  Ten years ago, coherent oscillations between two quantum states of a
superconducting circuit differing by the presence or absence of a single Cooper
pair on a metallic island were observed for the first time. This result
immediately stimulated the development of several other types of
superconducting quantum circuits behaving as artificial atoms, thus bridging
mesoscopic and atomic physics. Interestingly, none of these circuits fully
implements the now almost 30 year old proposal of A. J. Leggett to observe
coherent oscillations between two states differing by the presence or absence
of a single fluxon trapped in the superconducting loop interrupted by a
Josephson tunnel junction. This phenomenon of reversible quantum tunneling
between two classically separable states, known as Macroscopic Quantum
Coherence (MQC), is regarded crucial for precision tests of whether macroscopic
systems such as circuits fully obey quantum mechanics. In this article, we
report the observation of such oscillations with sub-GHz frequency and quality
factor larger than 500. We achieved this result with two innovations. First,
our ring has an inductance four orders of magnitude larger than that considered
by Leggett, combined with a junction in the charging regime, a parameter choice
never addressed in previous experiments. Second, readout is performed with a
novel dispersive scheme which eliminates the electromagnetic relaxation process
induced by the measurement circuit (Purcell effect). Moreover, the reset of the
system to its ground state is naturally built into this scheme, working even if
the transition energy is smaller than that of temperature fluctuations. As we
argue, the MQC transition could therefore be, contrary to expectations, the
basis of a superconducting qubit of improved coherence and readout fidelity.
