The phenomenon you are referring to is called the "single-photon double-slit experiment," and it is indeed a counterintuitive aspect of quantum mechanics. In this experiment, when individual photons are sent through a double-slit apparatus, they can exhibit an interference pattern on a screen, suggesting wave-like behavior, despite being detected one at a time.
To understand this behavior, it's crucial to distinguish between the wavefunction, which describes the probabilistic behavior of the photon, and the actual physical position of the photon.
The wavefunction of the photon represents the probability distribution of its possible positions when it is not being measured. It describes the likelihood of finding the photon at different positions on the screen. This probability distribution can exhibit interference patterns when multiple possible paths of the photon interfere constructively or destructively.
When a single photon is sent through the double-slit apparatus, the wavefunction associated with the photon interacts with both slits simultaneously. It effectively "splits" into two parts, each passing through one of the slits. The two portions of the wavefunction then overlap and interfere with each other, creating an interference pattern on the screen.
However, it's important to note that the photon itself is not physically present in multiple places at the same time. The interference pattern arises from the probabilistic nature of the wavefunction and the resulting distribution of detection events on the screen.
When the photon is measured or interacts with a detector, its wavefunction "collapses" to a single position, and the photon is detected at a specific location on the screen. This collapse of the wavefunction is often described as the photon "choosing" one particular path or position during the measurement process.
In summary, the single-photon double-slit experiment showcases the wave-particle duality of quantum objects. The wavefunction of the photon represents the probability distribution of its position, and when multiple possible paths are involved, interference effects can be observed. However, the photon itself is not physically present in multiple places simultaneously but behaves in a probabilistic manner, exhibiting wave-like behavior as described by its wavefunction.