Optimal control of molecular spin qudits

Abstract

We demonstrate, numerically, the possibility of manipulating the spin states of molecular nanomagnets with shaped microwave pulses designed with quantum optimal control theory techniques. The state-to-state or full gate transformations can be performed in this way in shorter times than using simple monochromatic resonant pulses. This enhancement in the operation rates can therefore mitigate the effect of decoherence. The optimization protocols and their potential for practical implementations are illustrated by simulations performed for a simple molecular cluster hosting a single Gd 3+ ion. Its eight accessible levels (corresponding to a total spin S = 7/2) allow encoding an 8-level qudit or a system of three coupled qubits. All necessary gates required for universal operation can be obtained with optimal pulses using the intrinsic couplings present in this system. The application of optimal control techniques can facilitate the implementation of quantum technologies based on molecular spin qudits.

Floquet engineering with quantum optimal control theory

“…If the gradient of G can be computed at a reasonable cost, the algorithms that use the gradient will be more effective than the gradient-free ones. QOCT provides one expression for this gradient [18,19]:…”

“…Let us now assume that one cannot, as in the previous section, write F as an explicit function of U, which would lead to a simple expression for the boundary condition (24) such as equation (19). We face the problem of computing the gradient elements ∂F ∂U * ij .…”

Quantum optimal control in quantum technologies. Strategic report on current status, visions and goals for research in Europe

“…For example, triply charged lanthanide ions are a popular quantum platform. Pulses to carry out all quantum gate operations have recently been calculated for the example of gadolinium ions [119]. Microwave coherent control of the initialization, operation, and readout of the electronic spin state in erbium dopants has been demonstrated [148].…”

“…The generation of NOT and CNOT gates with different optimization methods has been compared [486]. Specific gate transformations have also been optimized for qudits, for example encoded in atomic [439] or molecular spins [119]. Another generalization beyond targeting specific gates is to optimize for a continuous family of gates [471,502], for example as a function of continuous system or gate parameters.…”