braket.default_simulator.density_matrix_simulation module

class braket.default_simulator.density_matrix_simulation.DensityMatrixSimulation(qubit_count: int, shots: int)

Bases: Simulation

This class tracks the evolution of the density matrix of a quantum system with qubit_count qubits. The state of system evolves by applications of GateOperation`s and `KrausOperation`s using the `evolve() method.

Parameters:

• qubit_count (int) – The number of qubits being simulated.

• shots (int) – The number of samples to take from the simulation. If set to 0, only results that do not require sampling, such as density matrix or expectation, are generated.

evolve(operations: list[GateOperation | KrausOperation]) → None

Evolves the state of the simulation under the action of the specified gate operations.

Parameters:

operations (list[GateOperation]) – Gate operations to apply for evolving the state of the simulation.

Note

This method mutates the state of the simulation.

apply_observables(observables: list[Observable]) → None

Applies the diagonalizing matrices of the given observables to the state of the simulation.

This method can only be called once.

Parameters:

observables (list[Observable]) – The observables to apply

Raises:

RuntimeError – If this method is called more than once

retrieve_samples() → list[int]

Retrieves samples of states from the state of the simulation, based on the probabilities.

Returns:

list[int] – List of states sampled according to their probabilities in the state. Each integer represents the decimal encoding of the corresponding computational basis state.

property density_matrix: ndarray

The density matrix specifying the current state of the simulation.

Note

Mutating this array will mutate the state of the simulation.

Type:

np.ndarray

property state_with_observables: ndarray

The density matrix diagonalized in the basis of the measured observables.

Raises:

RuntimeError – If observables have not been applied

Type:

np.ndarray

expectation(observable: Observable) → float

The expected value of the observable in the given state.

Parameters:

observable (Observable) – The observable to measure.

Returns:

float – The expected value of the observable.

property probabilities: ndarray

The probabilities of each computational basis state of the current density matrix of the simulation.

Type:

np.ndarray