We are still at the very beginnings of physical implementations of quantum computers. The devices listed below have been used successfully to entangle two qubits (at most!), except for NMR which has gone up to 7 qubits. It is still premature to try to predict which device will prove most effective for building a quantum computer capable of dealing with several hundred qubits (if such a computer someday exists); perhaps it will be something new, not on this list at all. In any case, it would be as foolish to predict that such a computer will not be available by 2050 as to predict the contrary.

The storage and processing of quantum information requires physical systems possessing the following properties (di Vincenzo criteria):

1. (i) they must be scalable, that is, capable of being extended to a sufficient number of qubits, with well defined qubits;

2. (ii) they must have qubits which can be initialized in the state |0〉;

3. (iii) they must have qubits which are carried by physical states of sufficiently long lifetime, so as to ensure that the quantum states remain coherent throughout the calculation;

4. (iv) they must possess a set of universal quantum gates: unitary transformations on individual qubits and a cNOT gate, which are obtained by controlled manipulations;

5. (v) there must be an efficient procedure for measuring the state of the qubits at the end of the calculation (readout of the results).