Cat-state generation and stabilization for a nuclear spin through electric quadrupole interaction

Abstract

Spin cat states are superpositions of two or more coherent spin states (CSSs) that are distinctly separated over the Bloch sphere. Additionally, the nuclei with angular momenta greater than 1/2 possess a quadrupolar charge distribution. At the intersection of these two phenomena, we devise a simple scheme for generating various types of nuclear spin cat states. The native biaxial electric quadrupole interaction that is readily available in strained solid-state systems plays a key role here. However, the fact that built-in strain cannot be switched off poses a challenge for the stabilization of target cat states once they are prepared. We remedy this by abruptly diverting via a single rotation pulse the state evolution to the neighborhood of the fixed points of the underlying classical Hamiltonian flow. Optimal process parameters are obtained as a function of electric field gradient biaxiality and nuclear spin angular momentum. The overall procedure is seen to be robust under 5% deviations from optimal values. We show that higher level cat states with four superposed CSS can also be formed using three rotation pulses. Finally, for open systems subject to decoherence we extract the scaling of cat state fidelity damping with respect to the spin quantum number. This reveals rates greater than the dephasing of individual CSSs. Yet, our results affirm that these cat states can preserve their fidelities for practically useful durations under the currently attainable decoherence levels.

Quantum information processing with bosonic qubits in circuit QED

“…These ideas are brought to reality by developing robust quantum hardware with both coherent harmonic modes for encoding information as well as non-linear ancillae for effective control and tomography of the encoded information. Such systems have been realized with the motional degree of freedom in trapped ions [72,73], electromagnetic fields of microwave [106] and optical cavities [107], Rydberg atom arrays [108], and flying photons [109], and there are proposals that use other physical systems [110,111] as well. Among the various platforms, the cQED architecture consisting of superconducting microwave cavities and Josephson junction-based non-linear ancillae have enabled many prominent experimental milestones toward realizing QEC using bosonic codes.…”