High-fidelity quantum control and quantum information processing with composite pulses

 The technique of composite pulses, developed originally in polarization optics and nuclear magnetic resonance (NMR), is a powerful tool for quantum state manipulation. It 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 SU(2) approach, which allows the construction of composite sequences of pulses with smooth shapes and time-dependent detuning that can create ultrahigh-fidelity excitation profiles. 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. 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. We have also developed composite STIRAP – a hybrid technique which uses a sequence of pulse pairs to make STIRAP work at ultrahigh fidelity. We have used composite pulses to design new, more efficient implementations of highly entangled states and highly-conditional quantum gates, such as CNOT and Toffoli gates. We have constructed also composite pulse sequences which allow to suppress dynamically unwanted transition channels in complex systems with branched linkage patterns even when the relevant couplings are unknown. Some of these composite sequences have already been demonstrated experimentally in doped solids.