Figure 2
From: Characterization of how dissipation and dephasing errors accumulate in quantum computers
The Toffoli gate:
(a) The three-qubit Toffoli gate on the left may be reproduced by a circuit of C-NOT, Hadamard (H), 
and 
gates, shown to the right. (b) The process matrix χ (left), characterizing the Toffoli gate may be calculated by concatenation of one- and two-qubit process matrices (right). The process matrices 
, 
, 
and 
characterize the C-NOT, Hadamard, T and gates respectively. The ‘identity’ process matrix 
indicates an idle qubit.