  We have analyzed a dissipative two level quantum system (TLS) which is
continuously and simultaneously irradiated by a high and low frequency
excitation. The interaction of the TLS with a high frequency excitation is
considered in the frame of the dressed state approach. A linear response of the
coupled TLS and corresponding photon field system to a signal whose frequency
is of the order of the Rabi frequency is found. The response exhibits undamped
low frequency oscillations, whose amplitude has a clear resonance at the Rabi
frequency with the width being dependent on the damping rates of the TLS. The
method can be useful for low-frequency Rabi spectroscopy in various physical
systems described by a two-level Hamiltonian, such as nuclei spins in NMR,
double well quantum dots, superconducting flux and charge qubits, etc. The
application of the method to a superconducting flux qubit and to the detection
of NMR is considered in detail.
