  The dynamics of a two-level spin system dressed by bichromatic radiation is
studied under the conditions of double resonance when the frequency of one
(microwave) field is equal to the Larmor frequency of the spin system and the
frequency of the other (radio-frequency) field \omega_{rf} is close to the Rabi
frequency \omega_{1} in a micro-wave field. It is shown theoretically that Rabi
oscillations between dressed-spin states with the frequency \epsilon are
accompanied by higher-frequency oscillations at frequencies n\omega_{rf} and
n\omega_{rf}\pm \epsilon, where n = 1, 2, .... The most intense among these are
the signals corresponding to n = 1. The counter-rotating (antiresonance)
components of the RF field give rise to a shift of the dressed-state energy,
i.e., to a frequency shift similar to the Bloch-Siegert shift. In particular,
this shift is manifested as the dependence of the Rabi-oscillation frequency
\epsilon on the sign of the detuning \omega{1} -\omega{rf} from resonance. In
the case of double resonance, the oscillation amplitude is asymmetric; i.e.,
the amplitude at the sum frequency \omega_{rf} +\epsilon increases, while the
amplitude at the difference frequency \omega_{rf} -\epsilon decreases. The
predicted effects are confirmed by observations of the nutation signals of the
electron paramagnetic resonance (EPR) of centers in quartz and should be taken
into account to realize qubits with a low Rabi frequency in solids.
