  We propose a superconducting phase qubit on the basis of the radio-frequency
SQUID with the screening parameter value $\beta_L = (2\pi/\Phi_0)LI_c \approx
1$, biased by a half flux quantum $\Phi_e=\Phi_0/2$. Significant anharmonicity
($> 30%$) can be achieved in this system due to the interplay of the cosine
Josephson potential and the parabolic magnetic-energy potential that ultimately
leads to the quartic polynomial shape of the well. The two lowest eigenstates
in this global minimum perfectly suit for the qubit which is insensitive to the
charge variable, biased in the optimal point and allows an efficient dispersive
readout. Moreover, the transition frequency in this qubit can be tuned within
an appreciable range allowing variable qubit-qubit coupling.
