DensityMatrix

class DensityMatrix(data, dims=None)

Bases: QuantumState, TolerancesMixin

DensityMatrix class

Initialize a density matrix object.

Parameters

• or (data (np.ndarray or list or matrix_like or QuantumCircuit) – qiskit.circuit.Instruction): A statevector, quantum instruction or an object with a to_operator or to_matrix method from which the density matrix can be constructed. If a vector the density matrix is constructed as the projector of that vector. If a quantum instruction, the density matrix is constructed by assuming all qubits are initialized in the zero state.

• dims (int or tuple or list) – Optional. The subsystem dimension of the state (See additional information).

Raises

QiskitError – if input data is not valid.

Additional Information:

The dims kwarg can be None, an integer, or an iterable of integers.

• Iterable – the subsystem dimensions are the values in the list with the total number of subsystems given by the length of the list.

• Int or None – the leading dimension of the input matrix specifies the total dimension of the density matrix. If it is a power of two the state will be initialized as an N-qubit state. If it is not a power of two the state will have a single d-dimensional subsystem.

Methods

conjugate	Return the conjugate of the density matrix.
copy	Make a copy of current operator.
dims	Return tuple of input dimension for specified subsystems.
draw	Return a visualization of the Statevector.
evolve	Evolve a quantum state by an operator.
expand	Return the tensor product state other ⊗ self.
expectation_value	Compute the expectation value of an operator.
from_instruction	Return the output density matrix of an instruction.
from_int	Return a computational basis state density matrix.
from_label	Return a tensor product of Pauli X,Y,Z eigenstates.
is_valid	Return True if trace 1 and positive semidefinite.
measure	Measure subsystems and return outcome and post-measure state.
partial_transpose	Return partially transposed density matrix.
probabilities	Return the subsystem measurement probability vector.
probabilities_dict	Return the subsystem measurement probability dictionary.
purity	Return the purity of the quantum state.
reset	Reset state or subsystems to the 0-state.
reverse_qargs	Return a DensityMatrix with reversed subsystem ordering.
sample_counts	Sample a dict of qubit measurement outcomes in the computational basis.
sample_memory	Sample a list of qubit measurement outcomes in the computational basis.
seed	Set the seed for the quantum state RNG.
tensor	Return the tensor product state self ⊗ other.
to_dict	Convert the density matrix to dictionary form.
to_operator	Convert to Operator
to_statevector	Return a statevector from a pure density matrix.
trace	Return the trace of the density matrix.

Attributes

atol = 1e-08

data

Return data.

dim

Return total state dimension.

num_qubits

Return the number of qubits if a N-qubit state or None otherwise.

rtol = 1e-05

settings

Return settings.