3,089 research outputs found
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
The interpretation of non-Markovian stochastic Schr\"odinger equations as a hidden-variable theory
Do diffusive non-Markovian stochastic Schr\"odinger equations (SSEs) for open
quantum systems have a physical interpretation? In a recent paper [Phys. Rev. A
66, 012108 (2002)] we investigated this question using the orthodox
interpretation of quantum mechanics. We found that the solution of a
non-Markovian SSE represents the state the system would be in at that time if a
measurement was performed on the environment at that time, and yielded a
particular result. However, the linking of solutions at different times to make
a trajectory is, we concluded, a fiction. In this paper we investigate this
question using the modal (hidden variable) interpretation of quantum mechanics.
We find that the noise function appearing in the non-Markovian SSE can
be interpreted as a hidden variable for the environment. That is, some chosen
property (beable) of the environment has a definite value even in the
absence of measurement on the environment. The non-Markovian SSE gives the
evolution of the state of the system ``conditioned'' on this environment hidden
variable. We present the theory for diffusive non-Markovian SSEs that have as
their Markovian limit SSEs corresponding to homodyne and heterodyne detection,
as well as one which has no Markovian limit.Comment: 9 page
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
Implementing optimal control pulse shaping for improved single-qubit gates
We employ pulse shaping to abate single-qubit gate errors arising from the
weak anharmonicity of transmon superconducting qubits. By applying shaped
pulses to both quadratures of rotation, a phase error induced by the presence
of higher levels is corrected. Using a derivative of the control on the
quadrature channel, we are able to remove the effect of the anharmonic levels
for multiple qubits coupled to a microwave resonator. Randomized benchmarking
is used to quantify the average error per gate, achieving a minimum of
0.007+/-0.005 using 4 ns-wide pulse.Comment: 4 pages, 4 figure
Oil Presses.
1. Introduction; 2. Processes for obtaining vegetable oils; 3. Detailing the continuous mechanical pressing; 4. Examples on the application of pressing for obtaining oil from cotton, peanut and sunflower; 5. Conclusion
A perturbative approach to non-Markovian stochastic Schr\"odinger equations
In this paper we present a perturbative procedure that allows one to
numerically solve diffusive non-Markovian Stochastic Schr\"odinger equations,
for a wide range of memory functions. To illustrate this procedure numerical
results are presented for a classically driven two level atom immersed in a
environment with a simple memory function. It is observed that as the order of
the perturbation is increased the numerical results for the ensembled average
state approach the exact reduced state found via
Imamo\=glu's enlarged system method [Phys. Rev. A. 50, 3650 (1994)].Comment: 17 pages, 4 figure
Randomized benchmarking and process tomography for gate errors in a solid-state qubit
We present measurements of single-qubit gate errors for a superconducting
qubit. Results from quantum process tomography and randomized benchmarking are
compared with gate errors obtained from a double pi pulse experiment.
Randomized benchmarking reveals a minimum average gate error of 1.1+/-0.3% and
a simple exponential dependence of fidelity on the number of gates. It shows
that the limits on gate fidelity are primarily imposed by qubit decoherence, in
agreement with theory.Comment: 4 pages, 4 figures, plus supplementary materia
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