The double-slit experiment is a fundamental experiment in quantum physics that investigates the wave-particle duality of light and matter. In its simplest form, the experiment involves a barrier with two slits, and light or particles are sent through the slits onto a screen, creating an interference pattern.
The crucial aspect of the double-slit experiment is the observation of interference patterns when particles or light pass through both slits simultaneously. When the experiment is conducted without any detectors at the slits, a wave-like interference pattern emerges on the screen, suggesting that the particles behave as waves and exhibit wave-like properties.
However, when detectors are placed at the slits to determine which path the particles take (also known as "which-way" information), the interference pattern disappears, and the particles behave more like classical particles. The act of observing or measuring which path the particles take collapses the wavefunction and destroys the interference pattern.
To answer your question, in traditional double-slit experiments, only one which-way detector is typically used, and it is placed at one of the slits. By measuring which path the particles take, the experimenters gain knowledge about the particles' behavior and destroy the interference pattern.
However, it is worth noting that variations of the double-slit experiment have been conducted, exploring different scenarios and configurations. In some cases, researchers have used two detectors, one at each slit, to investigate the quantum behavior and the role of measurement more thoroughly. These variations aim to examine specific aspects of quantum mechanics, such as entanglement or delayed-choice experiments.
So, while the classical version of the double-slit experiment typically involves a single which-way detector, there have been experiments with dual detectors to explore more complex quantum phenomena.