The D-Wave QPU is a lattice of interconnected qubits. While some qubits connect to others via couplers, the D-Wave QPU is not fully connected. Instead, the qubits interconnect in an architecture known as Chimera.
The Chimera architecture comprises sets of connected unit cells, each with four horizontal
qubits connected to four vertical qubits via couplers. Unit cells are tiled vertically and
horizontally with adjacent qubits connected, creating a lattice of sparsely connected qubits.
The notation CN refers to a Chimera graph consisting of an NxN grid of unit cells. The
D-Wave 2000Q QPU supports a C16 Chimera graph: its 2048 qubits are logically mapped
into a 16x16 matrix of unit cells of 8 qubits.
In a D-Wave QPU, the set of qubits and couplers that are available for computation is
known as the working graph. The yield of a working graph is typically less than the total
number of qubits and couplers that are fabricated and physically present in the QPU.
See also: What is the QPU Working Graph?