Scientists turn air into 'Optical Fibre'

Scientists say they have turned thin air into an 'optical fibre' that can transmit and amplify light signals without the need for any cables.

In a proof-of-principle experiment they created an "air waveguide" that could one day be used as an instantaneous optical fibre to any point on earth, or even into space.

The findings, reported in the journal Optica, have applications in long range laser communications, high-resolution topographic mapping, air pollution and climate change research, and could also be used by the military to make laser weapons.

"People have been thinking about making air waveguides for a while, but this is the first time it's been realised," says Professor Howard Milchberg of the University of Maryland, who led the research, which was funded by the US military and National Science Foundation.

Lasers lose intensity and focus with increasing distance as photons naturally spread apart and interact with atoms and molecules in the air.

Fibre optics solves this problem by beaming the light through glass cores with a high refractive index, which is good for transmitting light.

The core is surrounded by material with a lower refractive index that reflects light back in to the core, preventing the beam from losing focus or intensity.

Fibre optics, however, are limited in the amount of power they can carry and the need for a physical structure to support them.

Light and air

Milchberg and colleagues' made the equivalent of an optical fibre out of thin air by generating a laser with its light split into a ring of multiple beams forming a pipe.

They used very short and powerful pulses from the laser to heat the air molecules along the beam extremely quickly.

Such rapid heating produced sound waves that took about a microsecond to converge to the centre of the pipe, creating a high-density area surrounded by a low-density area left behind in the wake of the laser beams.

"A microsecond is a long time compared to how far light propagates, so the light is gone and a microsecond later those sound waves collide in the centre, enhancing the air density there," says Milchberg.

The lower density region of air surrounding the centre of the air waveguide had a lower refractive index, keeping the light focused.

"Any structure [even air] which has a higher density will have a higher index of refraction and thereby act like an optical fibre," says Milchberg.

Amplified signal

Once Milchberg and colleagues created their air waveguide, they used a second laser to spark the air at one end of the waveguide turning it into plasma.

An optical signal from the spark was transmitted along the air waveguide, over a distance of a metre to a detector at the other end.

The signal collected by the detector was strong enough to allow Milchberg and colleagues to analyse the chemical composition of the air that produced the spark.

The researchers found the signal was 50 per cent stronger than a signal obtained without an air waveguide.

The findings show the air waveguide can be used as a "remote collection optic," says Milchberg.

"This is an optical fibre cable that you can reel out at the speed of light and place next to [something] that you want to measure remotely, and have the signal come all the way back to where you are."

Australian expert Professor Ben Eggleton of the University of Sydney says this is potentially an important advance for the field of optics.

"It's sort of like you have an optical fibre that you can shine into the sky, connecting your laser to the top of the atmosphere," says Eggleton.

"You don't need big lenses and optics, it's already guided along this channel in the atmosphere."

How to weigh a Galaxy ?

Step on a scale and you’ll get a quick measure of your weight. Weighing galaxy clusters, groups of hundreds or thousands of galaxies bound together by gravity, isn’t so easy.

But scientists have many ways to do it. This is fortunate for particle astrophysics; determining the mass of galaxy clusters led to the discovery of dark matter, and it’s key to the continuing study of the “dark” universe: dark matter and dark energy.

“Galaxy cluster measurements are one of the most powerful probes of cosmology we have,” says Steve Allen, an associate professor of physics at SLAC National Accelerator Laboratory and Stanford University.

When you weigh a galaxy cluster, what you see is not all that you get. Decades ago, when scientists first estimated the masses of galaxy clusters based on the motions of the galaxies within them, they realized that something strange was going on. The galaxies were moving faster than expected, which implied that the clusters were more massive than previously thought, based on the amout of light they emitted. The prevailing explanation today is that galaxy clusters contain vast amounts of dark matter. 

Measurements of the masses of galaxy clusters can tell scientists about the sizes and shapes of the dark matter “halos” enveloping them and can help them determine the effects of dark energy, which scientists think is driving the universe’s accelerating expansion.

Today, researchers use a combination of simulations and space- and ground-based telescope observations to estimate the total masses of galaxy clusters.

Redshift, blueshift: Just as an ambulance’s siren seems higher in pitch as it approaches and lower as it speeds into the distance, the light of objects traveling away from us is shifted to longer, “redder” wavelengths, and the light of those traveling toward us is shifted to shorter, “bluer” wavelengths. Measurements of these shifts in light coming from galaxies orbiting a galaxy cluster can tell scientists how much gravitational pull the cluster has, which is related to its mass.

Gravitational lensing: Gravitational lensing, theorized by Albert Einstein, occurs when the light from a distant galaxy is bent by the gravitational pull of a massive object between it and the viewer. This bending distorts the image of the background galaxy (pictured above). Where the effects are strong, the process can cause dramatic distortions; multiple images of the galaxy can appear. Typically, however, the effects are subtle and require careful measurements to detect. The greater the lensing effect caused by a galaxy cluster, the larger the galaxy cluster’s mass.

X-rays: Galaxy clusters are filled with superhot, 10- to 100-million-degree gas that shines brightly at X-ray wavelengths. Scientists use X-ray data from space telescopes to find and study massive galaxy clusters. They can use the measured properties of the gas to infer the clusters’ masses.

