In the double-slit experiment, the measurement of photons can be done using various techniques. One common method involves using a photon detector, such as a photographic plate, a CCD camera, or a photomultiplier tube. These detectors are capable of detecting individual photons or measuring the intensity of the light.
When the double-slit experiment is conducted without any additional measuring device, photons exhibit an interference pattern on a screen placed behind the slits. This pattern arises due to the wave-like nature of photons interfering with each other as they pass through the slits.
Now, let's consider the scenario where a measuring device is gradually introduced into the experiment:
Initially, if no measuring device is present, photons will create an interference pattern on the screen.
If a measuring device, such as a photon detector, is placed behind one of the slits, it will detect which slit each photon passes through. This detection process involves an interaction between the measuring device and the photon, which disturbs the photon's wave-like behavior. As a result, the interference pattern on the screen will begin to disappear. This is because the act of measurement collapses the photon's wavefunction into a definite state, eliminating the interference.
If the measuring device is removed before the photon reaches the screen, the interference pattern can reappear. The interaction with the measuring device is no longer present to collapse the wavefunction, and the photons can once again exhibit their wave-like behavior, leading to interference.
The key point at which the measuring device prevents the interference pattern is when it obtains information about which slit the photon passed through. This information disrupts the wave-like behavior and collapses the photon's wavefunction into a specific path, destroying the interference. Therefore, the interference pattern gradually diminishes as the measuring device gains information about the photons' paths.
It's important to note that the exact details of the experimental setup and the measuring device used can influence the specific outcomes and effects observed in the double-slit experiment. The gradual introduction or removal of a measuring device is just one way to illustrate the connection between measurement and the disappearance/reappearance of interference.