Measuring a classical bit does not change its value. If the bit was in state 0 before we measured it, it will remain 0 even after we measure it. However, if you measure a qubit that is in a superposition state, you will force it to ‘collapse’ in one of the states. Just like if you a touch a spinning coin (superposition), you force it to collapse into either heads or tails. Once a qubit loses its superposition state and collapses, it becomes an ordinary classical bit.
Acquiring knowledge about a quantum state, by observing or measuring it, changes its state. This is a behaviour that is alien to us. Looking at a moving train does not affect the state of the train, it will still be moving after you have looked at it. Although strange and unfamiliar, it is this quantum mechanical principle that ensures transmitted information on a quantum network is not intercepted. Since intercepting it (by reading or copying it) would change its state, and it would allow you to detect interception attempts.
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