8 Two Qubits, Three

Not only is the Universe stranger than we think, it is stranger than we can think.

Werner Heisenberg [2]

In the previous chapter we defined qubits and saw what we could do with just one of them. Things now start to get exponential with every additional qubit added, because entanglement allows the size of the working state space to double.

This chapter is about seeing how multiple qubits can behave together and then building a collection of tools to manipulate those qubits. These include the concept of entanglement, which is a requirement to do quantum computing. We also examine important 2-qubit gates like CNOT and SWAP. This will lead us into chapter 9 and chapter 10 where we look at algorithms and build circuits that ...

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