The Sunyaev-Zel’dovich effect: The Sunyaev-Zel’dovich effect is a shift in the wavelength of the Cosmic Microwave Background—light left over from the big bang—that occurs when this light passes through the hot gas in a galaxy cluster. The size of the wavelength shift can tell scientists the mass of the galaxy cluster it passed through.

“These methods are much more powerful in combination than alone,” says Aaron Roodman, a faculty member at the Kavli Institute for Particle Astrophysics and Cosmology at SLAC National Accelerator Laboratory. 

Forthcoming data from the Dark Energy Survey, the under-construction Large Synoptic Survey Telescope and Dark Energy Spectroscopic Instrument, improved Sunyaev-Zel’dovich effect measurements, and the soon-to-be-launched ASTRO-H and eRosita X-ray telescopes should further improve galaxy cluster mass estimates and advance cosmology. Computer simulations are also playing an important role in testing and improving mass estimates based on data from observations.

Even with an extensive toolkit, it remains a challenging business to weigh galaxy clusters,  says Marc Kamionkowski, a theoretical physicist and professor of physics and astronomy at Johns Hopkins University. They are constantly changing; they continue to suck in matter; their dark matter halos can overlap; and no two are alike.

“It’s like asking how many birds are in my backyard,” he says.

Despite this, Allen says he sees no roadblocks toward pushing mass estimates to within a few percent accuracy.

“We will be able to take full advantage of these amazing new data sets that are coming along,” he says. “We are going to see rapid advances.”

First Schrödinger's Cat ...and now Paradoxical Pigeons

First there was Schrödinger's cat, now an international team of physicists has come up with a new animal-related paradox involving "quantum pigeons".

For nearly a century students have struggled to understand the many counter-intuitive implications of quantum physics. Perhaps the most famous paradox is Schrödinger's cat, whereby a cat being both dead and alive at the same time illustrates the fact that a particle can exist simultaneously in two quantum states.

Now, Jeff Tollaksen of Chapman University in California and colleagues in Israel, Italy and the UK have proposed an equally bizarre scenario dubbed the "quantum-pigeonhole effect". The paradox begins with the observation that when you put three pigeons in two pigeonholes, there will always be at least two pigeons in the same hole. But according to the team's quantum analysis, it is possible for none of the pigeons to share a hole.

"It's one of those things that seem to be impossible," says Tollaksen. But it is a direct consequence of quantum mechanics and, he adds, "It really has immense implications."

Nondeterministic measurements

Classical physics is deterministic. This means that measuring the initial state of a system will, in principle, tell you everything you need to determine the final state. But in 1964 Yakir Aharonov of Chapman University and Tel Aviv University helped discover that in quantum mechanics, you can choose initial and final states that are entirely independent, Tollaksen says.

Now Aharonov has teamed up with Tollaksen and colleagues to use this and other concepts of quantum mechanics to postulate the quantum-pigeonhole effect. They reckon that the effect will arise when an observer makes a sequence of measurements while trying to fit three particles in two boxes. First, you make an initial, "pre-selection" measurement of the locations of the particles. Next, you can perform an intermediate measurement to see whether two particles share a box. Finally, you make a final, "post-selection" measurement of the locations. You can make the pre-selection and post-selection measurements such that they are completely independent. In the intermediate step, you can make what's called a weak measurement to look at all three particles simultaneously. And when you do, it turns out that no two particles share a box.

Spooky and profound

The implications of these results, Tollaksen says, complement the well-known Einstein–Podolsky–Rosen (EPR) paradox. In this scenario, two particles that start in the same place can become intimately correlated, a relationship called entanglement. Measuring the state of the first particle seems to influence the state of the second one, even if they are subsequently separated by distances so great that it would be impossible to explain the influence using classical physics. This unsettling conclusion led Einstein to call entanglement "spooky action at a distance".

"EPR is one of the most profound discoveries in science," Tollaksen says. "But that's only half the story." The quantum-pigeonhole principle creates a somewhat opposite situation, he explains. Three particles can begin separated with no connections or correlations at all. You bring them together and force them to interact by squeezing them in two boxes. During this intermediate stage, they are more strongly correlated than classically possible. But in the final stage, they are not correlated at all.

The implications of the EPR paradox are important and shape our understanding of information and the fundamental physics of matter. Although it is too early to predict every implication, he believes that the quantum-pigeonhole principle could prove to be just as influential – if not more so. "This is at least as equally profound, if not more profound," he says. It implies a new concept of correlation that is surprising.

Electronic pigeons

To verify their conclusions, Tollaksen and colleagues propose an experiment in which three electrons travel through an interferometer. This is essentially a beam splitter that creates two separate paths for the electrons, which then meet again.

Because there are only two possible paths, you would expect at least two electrons to share a path. If so, then the two will be close together and interact: their identical electric charges will repel each other, slightly deflecting their trajectories. Then physicists will be able to detect these deflections when all three electrons reunite after the paths converge. But, Tollaksen says, because their calculations show that no two of the three electrons will actually follow the same path, no deflections will be observed.

Physicists have not done these experiments yet, but Tollaksen is confident in their results. "I'm sure it will be confirmed experimentally very soon," he says.

The new results seem "fascinating," says Leonard Susskind of Stanford University. "I would guess that the new effect is a serious step in understanding quantum correlations."