Unconditionally Secure Bit Commitment with Flying Qudits

We present simple and practical protocols for bit and data commitment based on propagating a qudit in Minkowski space, and prove that they are unconditionally secure if relativistic quantum theory is correct. The security proof follows from the no-summoning theorem, a relativistic extension of the no-cloning theorem. This result highlights another previously unnoticed limitation of Mayers' and Lo-Chau's no-go theorems for quantum bit commitment. In relativistic quantum theory, the unitary operation required for a Mayers-Lo-Chau attack can be known to both parties but impossible to implement physically, as it violates causality.