= Ising spin echo
{c}
{title2=$e^{-iH_I\tau/2}P e^{-iH_I\tau/2}P^\dagger=I$}
A $\pi$ <rotation about the x-axis> on one qubit conjugates $Z\otimes Z$ to its negative. With ideal instantaneous pulses, two equal free-evolution intervals separated by this pulse and its inverse cancel an <Ising coupling> exactly. Finite-duration pulses require accounting for the coupling during the pulses; the error is small only when $|J|$ times the pulse duration is small, or when the control is explicitly designed to compensate it.
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