The SWAP gate, possibly raised to a power. Exchanges qubits.SwapPowGate()**t = SwapPowGate(exponent=t) and acts on two qubits in thecomputational basis as the matrix:
[[1, 0, 0, 0], [0, g·c, -i·g·s, 0], [0, -i·g·s, g·c, 0], [0, 0, 0, 1]]
c = cos(π·t/2) s = sin(π·t/2) g = exp(i·π·t/2).
cirq.SWAP, the swap gate, is an instance of this gate at exponent=1.