cirq.google.optimizers.convert_to_sqrt_iswap.SQRT_ISWAP

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/