A ug 2 00 3 Characterising a solid state qubit via environmental noise

We propose a method for characterising the energy level structure of a solid-state qubit by monitoring the noise level in its environment. We consider a model persistent-current qubit in a lossy resevoir and demonstrate that the noise in a classical bias field is a sensitive function of the applied field. In this paper, we propose a novel technique to characterise the energy level structure of a solid state qubit and its spontaneous emission rate into a lossy reservoir. The technique relies on the backreaction of a solid state qubit on its environment and the incoherent transfer of energy from a high frequency mode to a low frequency mode due to the stochastic transitions of the qubit between energy eigenstates. We consider a coupled system consisting of a model qubit and several classical degrees of freedom. The qubit is coupled to two main electromagnetic modes: a low frequency mode (classical bias field) that is used to control the operation of the qubit, and a higher frequency mode that is used to pump the qubit from the ground state to an excited state. In addition, the qubit is assumed to be coupled to a lossy reservoir, which represents the cavity that contains the qubit and control fields. The reservoir provides a mechanism to allow the qubit to dissipate energy and to induce spontaneous decays from an excited state into the ground state. The loss mechanism is modelled using a quantum trajec-tory approach [1,2], corresponding to an unravelling of the Markovian Master