.. _concepts_sdk:
========
Concepts
========
See the :ref:`ocean_glossary` for short definitions of terminology or learn Ocean concepts here:
.. list-table:: Ocean Concepts
:widths: auto
:header-rows: 1
* - Concepts
- Related terms
* - :ref:`bqm_sdk`
- BQM, Ising, QUBO
* - :ref:`cqm_sdk`
- CQM, constrained quadratic model
* - :ref:`csp_sdk`
- CSP, binary CSP
* - :ref:`dqm_sdk`
- DQM, discrete quadratic model
* - :ref:`hybrid_sdk`
- quantum-classical hybrid, Leap's hybrid solvers, hybrid workflows
* - :ref:`embedding_sdk`
- embedding, mapping logical variables to physical qubits, chains, chain strength
* - :ref:`penalty_sdk`
- Penalty Models
* - :ref:`qm_sdk`
- Quadratic Models
* - :ref:`topology_sdk`
- Chimera, Pegasus
* - :ref:`samplers_sdk`
- solver
* - :ref:`solutions_sdk`
- samples, sampleset, probabilistic, energy
* - :ref:`variables_sdk`
- binary, discrete, integer, real variables
.. toctree::
:hidden:
:maxdepth: 1
bqm
cqm
csp
dqm
hybrid
embedding
penalty
qm
topology
samplers
solutions
variables
.. _ocean_glossary:
Glossary
--------
.. glossary::
binary quadratic model
BQM
A collection of binary-valued variables (variables that can be assigned two values, for example -1, 1)
with associated linear and quadratic biases. Sometimes referred to in other tools as a problem.
See a fuller description under :doc:`Binary Quadratic Models `.
Chain
One or more nodes or qubits in a target graph that represent a single variable in
the source graph. See :term:`embedding`.
See a fuller description under :doc:`Minor-Embedding `.
Chain length
The number of qubits in a :term:`Chain`.
See a fuller description under :doc:`Minor-Embedding `.
Chain strength
Magnitude of the negative quadratic bias applied between variables to form a chain.
See a fuller description under :doc:`Minor-Embedding `.
Chimera
A D-Wave :term:`QPU` is a lattice of interconnected qubits. While some qubits
connect to others via couplers, D-Wave QPUs are not fully connected.
For D-Wave 2000Q QPUs, the qubits interconnect in an architecture known
as Chimera. See a fuller description under :doc:`QPU Topology `.
See also :term:`Pegasus` and :term:`Zephyr`.
Clique
Complete graph
Fully connected
See `complete graph`_ on Wikipedia or
:std:doc:`docs_dnx/reference/algorithms/clique`. A fully connected or
complete :term:`binary quadratic model` is one that has interactions
between all of its variables.
.. _complete graph: https://en.wikipedia.org/wiki/Complete_graph
Composed sampler
Samplers that apply pre- and/or post-processing to binary quadratic programs without
changing the underlying :term:`sampler` implementation by layering composite patterns
on the sampler. For example, a composed sampler might add spin transformations when
sampling from the D-Wave system.
Composite
A :term:`sampler` can be composed. The
`composite pattern `_
allows layers of pre- and post-processing to be applied to binary quadratic
programs without needing to change the underlying sampler implementation.
We refer to these layers as "composites". A composed sampler includes at least one
sampler and possibly many composites.
Connected graph
See `Connected graph `_
on the US NIST site. A connected graph has some path from any vertex
to any other. A graph that has at least two vertices without a path
between them is disconnected. Any :term:`Complete graph` is connected
(but not all connected graphs are complete).
Constrained quadratic model
CQM
A collection of variables with associated linear and quadratic biases
representing a problem modeled as an :term:`objective function` and
inequality and equality constraints.
See a fuller description under :doc:`Constrained Quadratic Models `.
Constraint satisfaction problem
CSP
A `constraint satisfaction problem (CSP) `_
requires that all the problem's variables be assigned values, out of a finite domain,
that result in the satisfying of all constraints.
See a fuller description under :doc:`QPU Topology `.
discrete quadratic model
DQM
A collection of discrete-valued variables (variables that can be
assigned the values specified by a set such as :math:`\{red, green, blue\}`
or :math:`\{33, 5.7, 3,14 \}` ) with associated linear and quadratic biases.
See a fuller description under :doc:`Discrete Quadratic Models `.
Embed
Embedding
Minor-embed
Minor-embedding
The nodes and edges on the graph that represents an objective function
translate to the qubits and couplers in :term:`Chimera`. Each logical qubit, in
the graph of the :term:`objective function`, may be represented by one or more
physical qubits. The process of mapping the logical qubits to physical
qubits is known as minor embedding.
See a fuller description under :doc:`Minor-Embedding `.
Excited state
States of a quantum system that have higher energy than the :term:`ground state`.
Such states represent non-optimal solutions for problems represented by an
:term:`Objective function` and infeasible configurations for problems
represented by a :term:`penalty model`.
Graph
A collection of nodes and edges. A graph can be derived
from a :term:`model`\ : a node for each variable and an edge for each pair
of variables with a non-zero quadratic bias.
Ground state
The lowest-energy state of a quantum-mechanical system and the global minimum
of a problem represented by an :term:`Objective function`.
Hamiltonian
A classical Hamiltonian is a mathematical description of some physical
system in terms of its energies. We can input any particular state of
the system, and the Hamiltonian returns the energy for that state.
For a quantum system, a Hamiltonian is a function that maps certain states,
called *eigenstates*, to energies. Only when the system is in an
eigenstate of the Hamiltonian is its energy well defined and called
the *eigenenergy*. When the system is in any other state, its energy
is uncertain.
