1,464 research outputs found
Quantum filter for a non-Markovian single qubit system
In this paper, a quantum filter for estimating the states of a non-Markovian
qubit system is presented in an augmented Markovian system framework including
both the qubit system of interest and multi-ancillary systems for representing
the internal modes of the non-Markovian environment. The colored noise
generated by the multi-ancillary systems disturbs the qubit system via a direct
interaction. The resulting non-Markovian dynamics of the qubit is determined by
a memory kernel function arising from the dynamics of the ancillary system. In
principle, colored noise with arbitrary power spectrum can be generated by a
combination of Lorentzian noises. Hence, the quantum filter can be constructed
for the qubit disturbed by arbitrary colored noise and the conditional state of
the qubit system can be obtained by tracing out the multi-ancillary systems. An
illustrative example is given to show the non-Markovian dynamics of the qubit
system with Lorentzian noise.Comment: arXiv admin note: text overlap with arXiv:1503.0799
Linear feedback stabilization of a dispersively monitored qubit
The state of a continuously monitored qubit evolves stochastically,
exhibiting competition between coherent Hamiltonian dynamics and diffusive
partial collapse dynamics that follow the measurement record. We couple these
distinct types of dynamics together by linearly feeding the collected record
for dispersive energy measurements directly back into a coherent Rabi drive
amplitude. Such feedback turns the competition cooperative, and effectively
stabilizes the qubit state near a target state. We derive the conditions for
obtaining such dispersive state stabilization and verify the stabilization
conditions numerically. We include common experimental nonidealities, such as
energy decay, environmental dephasing, detector efficiency, and feedback delay,
and show that the feedback delay has the most significant negative effect on
the feedback protocol. Setting the measurement collapse timescale to be long
compared to the feedback delay yields the best stabilization.Comment: 16 pages, 7 figure
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