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Preserving Known or Unknown Entangled States by Uhrig's Dynamical Decoupling Sequence


Decoherence effects can completely kill quantum entanglement within a very short time.  Hence it will be of vast importance if we can actively preserve quantum entanglement with a high efficiency and with a universal scenario that does not require our knowledge of the bath or of the system-bath coupling.  We show that this is indeed possible, by successfully extending Uhrig's dynamical decoupling sequence from one-qubit cases to general two-qubit systems.  In particular, we explicitly construct control sequences to lock a known but arbitrary two-qubit state to the Nth order of time, using only N control pulses.  We then show that three layers of Uhrig's dynamical decoupling sequence in a special order will be able to preserve an unknown two-qubit entangled state to the Nth order of time, using N^3 control pulses.  The results may be also extended to general multi-level quantum systems.