# Superposition

In contrast to classical bits, a qubit can be in a state ‘1’, ‘0’, or in a *superposition* of both states. Analogous to a coin, if heads represents ‘1’and tails ‘0’, then a spinning coin is a superposition state. Just like a spinning coin eventually lands on either heads or tails because it interacts with its surroundings, a qubit ultimately loses its superposition after a certain time, referred to as *coherence time*. Accordingly, the computations need to be completed faster than this coherence time otherwise the information is lost. Because qubits are very fragile, any interaction with their environment can easily knock them out of their superposition state.

### Visualising a qubit state

A single qubit state can be visualised using a so-called Bloch sphere. A Bloch sphere is often labeled with 0 on its north pole and 1 on its south pole. An arrow placed within the sphere with its tail at the center of the sphere and its head at the surface of the sphere represents the qubit state. If the arrow is pointing to the north pole, then the qubit is in state 0. If the arrow is pointing anywhere between the north and south poles, then the qubit state can be represented by a linear combination of 0 and 1, i.e., a superposition state.

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