A Single-Atom Light Switch

With just a single atom, light can be switched between two fibre optic cables at the Vienna University of Technology. Such a switch enables quantum phenomena to be used for information and communication technology.

The Quantum Light Switch: It can occupy both possible states at the same time.

The Quantum Light Switch: It can occupy both possible states at the same time.

Fibre optic cables are turned in to a quantum lab: scientists are trying to build optical switches at the smallest possible scale in order to manipulate light. At the Vienna University of Technology, this can now be done using a single atom. Conventional glass fibre cables, which are used for internet data transfer, can be interconnected by tiny quantum systems.

Light in a Bottle
Professor Arno Rauschenbeutel and his team at the Vienna University of Technology capture light in so-called “bottle resonators”. At the surface of these bulgy glass objects, light runs in circles. If such a resonator is brought into the vicinity of a glass fibre which is carrying light, the two systems couple and light can cross over from the glass fibre into the bottle resonator.

Light in a bottle: An optical fibre with a captured beam of light.

Light in a bottle: An optical fibre with a captured beam of light.

“When the circumference of the resonator matches the wavelength of the light, we can make one hundred percent of the light from the glass fibre go into the bottle resonator – and from there it can move on into a second glass fibre”, explains Arno Rauschenbeutel.

A Rubidium Atom as a Light Switch
This system, consisting of the incoming fibre, the resonator and the outgoing fibre, is extremely sensitive: “When we take a single Rubidium atom and bring it into contact with the resonator, the behaviour of the system can change dramatically”, says Rauschenbeutel. If the light is in resonance with the atom, it is even possible to keep all the light in the original glass fibre, and none of it transfers to the bottle resonator and the outgoing glass fibre. The atom thus acts as a switch which redirects light one or the other fibre.

Both Settings at Once: The Quantum Switch
In the next step, the scientists plan to make use of the fact that the Rubidium atom can occupy different quantum states, only one of which interacts with the resonator. If the atom occupies the non-interacting quantum state, the light behaves as if the atom was not there. Thus, depending on the quantum state of the atom, light is sent into either of the two glass fibres. This opens up the possibility to exploit some of the most remarkable properties of quantum mechanics: “In quantum physics, objects can occupy different states at the same time”, says Arno Rauschenbeutel. The atom can be prepared in such a way that it occupies both switch states at once. As a consequence, the states “light” and “no light” are simultaneously present in each of the two glass fibre cables.

For the classical light switch at home, this would be plain impossible, but for a “quantum light switch”, occupying both states at once is not a problem. “It will be exciting to test, whether such superpositions are also possible with stronger light pulses. Somewhere we are bound to encounter a crossover between quantum physics and classical physics”, says Rauschenbeutel.

This light switch is a very powerful new tool for quantum information and quantum communication. “We are planning to deterministically create quantum entanglement between light and matter”, says Arno Rauschenbeutel. “For that, we will no longer need any exotic machinery which is only found in laboratories. Instead, we can now do it with conventional glass fibre cables which are available everywhere.”

Read more at http://www.tuwien.ac.at/en/news/news_detail/article/8480/

Experimental quest to test Einstein’s speed limit

Physicists use dysprosium to put bounds on maximum speed of electrons

Energy levels of Dysprosium

Energy levels of Dysprosium

Albert Einstein’s assertion that there’s an ultimate speed limit – the speed of light – has withstood countless tests over the past 100 years, but that didn’t stop University of California, Berkeley, postdoc Michael Hohensee and graduate student Nathan Leefer from checking whether some particles break this law.

The team’s first attempt to test this fundamental tenet of the special theory of relativity demonstrated once again that Einstein was right, but Leefer and Hohensee are improving the experiment to push the theory’s limits even farther – and perhaps turn up a discrepancy that could help physicists fix holes in today’s main theories of the universe.

“As a physicist, I want to know how the world works, and right now our best models of how the world works – the Standard Model of particle physics and Einstein’s theory of general relativity – don’t fit together at high energies,” said Hohensee of the Department of Physics. “By finding points of breakage in the models, we can start to improve these theories.”

Hohensee, Leefer and Dmitry Budker, a UC Berkeley professor of physics, conducted the test using a new technique involving two isotopes of the element dysprosium. By measuring the energy required to change the velocity of electrons as they jumped from one atomic orbital to another while Earth rotated over a 12-hour period, they determined that the maximum speed of an electron – in theory, the speed of light, about 300 million meters per second – is the same in all directions to within 17 nanometers per second. Their measurements were 10 times more precise than previous attempts to measure the maximum speed of electrons.

Using the two isotopes of dysprosium as “clocks,” they also showed that as the Earth moved closer to or farther from the sun over the course of two years, the relative frequency of these “clocks” remained constant, as Einstein predicted in his general theory of relativity. Their limits on anomalies in the physics of electrons that produce deviations from Einstein’s gravitational redshift are 160 times better than previous experimental limits.

