Red shift of the superconductivity cavity resonance in Josephson junction qubits as a direct signature of TLS population inversion
arxiv(2024)
摘要
Quantum two-level systems (TLSs) limit the performance of superconducting
qubits and superconducting and optomechanical resonators breaking down the
coherence and absorbing the energy of oscillations. TLS absorption can be
suppressed or even switched to the gain regime by inverting TLS populations.
Here we theoretically explore the regime where the full inversion of TLS
populations is attained at energies below a pump field quantization energy by
simultaneously applying the pump field and the time varying bias. This regime
is attained changing the bias sufficiently slowly to fully invert TLS
populations when their energies cross resonance with the pump field and
sufficiently fast to avoid TLS relaxation between two resonance crossing
events. This population inversion is accompanied by a significant red shift of
cavity resonance due to quantum level repulsion. The red-shift in frequency
serves as a signature of the population inversion, as its re-entrant behavior
as function of bias sweep rate and of the magnitude of the pump field allows
the determination of the TLSs dipole moment and relaxation time. The predicted
behavior is qualitatively consistent with the recent experimental observations
in Al superconducting resonators.
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