# Types of Postprocessing¶

Solutions obtained from a D-Wave quantum processing unit (QPU) can be postprocessed to achieve higher quality.

When submitting a problem to a D-Wave 2000Q QPU, users choose from:

- No postprocessing (default)
- Optimization postprocessing
- Sampling postprocessing

For optimization problems, the goal is to find the state vector with the lowest energy. For sampling problems, the goal is to produce samples from a specific probability distribution. In both cases, a logical graph structure is defined and embedded into the QPU’s Chimera topology. Postprocessing methods are applied to solutions defined on this logical graph structure.

The flow of integrated postprocessing is shown in Figure 108.

## Optimization Postprocessing¶

The goal of optimization postprocessing is to obtain a set of samples with lowest energy on a graph \(G\).
For simplicity of discussion, assume that \(G\) is a logical graph. Pseudocode for optimization postprocessing
is shown below. The postprocessing method works by performing local updates to the state vector \(S\)
based on low treewidth graphs. Specifically, the *DecomposeGraph* function uses a set of algorithms based on
the minimum-degree [Mar1957] heuristic to decompose graph \(G\) into several low treewidth subgraphs that
cover the nodes and edges of \(G\). The *SolveSubgraph* function is then used to update the solution on each subgraph
to obtain a locally optimal solution \(s'\). *SolveSubgraph* is an exact solver for low treewidth
graphs based on belief propagation on junction trees [Jen1990].

## Sampling Postprocessing¶

In sampling mode, the goal of postprocessing is to obtain a set of samples that correspond to a target Boltzmann
distribution with inverse temperature \(\beta\) defined on the logical graph \(G\). The pseudocode for sampling
postprocessing is shown below. As with optimization postprocessing, the graph \(G\) is decomposed into
low treewidth subgraphs \(G'\) that cover \(G\). On each subgraph, the state variables \(S\) obtained from
the QPU are then updated using the *SampleSubgraph* function to match the exact Boltzmann distribution conditioned on the
states of the variables outside the subgraph. Users can set the number of subgraph update iterations \(n\); the choice of
\(\beta\) is also determined by the user. Heuristically, this inverse temperature depends on model parameters in the
Hamiltonian. As a general rule, \(\beta\) should be set to a value close to an inverse temperature corresponding to
raw QPU samples. Some variation from a classical Boltzmann distribution is expected in postprocessed samples.
See Sampling Tests and Results for discussion.