Support gate exponentiation by Gate ** x instead of only Gate.power(x)

[kevinsung]

What is the expected enhancement?

Exponentiating a gate using ** should work. It can just call gate.power. For example, it's surprising that the following code raises an error:

from qiskit.circuit.library import iSwapGate

gate = iSwapGate()
gate.power(0.5)
gate**0.5

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-9fc21b517202> in <module>
      3 gate = iSwapGate()
      4 gate.power(0.5)
----> 5 gate**0.5

TypeError: unsupported operand type(s) for ** or pow(): 'iSwapGate' and 'float'

[AshwinKul28]

HI @kevinsung,

In the above issue, you're trying to invoke ** or exponential function directly on the NumPy array. It doesn't matter even if you pass int, float or bigint it will throw an error.

for example, Below snippet is also throwing an error.

The reason behind that is, gate.power() function is not same as normal exponential **. Hence gate will not allow the combination using **.

[dmkunsman]

Should this be closed?

[kevinsung]

This can be done by implementing the __pow__ magic method. Ints should be supported too.