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Energy level spectroscopy of macroscopic quantum systems is of
large current interest. In conjunction with quantum computing, superposition
of macroscopic quantum states have recently been observed in potential wells
with two adjacent minima. In single Josephson junctions, with a periodic
potential for the phase difference across the junction, superposition of
states leads to delocalization of the macroscopic phase variable. A
consequence of this is the formation of energy bands, in a similar fashion
as for electrons in solids.
We have verified the existence of these bands experimentally using a
novel spectroscopic tool based on inelastic Cooper pair tunneling in a mesoscopic junction. According to the environmental fluctuation theory,
non-coherent Cooper pair tunneling is allowed only if energy is exchanged
with the surroundings. Thus, transitions between energy bands are seen as
current peaks in the IV-curve of a probe junction.
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Contour plot
of the measured current in the probe junction in the flux-voltage plane;
flux penetrating the SQUID loops is given in reduced units Phi/Phi0 where
Phi0 = h/2e is the flux quantum. A sequence of resonance peaks is seen in
the subgap-conductance. These peaks are identified as coming from
transitions between the excited bands of a Josephson junction |
