Quantum bit commitment is weaker than quantum bit seals

Unconditionally secure quantum bit commitment (QBC) and quantum bit seal (QBS) were both considered impossible. But relativistic QBC is not covered. Practically secure QBC also exists. On the other hand, QBS remains insecure as long as the cheater can apply collective measurements. Thus it seems that QBC is strictly weaker than QBS in security requirement. Here we prove this conjunction directly, by showing that secure QBC can be built upon secure QBS, but not vice versa.