit-resonator couplings than current typical experimental
realizations, reducing spectral crowding and potentially simplifying the
implementation of multi-qubit devices.Comment: 9 pages, 6 figure
Approaching Unit Visibility for Control of a Superconducting Qubit with Dispersive Readout
In a Rabi oscillation experiment with a superconducting qubit we show that a
visibility in the qubit excited state population of more than 90 % can be
attained. We perform a dispersive measurement of the qubit state by coupling
the qubit non-resonantly to a transmission line resonator and probing the
resonator transmission spectrum. The measurement process is well characterized
and quantitatively understood. The qubit coherence time is determined to be
larger than 500 ns in a measurement of Ramsey fringes.Comment: 4 pages, 5 figures, version with high resolution figures available at
http://www.eng.yale.edu/rslab/Andreas/content/science/PubsPapers.htm
Protocol for universal gates in optimally biased superconducting qubits
We present a new experimental protocol for performing universal gates in a
register of superconducting qubits coupled by fixed on-chip linear reactances.
The qubits have fixed, detuned Larmor frequencies and can remain, during the
entire gate operation, biased at their optimal working point where decoherence
due to fluctuations in control parameters is suppressed to first order.
Two-qubit gates are performed by simultaneously irradiating two qubits at their
respective Larmor frequencies with appropriate amplitude and phase, while
one-qubit gates are performed by the usual single-qubit irradiation pulses
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