Josephson-qubit oscillations resonantly activated by axions.

In the last years Josephson junctions (JJs) have been supposed to interact with axions, the hypothetical elementary particles candidate as a possible component of cold dark matter. Unexplained experimental effects on Josephson systems can be well justified on the basis of the axion-JJ theory. This hypothesis, thus, has paved the way for the possibility of thinking of JJs as possible axion detectors. Further, the interaction of axion-induced photons with Josephson qubit in a cavity has been recently proposed. In this contribution, an effective quantum description of the axion as a two-level dynamic system is proposed. The direct coupling (not mediated by photons in a cavity) with a Josephson qubit is studied through an effective spin-spin Hamiltonian model. When the axion and the Josephson frequencies match, the axion-Josephson qubit interaction can be responsible for a resonance effect. Thus, experimentally detectable oscillations induced in the Josephson qubit by the axion are clearly derived. This phenomenon, causing a periodic magnetization reversal in the junction, can be exploited for the axion detection in the quantum limit of low noise intensity.