The UC Berkeley physicists and colleagues at the University of New South Wales in Sydney, Australia, who provided crucial theoretical calculations, published their results this week in the journal Physical Review Letters.

Hohensee noted that similar tests of Einstein’s theories can be conducted in huge accelerators like the Large Hadron Collider (LHC) in Switzerland, but such experiments are expensive, the colliders take a long time to build and still don’t reach energies high enough to where the theories could break down.

“You can try to probe these theories using big accelerators, but you would need to produce electrons with seven times the energy of the protons at the LHC. Or you can look at high energy phenomena in distant stars or black holes, but those are not in the lab and not fully understood,” he said. “Instead, We can look for evidence that the standard model or general relativity break at low energy scales in small ways in a tabletop experiment.”

Compared with existing tests, the revamped experiment by UC Berkeley physicists will potentially be a thousand times more sensitive, the level at which some theorists predict special relativity might break down.

“This technique will open the door to studying a whole other set of parameters that could be even more interesting and important,” said Budker, who was among the first to use dysprosium’s unusual electronic structure to test fundamental aspects of particle physics.

Budker and his team also report in a newly accepted paper in Physical Review Letters [Limits on violations of Lorentz symmetry and the Einstein equivalence principle using radio-frequency spectroscopy of atomic dysprosium, M. A. Hohensee, N. Leefer, D. Budker, C. Harabati, V. A. Dzuba, V. V. Flambaum] that they used the same experimental apparatus to show that a fundamental constant of nature, the fine structure constant, does not vary over time or in different gravitational fields.

Hohensee is part of a group led by UC Berkeley physics professor Holger Müller that focuses on precision measurements to test aspects of Einstein’s theories, including gravitational redshift. The new results complement findings from one of Müller’s 2010 experiments, which put the tightest limits yet on the gravitational redshift for matter waves.

“This experiment introduces a new technology using dysprosium to the field of testing Einstein. That is the major new trick. That makes it especially interesting to me,” Müller said.

Read more at http://www.eurekalert.org/pub_releases/2013-07/uoc–eqt072813.php

Read also: Testing Relativity Using Earth’s Motion

Elementary Physics in a Single Molecule

The molecule of about 2 nm in size is kept stable between two metal electrodes for several days. (Figure: Christian Grupe/KIT)

The molecule of about 2 nm in size is kept stable between two metal electrodes for several days. (Figure: Christian Grupe/KIT)

A team of physicists has succeeded in performing an extraordinary experiment: They demonstrated how magnetism that generally manifests itself by a force between two magnetized objects acts within a single molecule. This discovery is of high significance to fundamental research and provides scientists with a new tool to better understand magnetism as an elementary phenomenon of physics. The researchers published their results in the latest issue of Nature Nanotechnology (doi: 10.1038/nnano.2013.133).

The smallest unit of a magnet is the magnetic moment of a single atom or ion. If two of these magnetic moments are coupled, two options result: Either the magnetic moments add up to a stronger moment or they compensate each other and magnetism disappears. From the quantum physics point of view, this is referred to as a triplet or singlet. A team of researchers around Professor Mario Ruben from Karlsruhe Institute of Technology and Professor Heiko B. Weber from the Friedrich-Alexander-Universität Erlangen-Nürnberg now wanted to find out whether the magnetism of a pair of magnetic moments can be measured electrically in a single molecule.

For this purpose, the team headed by Mario Ruben used a customized molecule of two cobalt ions for the experiment. At Erlangen, Heiko B. Weber and his team studied the molecule in a so-called single-molecule junction. This means that two metal electrodes are arranged very closely to each other, such that the molecule of about 2 nm in length is kept stable between these electrodes for many days, while current through the junction can be measured. This experimental setup was then exposed to various, down to very deep, temperatures.

The scientists found that magnetism can be measured in this way. The magnetic state in the molecule became visible as Kondo anomaly. This is an effect that makes electric resistance shrink towards deep temperatures. It occurs only when magnetism is active and, hence, may be used as evidence. At the same time, the researchers succeeded in switching this Kondo effect on and off via the applied voltage. A precise theoretical analysis by the group of Assistant Professor Karin Fink from Karlsruhe Institute of Technology determines the various complex quantum states of the cobalt ion pair in more detail. Hence, the researchers succeeded in reproducing elementary physics in a single molecule…..

Read more at http://www.kit.edu/visit/pi_2013_13701.php

Experimental realization of an optical second …

… with strontium lattice clocks

LNE-SYRTE optical to microwave measurement chain.

LNE-SYRTE optical to microwave measurement chain.

