In quantum mechanics, the act of measuring a physical property of a quantum system can cause a change in the system's state, including the position of an atom. This phenomenon is known as the "collapse" or "projection postulate" of the wavefunction.
According to the principles of quantum mechanics, prior to measurement, the state of a quantum system is described by a superposition of multiple possible states. This superposition is represented by the wavefunction, which encodes the probabilities of different outcomes. However, when a measurement is performed, the wavefunction "collapses" into one of the possible states corresponding to the measurement outcome.
The specific outcome of the measurement is probabilistic, and the probabilities are determined by the squared magnitudes of the amplitudes of the different states in the superposition. After the measurement, the system is found to be in one of the eigenstates (also called eigenvalues) associated with the measured property.
In the case of position measurement, if you measure the position of an atom, it collapses into one of the eigenstates of position, giving a specific position value. The act of measurement effectively localizes the atom at a particular position.
It's important to note that the collapse of the wavefunction is a fundamental aspect of quantum mechanics, and it introduces an element of randomness into the measurement process. The precise mechanism behind the wavefunction collapse is a topic of interpretation and debate in quantum mechanics, with different interpretations offering various explanations for the collapse phenomenon.