ИСТИНА

We analyze the control of qubit states by short unipolar magnetic field pulses. In superconducting technologies, such an control with a picosecond timescale can be realized using fluxons in rapid single-flux-quantum logic circuits. We proposed a new approach to the theoretical analysis of such a procedure, based on the Magnus representation for the evolution operator (propagator) of qubits. An expression was obtained for the propagator in the form of a polynomial in powers of the Magnus matrix. This allows us to find a solution to the problem of fast quantum control outside the framework of perturbation theory. As an example, the switching between single qubit states was considered. An analog of the known pi-pulses in the Rabi-technique is found. This work was supported by the Ministry of Education of the Russian Federation (contract # 3.3026.2017), by the RFBR (grants No. 17-12-01079, 16-29-09515, and 16-07-01012 NIF), and by the RNF No. 18-72-00158.