There is no such thing as a one-photon-thick beam of light. Photons are not solid little balls that can be lined up in a perfectly straight beam that is one photon wide. Instead, photons are quantum objects. As such, photons act somewhat like waves and somewhat like particles at the same time. When traveling through free space, photons act mostly like waves. Waves can take on a variety of beam widths. But they cannot be infinitely narrow since waves are, by definition, extended objects. The more you try to narrow down the beam width of a wave, the more it will tend to spread out as it travels due to diffraction. This is true of water waves, sound waves, and light waves. The degree to which a light beam diffracts and diverges depends on the wavelength of the light. Light beams with larger wavelengths diverge more strongly than light beams with smaller wavelengths, all else being equal. As a result, smaller-wavelength beams can be made much narrower than larger-wavelength beams. The narrowness of a light beam therefore is ultimately limited by wave diffraction, which depends on wavelength, and not by a physical width of photon particles