Guest post by Congcong Huang, Associated Editor, Nature Communications and Nicky Dean, Team Manager Physics, Nature Communications.

This week we conclude our series of ‘beautiful experiments with light’ featured in our poll and finally reach the new millennium in which lasers continue to enable powerful and diverse experiments.

Our story begins with the generation of ultrafast laser pulses. Following the invention of lasers in 1960 enormous efforts were made to shorten the pulse duration which led to femtosecond lasers in the late 1980s and finally in 2001 to the first reports of attosecond laser pulses. In one attosecond (10^-18 s), light travels slightly more than the length of a water molecule, while molecules are essentially frozen during this time, with molecular vibration at femtosecond (10^-15 s) and rotation at picosecond (10^-12 s) timescales. This makes it possible to access the timescale of electron dynamics inside molecules.

As light pulses have been made ultrashort –short enough even to capture the motion of electrons – a natural question is whether the speed of light can be controlled to the same extent. It is not surprising that light slows down when it travels through glass or water, but this is only a modest effect. It was thus a stunning observation, made by Lene Hau and her group in Harvard in 1999, that light travels at a cycling speed – 7 orders of magnitude slower than c – in a sodium atom cloud right below its Bose-Einstein condensation temperature. The cold atoms alone cannot do the work; the use of a laser field that efficiently cancels light absorption, known as electromagnetically-induced transparency, makes the trick possible. The demonstration sparked a new chapter for laser controlled optical materials.

Meanwhile, more attempts at controlling the behavior of light were underway. As mentioned above, light slows down when it passes through a medium, an effect characterized by the medium’s refractive index. This index is normally positive, and it tells us how light rays will be bent when they move from one medium into another. You can see this effect by looking at a straw in a glass of water which appears to be sharply bent at the surface. In the late 1960s, Victor Veselago wondered what might happen if the refractive index was negative. He predicted that light entering such a medium should bend in the opposite sense to what we normally expect (as if the straw would bend the ‘wrong’ way). In 2001, David Smith and colleagues realized this prediction by constructing an artificial material, or ‘metamaterial’, made of an array of copper split-rings on circuit boards. Their metamaterial exhibited a negative refractive index at around 10 GHz. Following Smith’s demonstration, many more negative-index metamaterials have been made using all kinds of different structures, across a range of frequencies, including the visible spectrum.

Diffraction pattern of a virus particle, taken with an X-ray free electron laser.

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Guest post by Federico Levi, Associate Editor Nature Communications

The experiments in this week ’s blog entry accompanying our poll of ‘ the most beautiful experiment with light’ were carried out in the second half of the twentieth century, in which physicists were still struggling to accept the counter-intuitive implications of quantum physics.

One of the most bewildering embodiments of quantum theory is quantum entanglement.  When two particles are entangled, performing a measurement on one of them seems to instantaneously influence the other particle, even if it is light-years away. This paradox, famously termed ‘spooky action at a distance’ by Albert Einstein was formulated by Einstein, Podolsky and Rosen (EPR) in 1935. To restore reality, they argued that quantum physics could simply be our limited understanding of a deeper and less troubling theory, classically constructed over a set of ‘hidden variables’.

It took almost thirty years to devise a way to test their hypothesis. In 1964 John Stewart Bell demonstrated the famous theorem carrying his name, which showed how there would be an experimentally measurable difference between the prediction of quantum physics and that ‘less troubling theory’ imagined by EPR. Light provided the means to carry out this test. In 1972, by looking at the correlations in the linear polarization of photons emitted by an atomic cascade of calcium, Stuart J. Freedman and John F. Clauser tested Bell’s theorem at the Lawrence Berkeley Laboratory in California. The result was a landmark confirmation of quantum mechanics.

Jumping more than 10 years ahead, we find researchers dealing with the consequences of yet another quantum principle, namely the indistinguishability of fundamental particles. Two particles in exactly the same quantum state have to be considered essentially indistinguishable, and photons make no exception. In 1987 at Rochester University, New York, Chung Ki Hong, Zhe Yu Ou and Leonard Mandel showed what may be the most direct evidence of this principle. When a photon hits a beam splitter, it can continue in one of two ways with 50% probability. If two indistinguishable photons hit a beam splitter coincidentally, quantum interference forbids the outcome where they follow different output paths. The pronounced ‘dip’ in the measurement of simultaneous arrivals at the two outputs is a hallmark of the indistinguishability of a photon pair, a sought-after quality for quantum information applications.

