Steering Quantum Dynamics via Bang-Bang Control: Implementing optimal fixed point quantum search algorithm

摘要

A robust control over quantum dynamics is of paramount importance for quantumtechnologies. Many of the existing control techniques are based on smoothHamiltonian modulations involving repeated calculations of basic unitariesresulting in time complexities scaling rapidly with the length of the controlsequence. On the other hand, the bang-bang controls need one-time calculationof basic unitaries and hence scale much more efficiently. By employing a globaloptimization routine such as the genetic algorithm, it is possible tosynthesize not only highly intricate unitaries, but also certain nonunitaryoperations. Here we demonstrate the unitary control through the firstimplementation of the optimal fixed-point quantum search algorithm in athree-qubit NMR system. More over, by combining the bang-bang pulses with thetwirling process, we also demonstrate a nonunitary transformation of thethermal equilibrium state into an effective pure state in a five-qubit NMRsystem.