R. Le Targat et al
Progress in realizing the SI second had multiple technological impacts and enabled further constraint of theoretical models in fundamental physics.
Caesium microwave fountains, realizing best the second according to its current definition with a relative uncertainty of 2–4 × 10−16, have already been overtaken by atomic clocks referenced to an optical transition, which are both more stable and more accurate. Here we present an important step in the direction of a possible new definition of the second.
Our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.5 × 10−16.
Their comparison with three independent caesium fountains shows a degree of accuracy now only limited by the best realizations of the microwave-defined second, at the level of 3.1 × 10−16….
Read more at http://www.nature.com/ncomms/2013/130709/ncomms3109/full/ncomms3109.html

Read also: Optical lattice atomic clock could ‘redefine the second’

Precise measurements test quantum electrodynamics …

… constrain possible fifth fundamental force

A schematic layout of the experimental setup. The oscillator cavity is seeded by a cw Ti:Sa laser, the pulsed output of which makes multiple passes in an amplifier stage. The amplified output is frequency up-converted in two frequency doubling stages leading to fourth harmonic generation of ~211 nm. The deep UV radiation is sent to the experiment, where molecules in the X1 Σg+ ν´ = 1 state, populated by electrical discharge, are optically excited in a two-photon Doppler-free configuration . The cw-seed light is compared to a frequency comb while the frequency offset between pulsed and cw-seed light is measured via on-line chirp analysis to obtain an absolute frequency calibration. See text for further details. SHG: second harmonic generation; PMT: photomultiplier tube; and YAG: yttrium-aluminum garnet. Reproduced with permission from G. D. Dickenson et al, Phys. Rev. Lett. 110, 193601 (2013)

A schematic layout of the experimental setup. The oscillator cavity is seeded by a cw Ti:Sa laser, the pulsed output of which makes multiple passes in an amplifier stage. The amplified output is frequency up-converted in two frequency doubling stages leading to fourth harmonic generation of ~211 nm. The deep UV radiation is sent to the experiment, where molecules in the X1 Σg+ ν´ = 1 state, populated by electrical discharge, are optically excited in a two-photon Doppler-free configuration . The cw-seed light is compared to a frequency comb while the frequency offset between pulsed and cw-seed light is measured via on-line chirp analysis to obtain an absolute frequency calibration. See text for further details. SHG: second harmonic generation; PMT: photomultiplier tube; and YAG: yttrium-aluminum garnet. Reproduced with permission from G. D. Dickenson et al, Phys. Rev. Lett. 110, 193601 (2013)

Quantum electrodynamics (QED) – the relativistic quantum field theory of electrodynamics – describes how light and matter interact – achieves full agreement between quantum mechanics and special relativity.
(QED can also be described as a perturbation theory of the electromagnetic quantum vacuum.) QED solves the problem of infinities associated with charged pointlike particles and, perhaps more importantly, includes the effects of spontaneous particle-antiparticle generation from the vacuum.
Recently, scientists at VU University, The Netherlands, published two papers in quick succession that, respectively, tested QED to extreme precision by comparing values for the electromagnetic coupling constant1, and applied these measurements to obtain accurate results from frequency measurements on neutral hydrogen molecules that can be interpreted in terms of constraints on possible fifth-force interactions beyond the Standard Model of physics2.
In addition, the researchers point out that while the Standard Model explains physical phenomena observed at the microscopic scale, so-called dark matter and dark energy at the cosmological scale are considered as unsolved problems that hints at physics beyond the Standard Model.
Read more at: phys.org/news

A New Look at the Hydrogen Wave Function

hydrogen-wavefunction1The first direct observation of the orbital structure of an excited hydrogen atom has been made by an international team of researchers. The observation was made using a newly developed “quantum microscope”, which uses photoionization microscopy to visualize the structure directly. The team’s demonstration proves that “photoionization microscopy”, which was first proposed more than 30 years ago, can be experimentally realized and can serve as a tool to explore the subtleties of quantum mechanics….
Read more at physicsworld.com and physics.aps.org

New insights into what triggers lightning

What initiates a lightning strike? In the image above, multiple cloud-to-ground and cloud-to-cloud lightning strikes are observed during a night-time thunderstorm. (Courtesy: NOAA)

What initiates a lightning strike? In the image above, multiple cloud-to-ground and cloud-to-cloud lightning strikes are observed during a night-time thunderstorm. (Courtesy: NOAA)

Cosmic rays interacting with water droplets within thunderclouds could play an important role in initiating lightning strikes. That is the claim of researchers in Russia, who have studied the radio signals emitted during thousands of lightning strikes. The work could provide new insights into how and why lightning occurs in the first place.
Although most people have witnessed a flash of lightning during a thunderstorm at some point in their lives, scientists still do not completely understand what triggers the discharge in the first place. Lightning has been studied for hundreds of years, yet while many possibilities for observation are available – there are about 40 to 50 lightning strikes per second across the globe – predicting the onset of a strike is difficult…. Read more at http://physicsworld.com/cws/article/news/2013/may/07/new-insights-into-what-triggers-lightning