3,210 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
Robust randomized benchmarking of quantum processes
We describe a simple randomized benchmarking protocol for quantum information
processors and obtain a sequence of models for the observable fidelity decay as
a function of a perturbative expansion of the errors. We are able to prove that
the protocol provides an efficient and reliable estimate of an average
error-rate for a set operations (gates) under a general noise model that allows
for both time and gate-dependent errors. We determine the conditions under
which this estimate remains valid and illustrate the protocol through numerical
examples.Comment: 4+ pages, 1 figure, and 1 tabl
Tunable coupling in circuit quantum electrodynamics with a superconducting V-system
Recent progress in superconducting qubits has demonstrated the potential of
these devices for the future of quantum information processing. One desirable
feature for quantum computing is independent control of qubit interactions as
well as qubit energies. We demonstrate a new type of superconducting charge
qubit that has a V-shaped energy spectrum and uses quantum interference to
provide independent control over the qubit energy and dipole coupling to a
superconducting cavity. We demonstrate dynamic access to the strong coupling
regime by tuning the coupling strength from less than 200 kHz to more than 40
MHz. This tunable coupling can be used to protect the qubit from cavity-induced
relaxation and avoid unwanted qubit-qubit interactions in a multi-qubit system.Comment: 5 pages, 4 figure
Pure-state quantum trajectories for general non-Markovian systems do not exist
Since the first derivation of non-Markovian stochastic Schr\"odinger
equations, their interpretation has been contentious. In a recent Letter [Phys.
Rev. Lett. 100, 080401 (2008)], Di\'osi claimed to prove that they generate
"true single system trajectories [conditioned on] continuous measurement". In
this Letter we show that his proof is fundamentally flawed: the solution to his
non-Markovian stochastic Schr\"odinger equation at any particular time can be
interpreted as a conditioned state, but joining up these solutions as a
trajectory creates a fiction.Comment: 4 page
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