## 3 Discussion  
In summary, we showed that, as long as time can be treated as a continuous variable, then each infinite-dimensional system in the unconditionally secure QBC protocol proposed in ref. [56] can be realized using a single photon. Thus, we obtained an experimental implementation of this QBC protocol which is feasible under currently available technology. Other \"post-cold-war era\" multi-party cryptographic protocols are, therefore, made possible too, e.g., quantum coin tossing, as elaborated in Appendix B. The dimension of the systems cannot really be infinite in practice though, making the current experimental implementation a practically secure QBC only. Nevertheless, it still has an advantage over many other practically secure QBC protocols (e.g., refs. [61; 62; 63]). While these protocols could be more feasible than ours in practice, their security is based on certain practical limitations. For example, currently available quantum memory cannot store the quantum states for a long period of time, so that, as long as the holding phase of the protocol is longer than this period of time, we can be sure that [PERSON] can no longer cheat by storing the states and delaying the measurement until the unveil phase. However, as technology advances, the storage time of quantum memory will increase, making it harder and harder to keep the corresponding protocol secure. On the contrary, the security of our experimental protocol is based on the unconditionally secure theoretical protocol in ref. [56]. Practical limitation is the reason that weakens its security so that it can be practically secure only, not the reason that makes it secure. Therefore, with the advance in the technology on the precision of the delay time adjustment, we can expect the security of this experimental protocol to be constantly improved towards that of the theoretical protocol in ref. [56]. Meanwhile, it is also worth studying whether some new technologies can be adopted to implement the infinite-dimensional systems to make our protocol even more feasible, e.g., the continuous phase noise resulting from gain switching laser operation [64]. Our result may also contribute to the development of fundamental theories. There is a brilliant idea called the CBH theorem [65], which intends to deduce quantum theory by using three information-theoretic constraints as fundamental axioms: (I) the impossibility of superluminal information transfer, (II) the impossibility of perfect broadcasting of an unknown state, and (III) the impossibility of unconditionally secure BC. The reason for including the last constraint is that [PERSON]'s cheating strategy against BC requires the use of entangled states, as can be seen from Section 2.1 of the current work. That is, the impossibility of unconditionally secure BC entails the existence of non-locality, which is one of the essential feature of quantum theory. However, in the three QBC protocols we previously proposed in refs. [45; 46; 47], which manage to evade the MLC no-go theorem, non-locality is necessary even for honest participants. This observation implies that, if the constraint (III) is wrong, i.e., unconditionally secure QBC exists, then non-locality is also entailed. For this reason, we tend to believe that the (in)validity of constraint (III) has nothing to do with the existence of non-locality. The latter has to exist in our physical world. To complete the deduction of quantum theory from information-theoretic axioms, we should look for another constraint to replace constraint (III). Nevertheless, the finding of the unconditionally secure QBC protocol in ref. [56] may blur the above picture at first glance. This is because the protocol makes use of infinite-dimensional systems instead of entangled states, so that it seems to indicate that, in addition to non-locality, infinity should also be taken into account as the quantum resources that need to be entailed if we want to build quantum theory completely on top of information-theoretic axioms.