The BB84 protocol was one of the first ideas developed for secure quantum communication. From there, research expanded, trying to find out what could be done using similar schematics. Can you expand this protocol and use the 3 axis of the Bloch Sphere?
Yes, instead of using only states |0>, |1>, |+>, |->, we will now add the eigenstates of the Y basis, |+i> and |-i>. These two last states can be understood as the polarization on the Y axis in a sphere, with eigenvalues +1 (blue) and -1 (red). This protocol has been studied and it is called the six-state protocol.
In quantum networks, we always talk about distributing entanglement, why don’t we need it here? Could we implement a similar protocol using entanglement?
The explained protocol in the video is based on preparing and measuring quantum states. Where measuring an unknown quantum state may alter that state. This is a consequence of quantum indeterminacy and can be exploited to detect any eavesdropping on communication (which necessarily involves measurement) and, more importantly, to calculate the amount of information that has been intercepted.
Similar protocols based on quantum entanglement have been proposed, some of them are the E91 protocol and the BBM92 protocol. In these protocols Alice and Bob share an entangled pair and if anyone intercepts any of the two parts of the states will alter the overall system, revealing the presence of the third party (and the amount of information they have gained).