cirq.sim.DensityMatrixStepResult

A single step in the simulation of the DensityMatrixSimulator.

Inherits From: StepResult

cirq.sim.DensityMatrixStepResult(
    density_matrix: np.ndarray,
    measurements: Dict[str, np.ndarray],
    qubit_map: Dict[cirq.ops.Qid, int],
    dtype: "DTypeLike" = np.complex64
)

density_matrix The density matrix at this step. Can be mutated.
measurements The measurements for this step of the simulation.
qubit_map A map from qid to index used to define the ordering of the basis in density_matrix.
dtype The numpy dtype for the density matrix.

qubit_map A map from the Qubits in the Circuit to the the index of this qubit for a canonical ordering. This canonical ordering is used to define the state vector (see the state_vector() method).
measurements A dictionary from measurement gate key to measurement results, ordered by the qubits that the measurement operates on.

Methods

density_matrix

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density_matrix(
    copy=True
)

Returns the density matrix at this step in the simulation.

The density matrix that is stored in this result is returned in the computational basis with these basis states defined by the qubit_map. In particular the value in the qubit_map is the index of the qubit, and these are translated into binary vectors where the last qubit is the 1s bit of the index, the second-to-last is the 2s bit of the index, and so forth (i.e. big endian ordering). The density matrix is a 2 ** num_qubits square matrix, with rows and columns ordered by the computational basis as just described.

Example:

  • qubit_map: {QubitA: 0, QubitB: 1, QubitC: 2} Then the returned density matrix will have (row and column) indices mapped to qubit basis states like the following table

    QubitA QubitB QubitC
    0 0 0 0
    1 0 0 1
    2 0 1 0
    3 0 1 1
    4 1 0 0
    5 1 0 1
    6 1 1 0
    7 1 1 1

Args
copy If True, then the returned state is a copy of the density matrix. If False, then the density matrix is not copied, potentially saving memory. If one only needs to read derived parameters from the density matrix and store then using False can speed up simulation by eliminating a memory copy.

sample

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sample(
    qubits: List[cirq.ops.Qid],
    repetitions: int = 1,
    seed: "cirq.RANDOM_STATE_OR_SEED_LIKE" = None
) -> np.ndarray

Samples from the system at this point in the computation.

Note that this does not collapse the state vector.

Args
qubits The qubits to be sampled in an order that influence the returned measurement results.
repetitions The number of samples to take.
seed A seed for the pseudorandom number generator.

Returns
Measurement results with True corresponding to the |1⟩ state. The outer list is for repetitions, and the inner corresponds to measurements ordered by the supplied qubits. These lists are wrapped as a numpy ndarray.

sample_measurement_ops

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sample_measurement_ops(
    measurement_ops: List[cirq.ops.GateOperation],
    repetitions: int = 1,
    seed: "cirq.RANDOM_STATE_OR_SEED_LIKE" = None
) -> Dict[str, np.ndarray]

Samples from the system at this point in the computation.

Note that this does not collapse the state vector.

In contrast to sample which samples qubits, this takes a list of cirq.GateOperation instances whose gates are cirq.MeasurementGate instances and then returns a mapping from the key in the measurement gate to the resulting bit strings. Different measurement operations must not act on the same qubits.

Args
measurement_ops GateOperation instances whose gates are MeasurementGate instances to be sampled form.
repetitions The number of samples to take.
seed A seed for the pseudorandom number generator.

Returns: A dictionary from measurement gate key to measurement results. Measurement results are stored in a 2-dimensional numpy array, the first dimension corresponding to the repetition and the second to the actual boolean measurement results (ordered by the qubits being measured.)

Raises
ValueError If the operation's gates are not MeasurementGate instances or a qubit is acted upon multiple times by different operations from measurement_ops.

set_density_matrix

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set_density_matrix(
    density_matrix_repr: Union[int, np.ndarray]
)

Set the density matrix to a new density matrix.

Args
density_matrix_repr If this is an int, the density matrix is set to the computational basis state corresponding to this state. Otherwise if this is a np.ndarray it is the full state, either a pure state or the full density matrix. If it is the pure state it must be the correct size, be normalized (an L2 norm of 1), and be safely castable to an appropriate dtype for the simulator. If it is a mixed state it must be correctly sized and positive semidefinite with trace one.