  The two-qubit interaction Hamiltonian of a given physical implementation
determines whether or not a two-qubit gate such as the CNOT gate can be
realized easily. It can be shown that, e.g., with the XY interaction more than
one two-qubit operation is required in order to realize CNOT. Here we propose a
two-qubit gate for the XY interaction which combines CNOT with the SWAP
operation. By using this gate quantum circuits can be implemented efficiently,
even if only nearest-neighbor coupling between the qubits is available.
