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3. Low decoherence 〜 the key issue, many candidate systems for quantum hardwares are ruled out by this character. In general, quantum-coherent oscillations are caused at τ dec Δ / h ≫ 1 .
An approximate benchmark for sufficiently low decoherence is less than 10＾4 per elementary quantum gate operation in faithful loss.
4. Quantum gates. The universal set of gates is needed in order to control the system ( for example, Hamiltonian ). After preparing an optional state, we have to be able to switch an interaction between them on & off to make qubits act together and make computation useful.
5. Measurement ; the final requirement for quantum computation is the ability to perform quantum measurements on the qubits for obtaining a calculated result. Such readout transfers information to the external world to make the information useful.
