Quantum Zeno Dragging of a Qubit State

26 May 2020

S. Hacohen-Gourgy, L. Martin, L. P. Garcia-Pintos, J. Dressel, I. Siddiqi

The quantum Zeno effect is the suppression of Hamiltonian evolution by repeated observation, which pins the system to an eigenstate of the measurement observable. Using measurement alone, control of the state can be achieved if the observable is slowly varied, so that the state tracks the now time-dependent eigenstate. We demonstrate this using a circuit-QED readout technique that couples to a dynamically controllable observable of a qubit. Continuous monitoring of the measurement record allows us to detect an escape from the eigenstate, thus serving as a built-in form of error detection. We show this by post selecting on realizations with high fidelity with respect to the target state. Our dynamical measurement operator technique offers a new tool for numerous forms of quantum feedback protocols, including adaptive measurements and rapid state purification.

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Two example trajectories in the X-Y plane of the Bloch sphere: one illustrating successful Zeno dragging of the qubit state, whose state remains pure, while the other undergoes a quantum jump and continues to get dragged along the opposite pole. Colors in the figure correspond to time evolution. The colored lines outside the Bloch sphere indicate the time axis going from blue for 0 μs to red for 5 μs; these illustrate the position of the measurement axis as function of time. The same colors correspond to the time evolution of the two trajectories shown.