  In a direct two-level qubit system, when the Rabi frequency is comparable to
the resonance frequency, the rotating wave approximation is not appropriate. In
this case, the Rabi oscillation is accompanied by another oscillation at twice
the frequency of the driving field, the so called Bloch-Siegert oscillation
(BSO), which depends on the initial phase of the driving field. This
oscillation may restrict the precise rotation of a qubit made of direct
two-level system. Here, we show that in case of an effectively two-level lambda
system, the BSO is inherently negligible, implying a greater precision for
rotation of a qubit made of such a lambda system when compared to a direct
two-level qubit in a strong driving field.
