Rotates the |01⟩ vs |10⟩ subspace of two qubits around its Bloch X-axis.

When exponent=1, swaps the two qubits and phases |01⟩ and |10⟩ by i. More generally, this gate's matrix is defined as follows:

``````ISWAP**t ≡ exp(+i π t (X⊗X + Y⊗Y) / 4)
``````

which is given by the matrix:

``````[[1, 0, 0, 0],
[0, c, i·s, 0],
[0, i·s, c, 0],
[0, 0, 0, 1]]
``````

where:

``````c = cos(π·t/2)
s = sin(π·t/2)
``````

`cirq.ISWAP`, the swap gate that applies i to the |01⟩ and |10⟩ states, is an instance of this gate at exponent=1.

#### References:

"What is the matrix of the iSwap gate?" https://quantumcomputing.stackexchange.com/questions/2594/

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