qiskit/tools/qi/qi.py
def partial_trace(state, trace_systems, dimensions=None, reverse=True):
    """
    Partial trace over subsystems of multi-partite matrix.

    Note that subsystems are ordered as rho012 = rho0(x)rho1(x)rho2.

    Args:
        state (matrix_like): a matrix NxN
        trace_systems (list(int)): a list of subsystems (starting from 0) to
                                  trace over.
        dimensions (list(int)): a list of the dimensions of the subsystems.
                                If this is not set it will assume all
                                subsystems are qubits.
        reverse (bool): ordering of systems in operator.
            If True system-0 is the right most system in tensor product.
            If False system-0 is the left most system in tensor product.

    Returns:
        matrix_like: A density matrix with the appropriate subsystems traced
            over.
    Raises:
        Exception: if input is not a multi-qubit state.
    """
    state = np.array(state)  # convert op to density matrix

    if dimensions is None:  # compute dims if not specified
        num_qubits = int(np.log2(len(state)))
        dimensions = [2 for _ in range(num_qubits)]
        if len(state) != 2 ** num_qubits:
            raise Exception("Input is not a multi-qubit state, "
                            "specify input state dims")
    else:
        dimensions = list(dimensions)

    if isinstance(trace_systems, int):
        trace_systems = [trace_systems]
    else:  # reverse sort trace sys
        trace_systems = sorted(trace_systems, reverse=True)

    # trace out subsystems
    if state.ndim == 1:
        # optimized partial trace for input state vector
        return __partial_trace_vec(state, trace_systems, dimensions, reverse)
    # standard partial trace for input density matrix
    return __partial_trace_mat(state, trace_systems, dimensions, reverse)
def partial_trace(state, trace_systems, dimensions=None, reverse=True):
    """
    Partial trace over subsystems of multi-partite matrix.

    Note that subsystems are ordered as rho012 = rho0(x)rho1(x)rho2.

    Args:
        state (matrix_like): a matrix NxN
        trace_systems (list(int)): a list of subsystems (starting from 0) to
                                  trace over.
        dimensions (list(int)): a list of the dimensions of the subsystems.
                                If this is not set it will assume all
                                subsystems are qubits.
        reverse (bool): ordering of systems in operator.
            If True system-0 is the right most system in tensor product.
            If False system-0 is the left most system in tensor product.

    Returns:
        matrix_like: A density matrix with the appropriate subsystems traced
            over.
    Raises:
        Exception: if input is not a multi-qubit state.
    """
    state = np.array(state)  # convert op to density matrix

    if dimensions is None:  # compute dims if not specified
        num_qubits = int(np.log2(len(state)))
        dimensions = [2 for _ in range(num_qubits)]
        if len(state) != 2 ** num_qubits:
            raise Exception("Input is not a multi-qubit state, "
                            "specify input state dims")
    else:
        dimensions = list(dimensions)

    if isinstance(trace_systems, int):
        trace_systems = [trace_systems]
    else:  # reverse sort trace sys
        trace_systems = sorted(trace_systems, reverse=True)

    # trace out subsystems
    if state.ndim == 1:
        # optimized partial trace for input state vector
        return __partial_trace_vec(state, trace_systems, dimensions, reverse)
    # standard partial trace for input density matrix
    return __partial_trace_mat(state, trace_systems, dimensions, reverse)
Partial trace over subsystems of multi-partite matrix.

    Note that subsystems are ordered as rho012 = rho0(x)rho1(x)rho2.

    Args:
        state (matrix_like): a matrix NxN
        trace_systems (list(int)): a list of subsystems (starting from 0) to
                                  trace over.
        dimensions (list(int)): a list of the dimensions of the subsystems.
                                If this is not set it will assume all
                                subsystems are qubits.
        reverse (bool): ordering of systems in operator.
            If True system-0 is the right most system in tensor product.
            If False system-0 is the left most system in tensor product.

    Returns:
        matrix_like: A density matrix with the appropriate subsystems traced
            over.
    Raises:
        Exception: if input is not a multi-qubit state.