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Popular Information. Read the popular information on the 2005 Nobel Prize in Physics and match the paragraphs A-G with the headings: “The birth of Quantum Optics”, “How long is a metre?”, “Laser Based Precision Spectroscopy”, “Future prospects”, “The freq
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Popular Information
Read the popular information on the 2005 Nobel Prize in Physics and match the paragraphs A-G with the headings: “The birth of Quantum Optics”, “How long is a metre? ”, “Laser Based Precision Spectroscopy”, “Future prospects”, “The frequency comb — a new measuring stick”, “Waves or particles? ”, “Organised and random light”.
What limits the measurable?
A
We get most of our knowledge of the world around us through light, which is composed of electromagnetic waves. With the aid of light we can orient ourselves in our daily lives or observe the most distant galaxies of the universe. Optics has become the physicist’s tool for dealing with light phenomena. But what is light and how do various kinds of light differ from each other? How does light emitted by a candle differ from the beam produced by a laser in a CD player?
According to Albert Einstein, the speed of light in empty space is constant. Is it possible to use light to measure time with greater precision than with the atomic clocks of today? It is questions like these that have been answered by this year’s Nobel Laureates in Physics.
In the late 19th century it was believed that the electromagnetic phenomena could be explained by means of the theory that the Scottish physicist James Clerk Maxwell had presented; he viewed light as waves. But an unexpected problem arose when one tried to understand the radiation from glowing matter like the sun, for example. The distribution of the strength of the colours did not agree at all with the theories that had been developed based on Maxwell’s original work. There should be much more violet and ultraviolet radiation from the sun than had actually been observed.
This dilemma was solved in 1900 by Max Planck (Nobel Prize, 1918), who discovered a formula that matched the observed spectral distribution perfectly. Planck described the distribution as the result of the inner vibrational state of the heated matter. In one of his famous works a hundred years ago, in 1905, Einstein proposed that radiation energy, i. e. light, also occurs as individual energy packets, so-called quanta. When such an energy packet of this kind enters the surface of a metal, its energy is transferred to an electron, which is released and leaves the material — the photoelectric effect, which was included in Einstein’s Nobel Prize in 1921.
Einstein’s hypothesis means that a single energy packet, later called a photon, gives all its energy to just one electron. Thus we can count the quanta in the radiation by observing and counting the number of electrons, that is, the electric current that comes from the metal surface.
Almost all later light detectors are based on this effect. When the quantum theory was developed in the 1920s, it met with difficulties in the form of senseless, infinite expressions. This problem was not solved until after the Second World War, when quantum electrodynamics, QED, was developed (Nobel Prize in Physics 1965 to Tomonaga, Schwinger and Feynman). QED became the most precise theory in physics and was central to the development of particle physics. However, in the beginning it was judged unnecessary to apply QED to visible light. Instead it was to be treated as an ordinary wave motion with a number of random variations in intensity. A detailed quantum theoretical description was considered unnecessary.
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B
Until the development of the laser and similar devices, most light phenomena could be understood by Maxwell’s classical theory. A more realistic description is required when considering the light from a light bulb. Its light waves have different frequencies and wavelengths and are at the same time out of phase with each other. We see the source as affected by random noise. Thus, the incoherence makes the interference pattern less distinct.
Previously, most light sources were based on thermal radiation and special arrangements were required in order to observe the interference pattern. This changed when the laser, with its perfectly coherent light, was developed. Radiation with a well-defined phase and frequency was, of course, well known from radio technology. But somehow it seemed strange to see light from a thermal light source as a wave motion – it seemed easier to describe the disorder that stemmed from it as randomly distributed photons.
C
One half of this year’s Nobel Prize in Physics is awarded to Roy J. Glauber for his pioneering work in applying quantum physics to optical phenomena. In 1963 he reported his results; he had developed a method for using electromagnetic quantization to understand optical observations. He carried out a consistent description of photoelectric detection with the aid of quantum field theory. Now he was able to show that the “bunching” that R. Hanbury Brown and R. Twiss had discovered was a natural consequence of the random nature of thermal radiation.
An ideal coherent laser beam does not display the same effect at all. But how can a stream of photons, independent particles, give rise to interference patterns? Here we have an example of the dual nature of light. Electromagnetic energy is transmitted in patterns determined by classical optics. Energy distributions of this kind form the landscape into which the photons can be distributed. These are separate individuals, but they have to follow the paths prescribed by optics. This explains the term Quantum Optics. For low light intensities, the state will be described by only a few photons. The individual particle observations will build the patterns of optics after a sufficient number of photoelectrons have been observed.
An essential feature of the theoretical quantum description of optical observations is that, when a photoelectron is observed, a photon has been absorbed and the state of the photon field has undergone a change. When several detectors are correlated, the system will becomes ensitive to quantum effects, which will be more evident if only a few photons are present in the field. Experiments involving several photo detectors have been carried out later, and they are all described by Glauber’s theory.
Glauber’s work in 1963 laid the foundations for future developments in the new field of Quantum Optics. It soon became evident that technical developments made it necessary to use the new quantum description of the phenomena.
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An observable effect of the quantum nature of light is the opposite of the above-mentioned “bunching” that photons display. This is called “anti-bunching”. The fact is that in some situations photons occur more infrequently in pairs than in a purely random signal. Such photons come from a quantum state that cannot in any way be described as classical waves.
This is because a quantum process can result in a state where photons are well separated, in contrast to the results of a purely random process. Quantum physics sets the ultimate limits and promises new applications. In technical applications, the quantum effects are often very small. The field state is chosen so that it can be assigned well-defined phase and amplitude properties. In laboratory measurements, too, the uncertainty of quantum physics seldom sets the limit. But the uncertainty that nevertheless exists appears as a random variation in the observations. This “quantum noise” sets the ultimate limit for the precision of optical observations. It is only the quantum nature of light that sets a limit for how precise our apparatuses can be.
Our knowledge about quantum states can also be utilised directly. We can get completely new technical applications of quantum phenomena, for example to enable safe encryption of messages within communication technology and information processing.
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