I have now a working solution. However the code amount is about 600 LOC.

This widget is derived from QDoubleSpinBox. It uses a decimal value of 1000 (that is more decimal points than a double can handle) and implements a new decimal value for the presentation in scientific notation. The Validator is realised by setting the LineEdit to a QDoubleValidator::ScientificNotation. However the most important part is the reimplementation of textFromValue and valueFromText. This unfortunately requires to copy the whole validation code of QDoubleSpinBox, which can not be borrowed and represents the major part of the code.

Since I can not paste the whole code I put it online at:
http://www.matthiaspospiech.de/blog/...ific-notation/

Can this code be shrinked by keeping the functionality?