Amplitude spectroscopy of a superconducting artificial atom

We introduce and demonstrate amplitude spectroscopy in a superconducting artificial atom [1]. A harmonic field at a fixed frequency drives the artificial atom through its energy-level avoided crossings. Spectroscopic information is obtained from the amplitude dependence of the system response. The resulting ``spectroscopy diamonds,'' regions of parameters space in which state transitions occur, exhibit quantum interference patterns and population inversion which serve as a fingerprint of the atom's energy spectrum. Using this approach, we determined the energy spectrum of a manifold of states with energies from $h $x 0.01 GHz to $h $x 120 GHz for a fixed driving frequency near only 0.16 GHz. The amplitude spectroscopy technique is complementary to frequency spectroscopy, providing a means to access, manipulate, and characterize quantum systems over broad bandwidths while using only a single drive frequency that may be orders of magnitude smaller than the energy scales being probed. [1] Berns et al., Nature 455, 51 (2008)