Illustration: Alan Stonebraker

Figure 1: (a) The projection of an object (i.e., its shadow) in general cannot be inverted to obtain the original object. (b) Simplified diagram of the phase qubit potential during the weak continuous measurement. State $|1〉$ tunnels out with probability $p$, resulting in a detector click. A null measurement with no click will lead to a partial collapse of the initial state towards $|0〉$. (c) After the first null measurement, a $π$-pulse applied to the qubit flips the amplitudes of states $|0〉$ and $|1〉$. The same measurement is now repeated. (d) If the result is also null, then a second $π$-pulse will restore the qubit to its original state—the partial collapse of the wave function has been undone.