Exploring the Observer Effect

A qubit is a two-level quantum system in a superposition of the quantum states $|0\rangle$ and $|1\rangle$, unless we observe it. Once we observe it, there are distinct probabilities of measuring 0 or 1. In physics, this is known as the observer effect. The observer effect states that the mere observation of a phenomenon inevitably changes that phenomenon itself. For instance, if you measure the temperature in your room, you’re taking away a little bit of the energy to heat the mercury in the thermometer. This loss of energy cools down the rest of your room. In the world we experience, the effects of observation are often negligible.

However, in the sub-atomic world of quantum mechanics, these effects matter. The mere observation of a quantum bit changes its state from a superposition of the states $|0\rangle$ and $|1\rangle$ to either one value. Therefore, even an observation manipulates the system that we need to consider when developing a quantum circuit.