[unitaryhack] Define decomposition for `qml.CSWAP`

[antalszava]

This issue has been tagged for contributions during unitaryHACK

Each PennyLane operation defines a decomposition. The decomposition of an operation is an ordered list of other operations. Applying this list of operations evolves quantum states the same way as applying the operation itself does. In most cases, this list includes elementary gates that are often supported by most quantum devices.

We would like to add the decomposition of the qml.CSWAP operation by:

1. Defining the decomposition method for the qml.CSWAP operation that has the following signature:

   @staticmethod
   def decomposition(wires):

   Other operations in the pennylane/ops/qubit.py file can serve as an example for defining this method (e.g., see qml.Hadamard).

2. Including a test case for the decomposition of qml.CSWAP in the tests/ops/test_qubit_ops.py file to verify its correctness.

[charmerDark]

Hey! I would like to have a crack at this. Would you have any tips on writing the tests? Thanks for the opportunity!

[josh146]

Hi @charmerDark - you can have a look at some of the decomposition tests we have in test_qubit_ops.py. Essentially, the idea is to:

1. Decompose the operation
2. Multiply together the matrix representations of each operation in the decomposition
3. Make sure it agrees with the matrix representation of the CSWAP gate

[co9olguy]

Another possible landing spot (and reference) for gate tests:

https://github.com/PennyLaneAI/pennylane/blob/7680a48a8a93c5a2f24acd7204b8de83e41bc02e/tests/test_operation.py#L1034

[MaldoAlberto]

Hi @antalszava I send a Pull requests about this issue, but I forgot to add the Unitaryhack label, is there any problem or should I resend it?

[josh146]

Hi @MaldoAlberto, I can add the label for you :)