### Polynomial ring with complex elements.

I am trying to define the polynomial ring $\Bbb{Z}[j]$ where $j$ is ~~the ~~a primitive 3rd root of unity, i.e. $j = e^{\frac{2}{3} \pi i} \in \Bbb C$.

I tried this:

```
P.<x> = PolynomialRing(QQ)
j = exp((2/3)*pi*i)
P[j]
```

But I get the error:

```
NotImplementedError: ring extension with prescribed embedding is not implemented
```