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Journal of Physics B Atomic Molecular and Optical Physics

Dynamical suppression of stochastic dephasing of a qubit

Masashi Ban, Fumiaki Shibata, Sachiko Kitajima

Published Apr 18, 2007• 6 citations
Publisher

Abstract

It is shown that the stochastic dephasing of a qubit caused by the random frequency modulation in which the stochastic variable obeys the stationary Gauss–Markov process can be suppressed by applying a sequence of π pulses to the qubit. It is found that the application of around ten π pulses within the correlation time of the stochastic variable is enough to suppress the decoherence of the qubit. The recovery of the entanglement of the Werner state by means of π pulses is also discussed.

DOI
10.1088/0953-4075/40/9/s15

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It is shown that the stochastic dephasing of a qubit caused by the random frequency modulation in which the stochastic variable obeys the stationary Gauss–Markov process can be suppressed by applying a sequence of π pulses to the qubit. It is found that the application of around ten π pulses within the correlation time of the stochastic variable is enough to suppress the decoherence of the qubit. The recovery of the entanglement of the Werner state by means of π pulses is also discussed.
Decoherence of qubits under the influence of random phase fluctuations is studied by means of the stochastic Liouville equation, where the phase fluctuations are characterized by means of the Gauss-Markov process and the two-state-jump Markov process. We investigate how an observation of a stochastic variable during the time evolution influences the decoherence of the qubit system. We examine the decays of distinguishability and entanglement of qubit states. It is shown that the decoherence of qubits can be suppressed by the observation of the stochastic variable. It is also found that the suppression of the decoherence does not depend on the observed value.
An irreversible time evolution of a quantum system interacting with a surrounding environment is described by a quantum master equation. When an external field is applied to a quantum system, a non-Markovian master equation is derived in a rigorous way, where the relaxation terms in the quantum master equation include the effect of the external field that is ignored in the conventional treatment. It is shown that when the external field is a sequence of phase-modulation π-pulses, the decoherence of the quantum system can be suppressed. In particular, when the pulse separation is sufficiently short in comparison with the correlation time of the environmental system, dynamical decoupling takes place. To see the effect of the phase-modulation π-pulses, the time evolution of qubit entanglement is investigated in detail.
The reduced time evolution of a single qubit under the influence of a classical environment is studied by means of the stochastic Liouville equation. The qubit initially correlates with the environment, which is created by a preparation process of an initial state. The phase fluctuation caused by the environment is characterized by means of the Gauss-Markov process and the two-state-jump Markov process. It is shown that depending on values of the parameters, the initial correlation enhances or reduces the decoherence of the qubit. In the case of the slow modulation, the effect of the initial preparation of the qubit becomes significant and it cannot be ignored in the reduced time evolution of the qubit.
The time evolution of the trace distance between quantum states of a qubit which is placed under the influence of stochastic dephasing is investigated within the framework of the stochastic Liouville equation. When stochastic dephasing is subject to the homogeneous Gauss-Markov process, the trace distance is exactly calculated in the presence of the initial correlation between the qubit and the stochastic process, where the stochastic process is inevitably a nonstationary process. It is found that even the initial correlation with the classical environment can make the trace distance greater than the initial value if stochastic dephasing causes the slow modulation of the qubit.
Time evolutions of quantum correlation, classical correlation and total correlation of two qubits are investigated when the two qubits are placed under the influence of classical phase fluctuations with correlation. Stochastic variables that describe the phase fluctuations are correlated and subject to the stationary Gauss–Markov process. The model includes the local and global dephasing models. It is shown that the quantum correlation measured by the quantum discord is increased by the correlation between the stochastic variables in the initial time region while the classical correlation and the total correlation are not. Furthermore the entanglement, the optimal fidelity of the quantum teleportation and the violation of the Bell inequality are investigated.
Detailed studies are carried out for the problem of decoherence control by a successive application of ?-pulses when a relevant spin-1/2 (qubit) system is surrounded by a stochastically fluctuating environment. Specifically, considerations are given to two representative stochastic processes: a two-state-jump Markov process and a Gauss?Markov process. Exact analytical solutions are obtained for the decoherence control by the ?-pulse application even for non-stationary stochastic processes as well as for stationary processes. In view of recent advances in dynamical decoupling experiments with artificial (engineered) stochastic noise, the results are useful in analysing qubit systems under the influence of a fluctuating environment.