Anyone who asks a quantum physicist about the feasibility of quantum computers will probably be one of the first answers to be given a list of ingredients that he needs. This list from 2000, which goes back to the Jülich physicist David DiVincenzo, names criteria that a quantum computer must offer.
The first item on DiVincenzo’s list is quite obvious and says that you need a physical system of well-defined quantum bits. Such qubits form the smallest unit of a quantum processor and correspond to the bits of a classic computer. The crux of the matter: the qubits must be scalable, their number and thus the performance of the quantum processor must be easy to increase. This detail is the only challenge that currently stands in the way of the development of useful quantum computers.
The next item on the DiVincenzo list must be initialization. This means that it must be possible to put the qubits used in a known state. An intrinsic property of qubits is that their quantum state is lost after a while. This loss of information is inevitable because it is a property of every quantum system. The time scale on which the information in a qubit is lost is called the decoherence time. So in order to build a quantum computer, the time it takes to obtain the qubit – the coherence time – must be longer than the time it takes to perform operations on the qubit. Furthermore, the state of a qubit must be readable so that the results of a quantum algorithm can be found out.
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