## SITUATION

Standard computers work using a binary system: units of information, bits, can have the value of 0 or 1. From a hardware point of view, this means that an electric current either passes or doesn’t. When you enter the infinitely small world of quantums, highly fascinating things occur. An object (it may be a photon or electron, etc.) can exist in two states at the same time: e.g. an electron can be located in two different orbits around a nucleus or it can pass through two places simultaneously. This state, called superposition, only exists when there is no observer. If we measure the given particle the superposition disappears The particle ‘selects’ its conditions and the oddities stop. The secret of quantum computing involves manipulating objects such as electrons or photons and make them represent information not by their status but by using all the available options. This technique allows the information to be multiplied exponentially. Consequently, a two bit computer can represent the four following numbers: 00, 01, 10 and 11, in other words 0, 1, 2 and 3 in binary. However a quantum computer with two qubits can store four times as many numbers at the same time! A 300 qubit computer would be capable of storing more bits than there are atoms in the whole universe.

Quantum computers will therefore be used for massive parallel calculations but with a single limit: once the ‘measuring’ is finally finished, there will no longer be any superposition and only one of the solutions will be available for the user. This is why we think that quantum computes will be primarily used in applications where a choice has to be made amongst multiple alternatives. Such a machine will be able to factorise huge numbers (which will allow them to break almost any code) and search in large databases, to navigate in an optimal fashion in an explosion of potential combinations as in the famous ‘travelling salesman’s dilemma’ or to provide very precise simulations especially in the field of physics itself. What’s more, according to Seth Lloyd, the famous researcher in the field and author of Programming the Universe, our universe will be nothing more than a massive quantum calculator!

However, there is a major problem: decoherence. The observer who breaks the superposition is not just a human but can be any system external to the particle’s behaviour that can interact with it. If we build a quantum computer that is too big, it will be difficult to isolate it sufficiently. As Seth Lloyd said, «we don’t have a problem with building quantum computers, we have built big ones». Currently, according to IT expert, Thierry Lombry, no one, not at Los Alamos, not at MIT, not at Princeton nor at IBM has perceived a solution beyond 10 qubits because of the decoherence makes the systems too fragile to be useable.

What will a quantum computer, which some people are forecasting will be in use from the twenties, actually look like, ? Will it be solid, liquid or gas? Will it be the size of a building or can you hold it in your hand?