Beam splitters and interferometers in .NET Make USS Code 128 in .NET Beam splitters and interferometers

6. using visual studio .net todraw code 128a on web,windows application Microsoft Office Development. Microsoft Office 2000/2003/2007/2010 Beam splitters and interferometers Experiments with single photons Central to the entire disc barcode standards 128 for .NET ipline of quantum optics, as should be evident from the preceding chapters, is the concept of the photon. Yet it is perhaps worthwhile to pause and ask: what is the evidence for the existence of photons Most of us rst encounter the photon concept in the context of the photo-electric effect.

As we showed in 5, the photo-electric effect is, in fact, used to indirectly detect the presence of photons the photo-electrons being the entities counted. But it turns out that some aspects of the photo-electric effect can be explained without introducing the concept of the photon. In fact, one can go quite far with a semiclassical theory in which only the atoms are quantized with the eld treated classically.

But we hasten to say that, for a satisfactory explanation of all aspects of the photo-electric effect, the eld must be quantized. As it happens, the other venerable proof of the existence of photons, the Compton effect, can also be explained without quantized elds. In an attempt to obtain quantum effects with light, Taylor, in 1909 [1], obtained interference fringes in an experiment with an extremely weak source of light.

His source was a gas ame and the emitted light was attenuated by means of screens made of smoked glass. The double slit in the experiment was, in fact, a needle whose shadow on a screen exhibited the fringes of a diffraction pattern when exposed to direct light from the source. But Taylor found that the fringes persisted upon attenuation of the source, even down to the lowest intensities where, one could naively conclude, on the basis of simple energy considerations, there was at most only one photon at a time between the source and the screen.

Apparently, photons passing by the needle one at a time give rise to interference. Presumably, this is the origin of Dirac s famous remark [2] that each photon interferes only with itself, interference between two photons does not occur . But we now know, as discussed in 5, that a thermal source, such as the gas ame used by Taylor, does not produce photons one-at-a-time but rather produces them in bunches.

Hence it is na ve and wrong to use energy considerations alone to determine the number of photons between the source and the screen at any given time; there is a strong likelihood that there are two photons present, the. Beam splitters and interferometers s-state Tw o-photon laser exci tation p-state s-state Fig. 6.1.

Energy-level dia gram of the single photon source for the experiments of Grangier et al. A calcium atom is irradiated by a laser, which excites the atom by two-photon absorption to a high-lying s-state. The atom then undergoes a cascade decay rst to a p-state, emitting a photon of frequency 1 , used as the trigger photon, and then to the original s-state emitting a photon of frequency 2 .

. photon bunching effect. A visual .net code-128c laser source produces photons randomly, so even when attenuated there is at least some chance that there may be more than one photon between the source and the screen.

To get as close as possible to having a single photon between source and screen requires a source of antibunched photons. This in turn requires a source consisting of only a very few atoms, ideally a single atom. Such a source was developed only relatively recently by Grangier et al.

[3] originally for the purpose of a fundamental test of quantum mechanics, namely a search for violations of Bell s inequality, and then used to demonstrate the indivisibility of photons. This source consists of a beam of calcium atoms irradiated by laser light exciting the atoms to a high-lying s-state. The s-state undergoes a rapid decay to a p-state emitting a photon of frequency 1 .

Subsequently the atom rapidly undergoes another rapid decay, this time to the ground s-state, by emitting a second photon, this one having a frequency 2 (see Fig. 6.1).

The photons, to conserve momentum, are emitted in opposite directions. In the experiment described in Reference [3], the rst photon, detected by Dtrig , was used as a trigger to alert a set of photo-detectors placed at the outputs of a 50:50 beam splitter upon which the second photon falls as illustrated in Fig. 6.

2. The trigger tells the photo-detectors to expect a photon to emerge from the beam splitter by gating the detection electronics for a brief time interval. This eliminates spurious counts due to photons entering the detectors from irrelevant sources.

The experimental setup, as pictured in Fig. 6.2, is such that only the particle nature of the photons will be manifested.

That is, a single photon falling on the beam splitter would be either re ected into detector Dref or transmitted into detector Dtran , i.e. it is a which path experiment and no interference effects are expected.

There should be no simultaneous counts (the counts should anti-correlated) of re ected and transmitted photons and, because the beam splitter is 50:50, repeated runs of the experiment should result in each of the two detectors ring approximately 50% of the time. These expectations were con rmed by the investigators..

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