A hundred years of the photon

In 1905 Albert Einstein developed the theory of the photoelectric effect. That was the beginning of quantum optics, and it helped to initiate quantum physics. For over 100 years, people have gradually come to appreciate the allure of photons, and to exploit its applications in quantum cryptography, quantum networks, quantum simulation, quantum metrology and quantum computing. But even today the nature of the photon is still an outstanding issue.