# QPU Topology¶

To solve a bqm on the D-Wave system, you
must map it to a graph that represents the topology of the system’s
qubits. For D-Wave 2000Q systems, this is the *chimera* topology; for
next-generation Advantage systems, this is the *Pegasus* topology.

Note

If you are sending your problem to a Leap quantum-classical hybrid solver, the solver handles all interactions with the QPU.

## Chimera¶

The Chimera architecture comprises sets of connected unit cells, each with four horizontal qubits connected to four vertical qubits via couplers (bipartite connectivity). Unit cells are tiled vertically and horizontally with adjacent qubits connected, creating a lattice of sparsely connected qubits. A unit cell is typically rendered as either a cross or a column.

Chimera qubits are considered to have a nominal length of 4 (each qubit is connected to 4 orthogonal qubits through internal couplers) and degree of 6 (each qubit is coupled to 6 different qubits).

The notation CN refers to a Chimera graph consisting of an \(N{\rm x}N\) grid of unit cells. The D-Wave 2000Q QPU supports a C16 Chimera graph: its 2048 qubits are logically mapped into a \(16 {\rm x} 16\) matrix of unit cells of 8 qubits.

## Pegasus¶

In Pegasus as in Chimera, qubits are “oriented” vertically or horizontally but similarly aligned qubits can also be also shifted by distances and in groupings that differ between Pegasus families. Pegasus qubits are also more densely connected and have three types of coupler:

*Internal couplers*. Internal couplers connect pairs of orthogonal (with opposite orientation) qubits. In Pegasus, each qubit is connected via internal coupling to 12 other qubits (versus four in the Chimera topology).*External couplers*. External couplers connect vertical qubits to adjacent vertical qubits and horizontal qubits to adjacent horizontal qubits. Each qubit has one or two external couplers.*Odd couplers*. Odd couplers connect similarly aligned pairs of qubits. Each qubit has one odd coupler.

Pegasus qubits are considered to have a nominal length of 12 (each qubit is connected to 12 orthogonal qubits through internal couplers) and degree of 15 (each qubit is coupled to 15 different qubits).

As we use the notation CN to refer to a Chimera graph with size parameter N, we refer to instances of Pegasus topologies by PN; for example, P3 is a graph with 144 nodes.