Photons mediate electromagnetic interactions between particles in quantum electrodynamics. Computer modeling is essential for quantum and relativistic physics. Quantization is one of the foundations of the much broader physics of quantum mechanics. They are the fundamental objects of quantum field theory. Entanglement has many applications in quantum information theory. Candidates for a future theory of quantum gravity include string theory. Kaku is the author of several textbooks on string theory and quantum field theory. There are related singular functions which are important in quantum field theory. Much research has been devoted to applications of photons in the field of quantum optics. The rules of quantum mechanics are fundamental. This result is important in understanding the limits of quantum computation. Examples abound in classical mechanics and quantum mechanics. They are relevant in research on unified theories such as quantum gravity. Entanglement is used in some protocols of quantum cryptography. Research on irreversibility in quantum mechanics takes several different directions.