The fundamental unit of a quantum computer is the quantum bit, or qubit (Krill, 2010). Like the bits used in classical computers, qubits are units with two distinguishable states, which can be translated as 0 or 1. Qubits can be made from a wide variety of things, such as photons, trapped atoms and electrons in a superconducting circuit. So long as the medium being used can have two different quantum states, it can potentially be used as a qubit. While it might seem that the advantage of qubits over bits is simply their smaller size, there is actually more to it than that. The main difference between a bit and a qubit is the notion of superposition.
Superposition is one of the ways in which things at the quantum level behave differently than things at the classical level. The superposition of a particle is the combination of all possible states that the particle could have. So whereas a classical bit is either 1 or 0, a qubit is both 1 and 0, and theoretically any value or combination of values of and between those two states (Hagar, 2011). This is possible because objects at the quantum level behave as both a wave and a particle. Before being measured, a qubit acts as a wave, occupying all its potential states--its superposition. However, when it is measured, it is forced to collapse into one position, behaving as a particle. So, during a computation, a series of qubits would occupy all the possible answers to the problem. Then, when a certain answer is requested they would be forced to collapse into the answer being requested. It is clear that this would be faster than a classical computation. The classical computation could only calculate one answer at a time, and require time arriving at the answer. A quantum computation would simply be picking the answer out of a storage of all the potential answers currently being occupied by the qubits. While this is a very general description of the way in which quantum computing works, it should serve to give the reader a basic understanding of what is meant by a quantum computer and what its advantages are over a classical computer.
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