  A novel rf-SQUID flux qubit that is robust against fabrication variations in
Josephson junction critical currents and device inductance has been
implemented. Measurements of the persistent current and of the tunneling energy
between the two lowest lying states, both in the coherent and incoherent
regime, are presented. These experimental results are shown to be in agreement
with predictions of a quantum mechanical Hamiltonian whose parameters were
independently calibrated, thus justifying the identification of this device as
a flux qubit. In addition, measurements of the flux and critical current noise
spectral densities are presented that indicate that these devices with Nb
wiring are comparable to the best Al wiring rf-SQUIDs reported in the
literature thusfar, with a $1/f$ flux noise spectral density at $1 $Hz of
$1.3^{+0.7}_{-0.5} \mu\Phi_0/\sqrt{\text{Hz}}$. An explicit formula for
converting the observed flux noise spectral density into a free induction decay
time for a flux qubit biased to its optimal point and operated in the energy
eigenbasis is presented.
