def _to_chi(rep, data, input_dim, output_dim):
    """Transform a QuantumChannel to the Chi representation."""
    if rep == 'Chi':
        return data
    # Check valid n-qubit input
    _check_nqubit_dim(input_dim, output_dim)
    if rep == 'Operator':
        return _from_operator('Chi', data, input_dim, output_dim)
    # Convert via Choi representation
    if rep != 'Choi':
        data = _to_choi(rep, data, input_dim, output_dim)
    return _choi_to_chi(data, input_dim, output_dim)
Transform a QuantumChannel to the Chi representation.