Decoherence in a double-slit experiment is primarily caused by interactions between the quantum system (such as a photon) and its surrounding environment. Decoherence occurs when the quantum system interacts with other particles or fields in its environment, causing it to lose its coherent quantum properties and behave more classically.
In the case of a double-slit experiment, the interference pattern that arises when a single photon is sent through the slits is a result of the photon's wave-like nature. The photon can interfere with itself, creating regions of constructive and destructive interference.
However, if the photon interacts with its environment before or during the experiment, such as through interactions with air molecules or other particles, it can experience decoherence. These interactions cause the quantum superposition of the photon's wavefunction to collapse into a classical state, destroying the interference pattern and producing a particle-like behavior.
So, while the photon can exhibit wave-particle duality and interfere with itself, interactions with the environment can disrupt the coherence necessary for interference. The photon itself does not bring decoherence upon itself; rather, it is the interactions with the environment that lead to decoherence.
In quantum physics, the concept of "environment" refers to external factors that can influence and disturb the behavior of quantum systems. The photon, as a quantum system, can indeed interact with its environment, but it is the interaction with external particles or fields that leads to decoherence, not interactions within the photon itself.