SolverConfig – Solver Configuration Reference

The SolverConfig class defines how a QUBO problem should be solved — specifying whether to use a quantum or classical approach, which backend to run on, and additional execution parameters.

This configuration is passed into any solver (e.g., QuboSolver) and guides its behavior. Note that SolverConfig uses four other configuration objects: 'BackendConfig', EmbeddingConfig, ClassicalConfig and PulseShapingConfig. Besides ClassicalConfig, the other configurations represents different parts of the solver when using a quantum approach:

Fields for SolverConfig

Generic parameters

Field	Type	Description
`config_name`	`str`	The name of the current configuration
`use_quantum`	`bool`	Whether to solve using a quantum approach (`True`) such as QAA or VQA or a classical approach (`False`).
`backend_config`	`BackendConfig`	Backend part configuration of the solver.
`n_calls`	`int` \| `None`	Number of optimization rounds taken to find the best set of parameters for the optimization process inside VQA. The minimum value is 20. Note the optimizer accepts a minimal value of 12.
`embedding`	`EmbeddingConfig`	Embedding part configuration of the solver.
`pulse_shaping`	`PulseShapingConfig`	Pulse-shaping part configuration of the solver.
`classical`	`ClassicalConfig`	Classical part configuration of the solver.
`num_shots`	`int`	Number of samples when using a quantum device. Defaults to 500.

Backend configuration

The backend configuration part (the backend_config field) is set via the BackendConfig class. It defines how we will run our quantum programs (via a local emulator, or via remote connection).

Field	Type	Description
`backend`	`BackendType`	(optional) Which backend to use (e.g., `'qutip'`, `'emu_mps'`, `'emu_sv'`, `'remote_qpu'`, `'remote_emumps'`).
`device`	`NamedDevice` \| `DeviceType` \| `None`	(optional) If `None`, the backend will pick a reasonable device. If `DeviceType`, choose a device by its capabilities, e.g. `DeviceType.DIGITAL_ANALOG`. If `NamedDevice`, requiest a specific device. Only remote backends make use of `NamedDevice`.
`project_id`	`str`	(optional) Project ID for accessing remote Pasqal services. Only used for remote backends.
`username`	`str`	(optional) Username for Pasqal Cloud authentication. Only used for remote backends.
`password`	`str`	(optional) Password for Pasqal Cloud authentication. Only used for remote backends.

Embedding configuration

When solving with a quantum approach, we need to define an embedding method, that is how we define the geometry (register) of atoms based on the QUBO instance and compatibility with a device. The embedding configuration part (the embedding field of SolverConfig) can be divided into two groups of parameters.

Method parameter

Field	Type	Description
`embedding_method`	`str` \| `EmbedderType` \| `Type[BaseEmbedder]`	The type of embedding method used to place atoms on the register according to the QUBO problem. (e.g., 'blade', 'greedy', but we can also create our own custom embedding method).
`draw_steps`	`bool`	Show generated graph at each step of the optimization. Defaults to False.

BLaDE embedding parameters

Field	Type	Description
`blade_dimensions`	`list[int]`	A list of dimension degrees to explore one after the other (default is [5, 4, 3, 2, 2, 2]).
`starting_positions`	`torch.Tensor`	The starting parameters according to the specified dimensions.
`blade_steps_per_round`	`int` \| `None`	The number of steps for each layer of dimension for the BLaDE embedder method. Defaults to 200.

Greedy embedding parameters

We made available a greedy embedding method (given a fixed lattice or layout, it will defines the register to minimize the incremental mismatch between the logical QUBO matrix Q and the physical device interactions), whose related fields are:

Field	Type	Description
`layout_greedy_embedder`	`str` \| `LayoutType` \| `None`	Type of layout to run the greedy embedder method on (e.g., 'SquareLatticeLayout', 'TriangularLatticeLayout').
`traps`	`int` \| `None`	The number of traps on the register.
`spacing`	`int` \| `None`	The minimum distance between atoms.
`density`	`int` \| `None`	The estimated density of the QUBO matrix for the greedy algorithm.

Pulse Shaping configuration

Quantum devices can be programmed by specifying a sequence of pulses. The pulse shaping configuration part (the pulse_shaping field of SolverConfig) is set via the PulseShapingConfig class, and defines how the pulse parameters are constructed (in an adiabatic fashion, via optimization, ...).

