Speeding up adiabatic passage with an optimal modified Roland-Cerf protocol

In this article we propose a novel method to accelerate adiabatic passage in a two-level system with only longitudinal field (detuning) control, while the transverse field is kept constant. The suggested method is a modification of the Roland-Cerf protocol, during which the parameter quantifying local adiabaticity is held constant. Here, we show that with a simple 'on-off' modulation of this local adiabaticity parameter, a perfect adiabatic passage can be obtained for every duration larger than the lower bound pi/Omega, where Omega is the constant transverse field. For a fixed maximum amplitude of the local adiabaticity parameter, the timings of the 'on-off' pulse-sequence which achieves perfect fidelity in minimum time are obtained using optimal control theory. The corresponding detuning control is continuous and monotonic, a significant advantage compared to the detuning variation at the quantum speed limit which includes non-monotonic jumps. The proposed methodology can be applied in several important core tasks in quantum computing, for example to the design of a high fidelity controlled-phase gate, which can be mapped to the adiabatic quantum control of such a qubit. Additionally, it is expected to find applications across all physics disciplines which exploit the adiabatic control of such a two-level system.