# Finally, treating photons like particles

At the dawn of the 20th century, two of Mankind's greatest minds, Max Planck and Albert Einstein, changed the world's view on the nature of light, in what is today regarded as the birth of Quantum Physics.

First, in 1900, Max Planck demonstrated that the Blackbody Radiation can only be explained by assuming that light is emitted in discrete units of energy (“quanta”) [1].

While Planck considered his own assumption as a mere mathematical tool to derive the correct answer, his result intrigued Einstein, who believed it reflected an actual, fundamental physical reality.

Five years later, Einstein presented more evidence, including the photoelectric effect, that indeed light is a stream of energy quanta [2]. Although the name "photon" was coined for these energy quanta only in 1926 [3], Einstein said already in 1905 that light "looks like particles". The experimental proof, though, arrived more than 70 years later.

While many refer to the photoelectric effect as the experimental proof of the existence of the photon, this effect actually reflects the discreteness, or indivisibility, of the electron, not of the photon. What is today regarded as the first experimental evidence of the indivisibility of the units of energy of light, and therefore of the existence of the photon, is actually the antibunching experiment of Kimble and Mandel in 1977 [4].

This experiment showed that when light is emitted from single atoms at short time scales it cannot be divided into two detectors. This effect was demonstrated again 30 years later in a much cleaner setting, by using single atoms next to a fiber-coupled photonic chip [5]. In fact, this is the first version of the same technology harnessed by Quantum Source today.

**Deterministic photon-atom interaction**

The experiment reported in Nature Photonics 2016 [6] follows the footsteps of these fundamental demonstrations. For the first time, scientists have been able to deterministically "pick-out", or extract, exactly a single photon from a regular, classical laser pulse, which inherently contains a statistical distribution of multiple photons.

As reported in detail in the paper, the experimental setup consisted of fiber-coupled chip-based resonators to which single ^{87}Rb atoms were coupled in a special 3-level configuration. This configuration of levels leads to an effect called Single-Photon Raman Interaction (SPRINT) [7], in which the atom deterministically reflects only one photon, and at the same time flips its own state to the other ground state, at which it is no longer interacting with the incoming light.

The result, as shown in Figure 2 below, is that no matter how many photons on average are in the incoming pulse (the x axis), the atom reflects only 1 photon, if such exists in the pulse (red line), and leaves all the rest to continue (blue curve).

This mechanism works as a "quantum filter" in some sense: in comes a classical pulse, most of it continues on, except for a single photon that is reflected back - a pulse that is no longer classical but as quantum as it goes - a single photon pulse. As such, the demonstration of this mechanism bears significance far beyond its scientific novelty. In fact, atom sites performing single-photon extraction play a vital role in Quantum Source's architecture that is based on deterministic generation of photons and entangling gates.

[1] M. Planck, Annalen der Physik 309, 553 (1901)

[2] A. Einstein, Annalen der Physik 6, 132 (1905)

[3] Lewis Gilbert Newton, Nature 118, 874 (1926)

[4] H. J. Kimble, M. Dagenais, and L. Mandel, Physical Review Letters 39, 691 (1977)

[5] B. Dayan, A.S. Parkins, T. Aoki, E. Ostby, K.J. Vahala and H.J. Kimble, Science 319, 1062 (2008)

[7] S. Rosenblum, A. Borne, and B. Dayan, Physical Review A 95, 033814 (2017)

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