  Unconditionally secure quantum bit commitment (QBC) was widely believed to be
impossible for more than two decades. But recently, based on an anomalous
behavior found in quantum steering, we proposed a QBC protocol which can be
unconditionally secure in principle. The protocol requires the use of
infinite-dimensional systems, therefore it may seem less feasible in practice.
Here we propose a quantum optical method based on Mach-Zehnder interferometer,
which gives a very good approximation to such infinite-dimensional systems.
Thus, it enables a proof-of-principle experimental implementation of our
protocol, which can also serve as a practically secure QBC scheme. Other
multi-party cryptographic protocols such as quantum coin tossing can be built
upon it too. Our approach also reveals a relationship between infinity and
non-locality, which may have an impact on the research of fundamental theories.
