1 research outputs found
Distinguishing quantum from classical oscillations in a driven phase qubit
Rabi oscillations are coherent transitions in a quantum two-level system
under the influence of a resonant perturbation, with a much lower frequency
dependent on the perturbation amplitude. These serve as one of the signatures
of quantum coherent evolution in mesoscopic systems. It was shown recently [N.
Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005)] that in
phase qubits (current-biased Josephson junctions) this effect can be mimicked
by classical oscillations arising due to the anharmonicity of the effective
potential. Nevertheless, we find qualitative differences between the classical
and quantum effect. First, while the quantum Rabi oscillations can be produced
by the subharmonics of the resonant frequency (multiphoton processes), the
classical effect also exists when the system is excited at the overtones.
Second, the shape of the resonance is, in the classical case,
characteristically asymmetric; while quantum resonances are described by
symmetric Lorentzians. Third, the anharmonicity of the potential results in the
negative shift of the resonant frequency in the classical case, in contrast to
the positive Bloch-Siegert shift in the quantum case. We show that in the
relevant range of parameters these features allow to confidently distinguish
the bona fide Rabi oscillations from their classical Doppelganger.Comment: 8 pages, 4 figures; v2: minor corrections, Fig.1 added, introduction
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