def _to_operator(rep, data, input_dim, output_dim):
    """Transform a QuantumChannel to the Operator representation."""
    if rep == 'Operator':
        return data
    if rep == 'Stinespring':
        return _stinespring_to_operator(data, input_dim, output_dim)
    # Convert via Kraus representation
    if rep != 'Kraus':
        data = _to_kraus(rep, data, input_dim, output_dim)
    return _kraus_to_operator(data, input_dim, output_dim)
Transform a QuantumChannel to the Operator representation.