High-fidelity quantum information processing with composite pulses
Prof. Dr. Nikolay Vitanov (Quantum Optics and Quantum Information, Sofia)

The technique of composite pulses, developed originally in polarization optics and nuclear magnetic resonance (NMR), is a powerful tool for quantum state manipulation. . This technique replaces the single pulse used traditionally for driving a two-state quantum transition by a sequence of pulses with suitably chosen phases, which are used as a control tool for shaping the excitation profile in a desired manner. This technique combines the accuracy of resonant excitation with a robustness similar to adiabatic techniques. We have developed a simple systematic approach, which allows the construction of composite sequences of pulses with smooth shapes and time-dependent detuning that can create ultrahigh-fidelity excitation profiles. Our method uses the SU(2) representation of the propagator of the two-state system, instead of the commonly used intuitive Bloch SO(3) rotations. We have designed arbitrarily accurate broadband, narrowband, passband and fractional-pi composite pulses. In one of the applications, composite sequences can reduce dramatically the addressing error in a lattice of closely spaced atoms or ions, and at the same time greatly enhance the robustness of qubit manipulations. One can thereby beat the diffraction limit, for only atoms situated in a small spatial range around the center of the laser beam are excited. We have used composite sequences of chirped pulses to optimize the technique of adiabatic passage between two quantum states: composite adiabatic passage (CAP), in which nonadiabatic losses can be canceled to any desired order.