The phenomenon you are referring to is known as "quantum wavefunction collapse" or "measurement collapse." In quantum mechanics, particles, such as qubits, are described by a wavefunction that represents a superposition of all possible states that the particle can be in. This superposition includes different probabilities for each possible state.
When a measurement is made on a quantum system, it interacts with the system and extracts information about its state. This interaction causes the wavefunction to "collapse" into a specific state corresponding to the measurement outcome. The collapse is a fundamental aspect of quantum mechanics and is not fully understood in terms of classical intuition.
The collapse of the wavefunction is often explained using the Copenhagen interpretation, which is one of the interpretations of quantum mechanics. According to this interpretation, the act of measurement forces the system to "choose" one of its possible states, and all other possibilities become inaccessible or irrelevant for further measurements.
It is important to note that the collapse of the wavefunction is not inherently caused by the act of observation by a conscious observer, as is sometimes mistakenly believed. In quantum mechanics, an observation or measurement can be any interaction that extracts information from the quantum system, whether it is detected by a human or a measuring instrument.
Alternative interpretations, such as the many-worlds interpretation and the consistent histories interpretation, propose different explanations for the apparent collapse of the wavefunction. However, the collapse itself remains a subject of ongoing debate and research in quantum mechanics.