6.7 Physical Realisations of Qubits and Quantum Gates
Now that the theoretical framework for quantum bits and quantum gates has been set, it is important to understand how these can actually be realised from a hardware point of view.
6.7.1 The DiVincenzo criteria
The modern approach to building quantum computing hardware was marked by the set of requirements for the physical implementation of quantum computation proposed in 2000 by DiVincenzo [87]. These requirements, now known as the "DiVincenzo criteria", are as follows:
1. A scalable physical system with well characterised qubits. A qubit being “well characterised” means the following:
- Its physical parameters should be accurately known, including the internal Hamiltonian of the qubit, which determines the qubit energy eigenstates. Typically, the ground state is taken as
and the first excited state is taken as 
. - The presence of and couplings to other states of the qubit.
- The couplings to external fields...