Large-area metallic photonic crystal layer rolled onto a glass rod.

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Guest post by Rachel Won International Editor, Nature Photonics

This week’s set of experiments featured in our poll  are all about the advent of the maser (microwave amplification by stimulated emission of radiation) and the optical maser, now known as the laser, and the remarkable wide impact these inventions had in science, technology and society.

The concept of stimulated emission was introduced in 1917 by Einstein, who found that the process of absorption by atoms must be accompanied by an amplification process such that the received radiation can stimulate the emission of the same kind of radiation. It was not until 1953 that the effect was experimentally demonstrated by Charles Townes and his two graduate students at Columbia University in New York. Their maser used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 GHz. The development of a maser was simultaneously carried out by Nikolay Basov and Alexander Prokhorov at the Lebedev Institute in Moscow.

The achievements led to the award of the Nobel Prize in Physics in 1964 to Townes, Basov and Prokhorov.

The invention of the maser kicked off a race to create a similar device for visible light, now known as a laser (with ‘microwave’ replaced by ‘light’). In 1958 Townes, together with Arthur Schawlow, then at Bell Labs, published a paper extending the maser techniques to the infrared and optical region. The first working laser, however, was built by Theodore Maiman at Hughes Research Laboratories in 1960. His laser used a solid-state synthetic ruby crystal pumped by a flashlamp to produce red laser light at 694 nm.

Theodore Maiman and his invention, the first laser. (Photo credit: HRL Laboratories, LLC)

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Guest post by Leonie Mueck, Associate Editor, Nature

In last week’s post you heard about beautiful experiments with light featured in our poll from the turn of the century. This week, we will talk about the time until the 1950s. And, while so many turning points in politics and history fall into that period, advances in optics and photonics were a bit betwixt and between. Scientists were modernizing their methods and instruments but still didn’t have modern-day tools like the laser, which in the 1960s would completely transform light-related research.

They did have highly advanced telescopes. When Edwin Hubble arrived at Mount Wilson Observatory in California it was 1919. By a lucky coincidence something else arrived there at around the same time: the Hooker Telescope, which allowed Hubble to perform detailed investigations on spiral nebulae. Thanks to his measurements, we now know that those nebulae are in fact distant galaxies. As jaw dropping as this finding was to his contemporaries, Hubble went on to show something even more ground-breaking. Looking at the Doppler shift of as many galaxies as possible, he found that the shift was proportional to the galaxies’ distance in 1929. The only plausible explanation for this phenomenon was that we live in an expanding Universe!

Andromeda Galaxy taken by Spitzer in infra-red, 24 micrometres. (Image: NASA/JPL–Caltech/K. Gordon, University of Arizona)

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Guest post by Maria Maragkou, Associate Editor at Nature Materials

This week’s entries for the poll of the most beautiful experiments with light occurred around the turn of the 19th century.

Wilhelm Roentgen, a physics professor in Wurzburg, changed the course of medicine when he accidentally discovered X-rays, energy waves at frequencies from 0.1 – 10 nanometres. In November 1895, while experimenting with an electron-discharge tube covered with black cardboard, he noticed that a fluorescent screen further away was illuminated. Eventually he realised that the tube emitted a type of ray – marked X for unknown – that was blocked by dense material, such as lead or bones, but could penetrate other objects. As he held a piece of lead in front of the X-rays, he could see the contrast between bones and flesh on the fluorescent screen. A few weeks later, Roentgen took an X-ray picture of his wife’s hand, who allegedly claimed “I have seen my death” upon seeing it. With the discovery of X-rays, it became possible to look inside the human body without surgery and Roentgen earned the first ever Nobel Prize in Physics in 1901 for this remarkable invention.

Roentgen’s first X-ray image featured in Nature in 1896.

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