  The destruction of superconducting phase coherence by quantum fluctuations
and the control of these fluctuations have been a problem of long-standing
interest, with recent impetus provided by its relevance to the pursuit of very
high temperature superconductivity. Building on the work of Little and Parks,
de Gennes predicted more than three decades ago that superconductivity could be
destroyed near half-integer-flux quanta in ultrasmall loops with a side branch,
resulting in a destructive regime. We report the experimental observation of
this Little-Parks-de Gennes effect in Al loops prepared by advanced e-beam
lithography. We show that the effect can be used to restore the lost phase
coherence through side branches.