Field	Type	Description
`pulse_shaping_method`	`str` \| `PulseType` \| `Type[BasePulseShaper]`	The type of pulse-shaping method used (e.g., 'adiabatic', 'optimized').
`initial_omega_parameters`	`list[float]`	The list of initial amplitude \(\Omega\) parameters (\(3\) floating numbers) to be used in the first round of optimization.
`initial_detuning_parameters`	`list[float]`	The list of global detuning \(\delta\) parameters (\(3\) floating numbers) to be used in the first round of optimization.
`re_execute_opt_pulse`	`bool`	Whether to re-run the optimal pulse sequence.
`custom_qubo_cost`	`callable` \| `None`	To apply a different qubo cost evaluation than the default. Must be defined as: `def custom_qubo_cost(bitstring: str, QUBO: torch.Tensor) -> float`.
`custom_objective_fn`	`callable` \| `None`	Change the bayesian optimization objective. Instead of using the best cost (`best_cost`) out of the samples, one can change the objective for an average, or any function out of the form `cost_eval = custom_objective_fn(bitstrings, counts, probabilities, costs, best_cost, best_bitstring)`
`callback_objective`	`callable` \| `None`	Apply a callback during bayesian optimization. Only accepts one input dictionary created during optimization `d = {"x": x, "cost_eval": cost_eval}` hence should be defined as: `def callback_fn(d: dict) -> None:`.

Classical solver configuration

For the classical solver, its configuration can be set via the ClassicalConfig class:

Field	Type	Description
`classical_solver_type`	`str`	Classical solver type.
`cplex_maxtime`	`float`	CPLEX maximum runtime.
`cplex_log_path`	`str`	CPLEX logging path.

Pre-Post processing parameters

We can also apply preprocessing of the QUBO instance (to reduce it to another smaller instance) or postprocessing the solution after solving.

Field	Type	Description
`do_postprocessing`	`bool`	Whether we apply post-processing (`True`) or not (`False`).
`do_preprocessing`	`bool`	Whether we apply pre-processing (`True`) or not (`False`).

Example

The SolverConfig is designed in such way that all parameters have a default value which fulfilled the minimum required configuration to execute the necessary steps to solve a QUBO.

All the parameters are Optional which allows for running SolverConfig without specifying any parameter:

from qubosolver.config import SolverConfig
from qoolqit._solvers.types import BackendType, DeviceType
from qubosolver.qubo_types import EmbedderType

config = SolverConfig()
config.print_specs()

which returns the following default specifications:

config_name: ''
use_quantum: False
backend: qutip
device: DeviceType.DIGITAL_ANALOG_DEVICE
project_id: ''
username: ''
password: ''
n_calls: 20
embedding: {'embedding_method': <EmbedderType.GREEDY: 'greedy'>, 'layout_greedy_embedder': <LayoutType.SQUARE: <class 'pulser.register.special_layouts.SquareLatticeLayout'>>, 'draw_steps': False, 'traps': 1, 'spacing': 5.0, 'density': None}
pulse_shaping: {'pulse_shaping_method': <PulseType.ADIABATIC: 'adiabatic'>, 'initial_omega_parameters': [5.0, 10.0, 5.0,], 'initial_detuning_parameters': [-10.0, 0.0, 10.0], 're_execute_opt_pulse': False}
classical: {'classical_solver_type': 'cplex', 'cplex_maxtime': 600.0, 'cplex_log_path': 'solver.log'}
do_postprocessing: False
do_preprocessing: False

Although the default configuration is straightforward, all parameters can be modified by the user to better suit the specific QUBO instance. Below is an example of a configuration that uses a different embedder with customized parameters on a specific device:

from qubosolver import QUBOInstance
from qubosolver.config import SolverConfig, EmbeddingConfig, BackendConfig
from qoolqit._solvers.types import DeviceType

coefficients = [[0, 1, 2], [1, 0, 3], [2, 3, 0]]
instance = QUBOInstance(coefficients=coefficients)

embedding_config = EmbeddingConfig(embedding_method="greedy", traps=instance.size)
backend_config = BackendConfig(backend="qutip", device=DeviceType.DIGITAL_ANALOG_DEVICE,)

config = SolverConfig(
    config_name="my_config",
    use_quantum=True,
    backend_config = backend_config,
    embedding = embedding_config,
)

Equivalently, one can instantiate a SolverConfig simply using the keyword arguments of the other configs via the SolverConfig.from_kwargs method:

from qubosolver import QUBOInstance
from qoolqit._solvers.types import DeviceType
from qubosolver.config import SolverConfig

coefficients = [[0, 1, 2], [1, 0, 3], [2, 3, 0]]
instance = QUBOInstance(coefficients=coefficients)

config = SolverConfig.from_kwargs(
    config_name="my_config",
    use_quantum=True,
    backend="qutip",
    device=DeviceType.ANALOG_DEVICE,
    embedding_method="greedy",
    traps=instance.size
)