Quadratic Models: Unconstrained

Unconstrained quadratic models are used to submit problems to samplers such as D-Wave quantum computers1 and some hybrid quantum-classical samplers2.

When using such samplers to handle problems with constraints, you typically formulate the constraints as penalties: see Getting Started with D-Wave Solvers. (Constrained models, such as the ConstrainedQuadraticModel, can support constraints natively.)

1

D-Wave quantum computers accept unconstrained binary quadratic models, such as quadratic unconstrained binary optimization (QUBO) models: binary because variables are represented by qubits that return two states and quadratic because polynomial terms of two variables can be represented by pairs of coupled qubits.

2

Some hybrid quantum-classical samplers accept constrained and non-binary models; for example, a quadratic model with an integer variable that must be smaller than some configured value.

Supported Models

See examples of using QPU solvers and Leap hybrid solvers on these models in Ocean documentation’s Getting Started examples and the dwave-examples GitHub repository.

Model Construction

dimod provides a variety of model generators. These are especially useful for testing code and learning.

Example: dimod BQM Generator

This example generates a BQM from a fully-connected graph (a clique) where all linear biases are zero and quadratic values are uniformly selected -1 or +1 values.

>>> bqm = dimod.generators.random.ran_r(1, 7)

Typically you construct a model when reformulating your problem, using such techniques as those presented in D-Wave’s system documentation’s Problem-Solving Handbook.

For learning and testing with small models, construction in Python is convenient.

Example: Python Formulation

The maximum cut problem is to find a subset of a graph’s vertices such that the number of edges between it and the complementary subset is as large as possible.

Four-node star graph

Star graph with four nodes.

The dwave-examples Maximum Cut example demonstrates how such problems can be formulated as QUBOs:

\[\begin{split}Q = \begin{bmatrix} -3 & 2 & 2 & 2\\ 0 & -1 & 0 & 0\\ 0 & 0 & -1 & 0\\ 0 & 0 & 0 & -1 \end{bmatrix}\end{split}\]
>>> qubo = {(0, 0): -3, (1, 1): -1, (0, 1): 2, (2, 2): -1,
...         (0, 2): 2, (3, 3): -1, (0, 3): 2}
>>> bqm = dimod.BQM.from_qubo(qubo)

For performance, especially with very large BQMs, you might read the data from a file using methods, such as from_file() or from NumPy arrays.

Example: Construction from NumPy Arrays

This example creates a BQM representing a long ferromagnetic loop with two opposite non-zero biases.

>>> import numpy as np
>>> linear = np.zeros(1000)
>>> quadratic = (np.arange(0, 1000), np.arange(1, 1001), -np.ones(1000))
>>> bqm = dimod.BinaryQuadraticModel.from_numpy_vectors(linear, quadratic, 0, "SPIN")
>>> bqm.add_quadratic(0, 10, -1)
>>> bqm.set_linear(0, -1)
>>> bqm.set_linear(500, 1)
>>> bqm.num_variables
1001

Example: Interaction Between Integer Variables

This example constructs a QM with an interaction between two integer variables.

>>> qm = dimod.QuadraticModel()
>>> qm.add_variables_from('INTEGER', ['i', 'j'])
>>> qm.add_quadratic('i', 'j', 1.5)

Data Structure

Quadratic models are implemented with an adjacency structure in which each variable tracks its own linear bias and its neighborhood. The figure below shows the graph and adjacency representations for an example BQM,

\[E(x) = .5 x_0 - 3 x_1 - x_0 x_1 + x_0 x_2 + 2 x_0 x_3 + x_2 x_3\]
Adjacency Structure

Adjacency structure of a 4-variable binary quadratic model.