J. Leppäkangas, S. E. de Graaf, A. Adamyan, M. Fogelström, A. V. Danilov, T. Lindström, S. E. Kubatkin, G. Johansson
We experimentally study a superconducting charge qubit coupled to a driven cavity, including drive strengths large enough to energetically allow for quasiparticle creation. To explain the observed effects we develop a reduced density matrix theory for the dressed charge states that accounts for tunneling of existing quasiparticles as well as the creation of new quasiparticles at the Josephson junction. Quasiparticle tunneling leads to incoherent transitions between the even and odd parity state of the island. The dressed charge state dynamics in each separate parity state can be reduced to a two-level system. At weak drives, the population of each two-level system is determined by the dissipation induced by the charge-noise environment. At dressed state resonances, there is population inversion in one of the parity subspaces. This gives a large sensitivity to changes in the gate charge, which can be used for charge detection. The gate sensitivity is reduced by non-equilibrium quasiparticles, which induces incoherent transitions between the two parities, thus reducing the time spent in the sensitive parity state. At higher drives, the dissipation from quasiparticle creation through photon-assisted tunneling dominates over the charge-noise environment. This establishes a new population inversion, as well as a very fast relaxation channel to the sensitive parity state, practically eliminating the probability for the island to be in the insensitive parity state. In this regime, the dynamics is thus robust against externally injected quasiparticles and the dynamics prevail over a wide range of temperatures. We find very good agreement between theory and experiment over a wide range of drive strengths and temperatures.
View original:
http://arxiv.org/abs/1306.4200
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