Are there any slit experiments where the photons only make two lines on the screen, as if they were little bullets fired though the slits? I have conducted many double and single slit experiments and they always show an interference pattern. I also know that when professional experiments are done in labs they get interference patterns too, even when only one photon at a time is sent though the slits; yet, many web sources talk about just two lines being formed in some test. Is that only for electrons and larger objects. It seems to me that photons never loose their interference pattern. Is this true?
Yes slits always produce diffraction unless as Arpad points out the slit is modified so that the photon has a possibility of being absorbed. But then this is not really a slit. Another example, all camera lenses diffract based on aperture size.
First, photons can lose their interference pattern. If you put a filter on one of the slits, that creates inelastic scattering, and the photons will be out of phase, and there will be no pattern.
Please see here:
Now you are asking when there will be just two lines. If there is no diffraction or interference, there will only be two lines.
Please see here:
You need more than one photon to create the pattern. But one photon is shot at a time and to have interference, you only need one photon. One photon will travel as wave, and parts of the wave will go through the slits and the partial waves will interfere with each other. That will create constructive interference, that will be the bright parts in the pattern, and destructive interference, that will be the dark parts in the pattern.
It is the same with electrons or larger not elementary particles.