Nobel Lecture: One hundred years of light quanta^*

Figure 1

Young’s experiment. Light passing through a pinhole in the first screen falls on two closely spaced pinholes in a second screen. The superposition of the waves radiated by those pinholes at ${\mathit{r}}_1$ and ${\mathit{r}}_2$ leads to interference fringes seen at points $\mathit{r}$ on the third screen.

Figure 2

The intensity interferometry scheme of Hanbury Brown and Twiss. Radio-frequency waves are received and detected at two antennas. The filtered low-frequency signals that result are sent to a device that furnishes an output proportional to their product.

Figure 3

The Hanbury Brown–Twiss photon correlation experiment. Light from an extremely monochromatic discharge tube falls on a half-silvered mirror which sends the split beam to two separate photodetectors. The random photocurrents from the two detectors are multiplied together and then averaged. The variable time delay indicated is actually achieved by varying the distance of the detector $D_2$ from the mirror.

Figure 4

The photon coincidence rate measured rises slightly above the constant background of accidental coincidences for sufficiently small time delays. The observed rise was actually weakened in magnitude and extended over longer time delays by the relatively slow response of the photodetectors. With ideal detectors it would take the more sharply peaked form shown.

Figure 5

Schematic picture of a gas laser. The standing light wave in the discharge tube generates an intense transverse polarization current in the gas. Its oscillation sustains the standing wave and generates the radiated beam.

Figure 6

Geometrical or fixed-ratio sequence of probabilities for the presence of $n$ quanta in a mode that is excited chaotically.

Figure 7

The quasiprobability function $\mathrm{P}\left(\mid \alpha \mid \right)$ for a chaotic excitation is Gaussian in form, while for a stable laser beam it takes on nonzero values only near a fixed value of $\mid \alpha \mid$.

Figure 8

The two $\mathrm{P}\left(\mid \alpha \mid \right)$ distributions of Fig. 7 lead to different photon occupation number distributions $p\left(n\right)$: for chaotic excitation a geometric distribution, for coherent excitation a Poisson distribution.

Figure 9

Time interval distributions for counting experiments in a chaotically excited mode: ${\mathrm{W}}_{\mathrm{ch}}\left(t\right)$ is the distribution of intervals from an arbitrary moment until the first photocount. ${\mathrm{W}}_{\mathrm{ch}}\left(0\mid t\right)$ is the distribution of intervals between two successive photocounts.

Figure 10

(Color) Professor Einstein, encountered in the spring of 1951 in Princeton, NJ.