There are two widely known interpretations of quantum theory: Bohr’s Copenhagen interpretation and Everett’s interpretation. The focus of Chapter 9 is to assess, within the context they provide, the interpretation-independent advances discussed in this book. We want to see whether the advances that include decoherence and quantum Darwinism fit these two established and widely known points of view. In fact, it is surprising that (with minor but significant adjustments) decoherence and quantum Darwinism fit very naturally, addressing questions that were recognized as open and important. We then discuss the existential interpretation. It can be seen as a continuation of the Copenhagen interpretation, with the Universe consisting of quantum and classical realms, but with classicality that is emergent, rather than preordained. It is also compatible with Everett’s interpretation, since quantum states and evolutions are all that is needed. However, unlike the Many Worlds interpretation (which regards the quantum state of the Universe as objectively existing, akin to a classical point in phase space or a classical electromagnetic field), the existential interpretation recognizes that quantum states combine information and existence—they are epiontic. The mix of existence and information they represent fits the relative states reading of Everett’s approach.

In this chapter we introduce a variant of the BB’84 quantum key distribution protocol, the E’91 protocol due to Ekert. We show that this protocol achieves a higher level of security called “device independent security.” What this means, informally, is that the new protocol’s security doesn’t rely on Alice and Bob performing trusted measurements on their qubit in each round. We sketch the proof of security of the E’91 protocol, which rests on the property of entanglement monogamy.