Phase qubit is a Josephson junction biased near the maximum of its potential energy, operating in the regime where tunneling through the barrier matters. The potential $-E_J \cos(\phi)$ has a barrier near $\phi = \pi$.

Phase qubits are less commonly used today but historically important (Delft, Yale experiments). They exhibit strong nonlinearity and can be read via quasiparticle escape over the potential barrier.

from scqubits import PhaseQubit
 
phase_qubit = PhaseQubit(
    EJ=1.0,     # Josephson energy
    EC=0.1,     # Charging energy (small, weakly quantized)
    ncut=20
)
 
evals = phase_qubit.eigenvals(n=4)
print(evals)

The potential barrier near $\phi = \pi$ enables readout: if the qubit is in state $|1\rangle$, it can tunnel out and dissipate a quasiparticle, making the escape rate state-dependent. This is used for fast readout.

Phase qubits are less robust than transmons (strong barrier tunneling causes dephasing) but demonstrate important physics. scqubits includes phase qubit models for completeness.