A possible quantum bit is “the Cooper-pair box”. It consists in a superconducting island (several million cubic nanometres, in aluminium for instance), separated of a superconducting wire by an insulating barrier (tunnel): it is hence a Josephson junction. The quantum variable is the number of superconducting pairs that are attracted in the island by a gate potential placed in front of the island (scheme).
In a classical world, since superconductivity is caused by electron pairs, only a whole number of pairs can be in the island: a pair is either inside the island or outside. However, if the tunnel barrier is wide enough and thanks to the uncertainty principle [PRINCIPE D’INCERTITUDE], a pair can be both inside the island and outside – a superposition of 0s and 1s.
Another possible quantum bit is composed of a superconducting loop with a Josephson junction inside. This loop carries a supercurrent (a current that does not create heat: since the resistance is equal to zero, there is no Joule effect). This supercurrent can flow in either direction and we can use the direction of the flow to make 0s or 1s (in either direction). In a quantum loop, the current flows in both directions at the same time, meaning we have both 0s and 1s: it is indeed a quantum bit.
Today’s most promising qubits are Josephson oscillators: they are composed of an adjustable Josephson junction (a squid) that behaves as an inductance (a superconducting coil), in parallel with a capacity. The record held by a group of scientists at the University of Santa Barbara in the United Stated is 5 qubits in the same circuit, and in France, 2 entangled qubits that can be both addressed and read (cf. figure of the 2-qubit quantum microprocessor).
The difficult part does not only consists in finding the adequate quantum bits, but also the ways to initialize, address and manipulate them while not exposing them too much to the outside world because they would lose their quantum characteristics.
