Discrepancy between Chimera Graphs

Hi,

Using a the graph provided by this function:

`dnx.chimera_graph(int, int)`

I have noticed that it's quite different from the one reported here https://docs.dwavesys.com/docs/latest/c_gs_4.html.

So, I was questioning myself: which is the correct one?

Thanks to everyone.

• Hi Thomas,

Thank you for your question. There seems to be some confusion, which our team would be happy to clarify.

The chimera_graph(m,n) function in the dwave_networkx package is used to generate a chimera lattice of m number of rows and n number of columns (https://docs.ocean.dwavesys.com/projects/dwave-networkx/en/latest/reference/generated/dwave_networkx.chimera_graph.html) with each block being a chimera unit cell.

The following is a code example that shows how to plot a single unit cell in a chimera lattice:

The graph plotted above represents a singular chimera unit cell with the nodes representing qubits and the edges representing couplers. Each of the 4 horizontal qubits connects to each of the 4 vertical qubits via internal couplers, forming a K4,4 graph (as seen in the picture above).

The chimera topology in the D-Wave DW2000Q QPU chip supports a 16x16 chimera lattice(referred by C16 notation) which consists of 16 rows * 16 columns  of those unit cells of 8 qubits and this is what you would see in the documentation present here (https://docs.dwavesys.com/docs/latest/c_gs_4.html).

(The pictures above shows how the Chimera unit cells can be represented in column or crossed representation, with the latter being the same as the one produced by the chimera_graph(m,n) function)

On the fabricated chip, unit cells are laid out as below, with each chimera unit cell connected by external couplers (represented by connected blue circles, which connect adjacent unit cells):

(The picture above shows three 8 qubit unit cells, connected horizontally via external couplers)

I hope this clarifies your question. If you have any further concerns, please feel free to reach out again on our community thread.

• Thank you a lot, now it's much clearer.