For the D-Wave system, the Hamiltonian may be represented as
.. math::
:nowrap:
\begin{equation}
{\cal H}_{ising} = \underbrace{\frac{A({s})}{2} \left(\sum_i {\hat\sigma_{x}^{(i)}}\right)}_\text{Initial Hamiltonian} + \underbrace{\frac{B({s})}{2} \left(\sum_{i} h_i {\hat\sigma_{z}^{(i)}} + \sum_{i>j} J_{i,j} {\hat\sigma_{z}^{(i)}} {\hat\sigma_{z}^{(j)}}\right)}_\text{Final Hamiltonian}
\end{equation}
where :math:`{\hat\sigma_{x,z}^{(i)}}` are Pauli matrices operating on
a qubit :math:`q_i`, and :math:`h_i` and :math:`J_{i,j}` are the qubit
biases and coupling strengths.
Hardware graph
See `hardware graph`_. The hardware graph is the physical lattice of
interconnected qubits. See also :term:`working graph`.
See a fuller description under :doc:`QPU Topology `.
.. _hardware graph: https://docs.dwavesys.com/docs/latest/c_gs_4.html
Hybrid
Quantum-classical hybrid is the use of both classical and quantum resources
to solve problems, exploiting the complementary strengths that each provides.
See :ref:`using_hybrid`.
Ising
Traditionally used in statistical mechanics. Variables are "spin up"
(:math:`\uparrow`) and "spin down" (:math:`\downarrow`), states that
correspond to :math:`+1` and :math:`-1` values. Relationships between
the spins, represented by couplings, are correlations or anti-correlations.
The :term:`objective function` expressed as an Ising model is as follows:
.. math::
:nowrap:
\begin{equation}
\text{E}_{ising}(\pmb{s}) = \sum_{i=1}^N h_i s_i + \sum_{i=1}^N \sum_{j=i+1}^N J_{i,j} s_i s_j
\end{equation}
where the linear coefficients corresponding to qubit biases
are :math:`h_i`, and the quadratic coefficients corresponding to coupling
strengths are :math:`J_{i,j}`.
See also `Ising Model on Wikipedia `_.
Minimum gap
The minimum distance between the :term:`ground state` and the first
:term:`excited state` throughout any point in the anneal.
Model
A collection of variables with associated biases. Sometimes referred to
as a **problem**.
Objective function
A mathematical expression of the energy of a system as a function of
binary variables representing the qubits.
Pegasus
A D-Wave :term:`QPU` is a lattice of interconnected qubits. While some qubits
connect to others via couplers, D-Wave QPUs are not fully connected.
For an Advantage QPU, the qubits interconnect in an architecture known
as Pegasus. See a fuller description under :doc:`QPU Topology `.
See also :term:`Chimera` and :term:`Zephyr`.
Penalty function
An algorithm for solving constrained optimization problems. In the context
of Ocean tools, penalty functions are typically employed to increase the energy
level of a problem’s :term:`objective function` by penalizing non-valid configurations.
See `Penalty method on Wikipedia `_
Penalty model
An approach to solving constraint satisfaction problems (CSP) using an :term:`Ising` model
or a :term:`QUBO` by mapping each individual constraint in the CSP to a ‘small’ Ising model
or QUBO.
Quadratic model
A collection of variables with associated linear and quadratic biases.
Sometimes referred to as a **problem**.
QPU
Quantum processing unit.
QUBO
Quadratic unconstrained binary optimization.
QUBO problems are traditionally used in computer science. Variables
are TRUE and FALSE, states that correspond to 1 and 0 values.
A QUBO problem is defined using an upper-diagonal matrix :math:`Q`,
which is an :math:`N` x :math:`N` upper-triangular matrix of real weights,
and :math:`x`, a vector of binary variables, as minimizing the function
.. math::
:nowrap:
\begin{equation}
f(x) = \sum_{i} {Q_{i,i}}{x_i} + \sum_{i`_.
Sampler
Samplers are processes that sample from low energy states of a problem's objective
function, which is a mathematical expression of the energy of a system. A binary
quadratic model (BQM) sampler samples from low energy states in models such as those
defined by an :term:`Ising` equation or a :term:`QUBO` problem and returns an iterable
of samples, in order of increasing energy.
SAPI
Solver API used by clients to communicate with a :term:`solver`.
Solver
A resource that runs a problem. Some solvers interface to the :term:`QPU`;
others leverage CPU and GPU resources.
Source
Source graph
In the context of :term:`embedding`, the model or induced :term:`graph` that we
wish to embed. Sometimes referred to as the **logical** graph/model.
See a fuller description under :doc:`Minor-Embedding `.
Structured sampler
Samplers that are restricted to sampling only binary quadratic models defined
on a specific :term:`graph`.
Subgraph
See subgraph_ on wikipedia.
.. _subgraph: https://en.wikipedia.org/wiki/Glossary_of_graph_theory_terms#subgraph
Target
Target graph
:term:`Embedding` attempts to create a target :term:`model` from a target
:term:`graph`. The process of embedding takes a source model, derives the source
graph, maps the source graph to the target graph, then derives the target
model. Sometimes referred to as the **embedded** graph/model.
See a fuller description under :doc:`Minor-Embedding `.
Working graph
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 100% of qubits and couplers that are fabricated
and physically present in the QPU. See :term:`hardware graph`.
Zephyr
A D-Wave :term:`QPU` is a lattice of interconnected qubits. While some
qubits connect to others via couplers, D-Wave QPUs are not fully connected.
For D-Wave's next-generation QPU currently under development, the qubits
interconnect in an architecture known as Zephyr. See a fuller description
under :doc:`QPU Topology `.
See also :term:`Pegasus` and :term:`Chimera`.