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Abstract
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The problem of combating de-coherence by weak measurements has already
been studied for the amplitude damping channel and for specific input states.
We generalize this to a large four-parameter family of qubit channels and for
the average fidelity over all pure states. As a by-product we classify all the
qubit channels which have one invariant pure state and show that the param-
eter manifold of these channels is isomorphic to S2 × S1 × S1 and contains
many interesting sub-classes of channels. By tuning the parameter of the weak
measurement, we show that it is possible to increase the average input–output
fidelity without blocking too many particles to pass through the channel. Quan-
titative analysis reveals that the increase of average fidelity can be up to 0.3, by
blocking less than half of the particles.
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