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There are at least five important criteria that must be satisfied by possible hardware for a quantum computer. They are as follows.
1. Identifiable qubits and the ability to scale them in number. This means that even if only a few qubits can be constructed, it’s not sufficient to realize useful quantum computation. Very many qubits are required in some controlled and reliable way for realizing practical quantum computation.
2. Ability to prepare the initial state of the whole system. All qubits first must be prepared in some state and only after that, quantum computation can be started.
