acknowledgements
- it is a pleasure to thank B.-G. Englert for his encouraging remarks which inspired this project
- we enjoyed fruitful collaboration on Hadamard matrices with I. Bengtsson, Å. Ericsson,
M. Kuś and W. Słomczyński
- we would like to thank C. D. Godsil, M. Grassl, M. Rötteler, and R. Werner for fruitful discussions
and P. Diţă, M. Matolcsi, R. Nicoară, W. Orrick, F. Szöllősi, T. Tao and V. Elser for helpful
correspondence
- K. Ż. is grateful for hospitality of the Perimeter Institute for
Theoretical Physics in Waterloo, where this work was initiated
- we acknowledge financial
support by Polish Ministry of Science and Information Technology under the grant
PBZ-MIN-008/P03/2003
and
the European Research project COCOS