  Current implementations of quantum bits (qubits) continue to undergo too many
errors to be scaled into useful quantum machines. An emerging strategy is to
encode quantum information in the two meta-stable pointer states of an
oscillator exchanging pairs of photons with its environment, a mechanism shown
to provide stability without inducing decoherence. Adding photons in these
states increases their separation, and macroscopic bit-flip times are expected
even for a handful of photons, a range suitable to implement a qubit. However,
previous experimental realizations have saturated in the millisecond range. In
this work, we aim for the maximum bit-flip time we could achieve in a
two-photon dissipative oscillator. To this end, we design a Josephson circuit
in a regime that circumvents all suspected dynamical instabilities, and employ
a minimally invasive fluorescence detection tool, at the cost of a two-photon
exchange rate dominated by single-photon loss. We attain bit-flip times of the
order of 100 seconds for states pinned by two-photon dissipation and containing
about 40 photons. This experiment lays a solid foundation from which the
two-photon exchange rate can be gradually increased, thus gaining access to the
preparation and measurement of quantum superposition states, and pursuing the
route towards a logical qubit with built-in bit-flip protection.
