# Phase Qubits

**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.

```python
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)
```

## Barrier Tunneling

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.