Institute of Theoretical Physics

Lausanne, Switzerland

Institute of Theoretical Physics

Lausanne, Switzerland

Time filter

Source Type

News Article | April 18, 2017
Site: phys.org

Such was the intent behind a new study by a team of researchers from Sao Paulo, Brazil. In essence, they consider how the Unruh effect could be confirmed using a simple experiment that relies on existing technology. Not only would this experiment prove once and for all if the Unruh effect is real, it could also help us plan for the day when interstellar travel becomes a reality. To put it in layman's terms, Einstein's Theory of Relativity states that time and space are dependent upon the inertial reference frame of the observer. Consistent with this is the theory that if an observer is traveling at a constant speed through empty vacuum, they will find that the temperature of said vacuum is absolute zero. But if they were to begin to accelerate, the temperature of the empty space would become hotter. This is what William Unruh – a theorist from the University of British Columbia (UBC), Vancouver – asserted in 1976. According to his theory, an observer accelerating through space would be subject to a "thermal bath" – i.e. photons and other particles – which would intensify the more they accelerated. Unfortunately, no one has ever been able to measure this effect, since no spacecraft exists that can achieve the kind of speeds necessary. For the sake of their study – which was recently published in the journal Physical Review Letters under the title "Virtual observation of the Unruh effect" – the research team proposed a simple experiment to test for the Unruh effect. Led by Gabriel Cozzella of the Institute of Theoretical Physics (IFT) at Sao Paulo State University, they claim that this experiment would settle the issue by measuring an already-understood electromagnetic phenomenon. Essentially, they argue that it would be possible to detect the Unruh effect by measuring what is known as Larmor radiation. This refers to the electromagnetic energy that is radiated away from charged particles (such as electrons, protons or ions) when they accelerate. As they state in their study: "A more promising strategy consists of seeking for fingerprints of the Unruh effect in the radiation emitted by accelerated charges. Accelerated charges should back react due to radiation emission, quivering accordingly. Such a quivering would be naturally interpreted by Rindler observers as a consequence of the charge interaction with the photons of the Unruh thermal bath." As they describe in their paper, this would consist of monitoring the light emitted by electrons within two separate reference frames. In the first, known as the "accelerating frame", electrons are fired laterally across a magnetic field, which would cause the electrons to move in a circular pattern. In the second, the "laboratory frame", a vertical field is applied to accelerate the electrons upwards, causing them to follow a corkscrew-like path. In the accelerating frame, Cozzella and his colleagues assume that the electrons would encounter the "fog of photons", where they both radiate and emit them. In the laboratory frame, the electrons would heat up once vertical acceleration was applied, causing them to show an excess of long-wavelength photons. However, this would be dependent on the "fog" existing in the accelerated frame to begin with. In short, this experiment offers a simple test which could determine whether or not the Unruh effect exists, which is something that has been in dispute ever since it was proposed. One of the beauties of the proposed experiment is that it could be conducted using particle accelerators and electromagnets that are currently available. On the other side of the debate are those who claim that the Unruh effect is due to a mathematical error made by Unruh and his colleagues. For those individuals, this experiment is useful because it would effectively debunk this theory. Regardless, Cozzella and his team are confident their proposed experiment will yield positive results. "We have proposed a simple experiment where the presence of the Unruh thermal bath is codified in the Larmor radiation emitted from an accelerated charge," they state. "Then, we carried out a straightforward classical-electrodynamics calculation (checked by a quantum-field-theory one) to confirm it by ourselves. Unless one challenges classical electrodynamics, our results must be virtually considered as an observation of the Unruh effect." If the experiments should prove successful, and the Unruh effect is proven to exist, it would certainly have consequences for any future deep-space missions that rely on advanced propulsion systems. Between Project Starshot, and any proposed mission that would involve sending a crew to another star system, the added effects of a "fog of photons" and a "thermal bath" will need to be factored in. Explore further: Chinese scientists realize quantum simulation of the Unruh effect More information: Virtual observation of the Unruh effect. arxiv.org/abs/1701.03446


News Article | December 21, 2016
Site: www.eurekalert.org

Once again, graphene has proven itself to be a rather special material: an international research team led by Professor Fritz Aumayr from the Institute of Applied Physics at TU Wien was able to demonstrate that the electrons in graphene are extremely mobile and react very quickly. Impacting xenon ions with a particularly high electric charge on a graphene film causes a large number of electrons to be torn away from the graphene in a very precise spot. However, the material was able to replace the electrons within some femtoseconds. This resulted in extremely high currents, which would not be maintained under normal circumstances. Its extraordinary electronic properties make graphene a very promising candidate for future applications in the field of electronics. The Helmholtz-Center Dresden-Rossendorf and the University of Duisburg-Essen participated in the experiment alongside TU Wien. The international team received theoretical support from Paris and San Sebastian as well as from in-house staff (Institute of Theoretical Physics at TU Wien). 'We work with extremely highly-charged xenon ions,' explains Elisabeth Gruber, a PhD student from Professor Aumayr's research team. 'Up to 35 electrons are removed from the xenon atoms, meaning the atoms have a high positive electric charge.' These ions are then fired at a free-standing single layer of graphene, which is clamped between microscopically small brackets. 'The xenon ion penetrates the graphene film, thereby knocking a carbon atom out of the graphene - but that has very little effect, as the gap that has opened up in the graphene is then refilled with another carbon atom,' explains Elisabeth Gruber. 'For us, what is much more interesting is how the electrical field of the highly charged ion affects the electrons in the graphene film.' This happens even before the highly charged xenon ion collides with the graphene film. As the highly charged ion is approaching it starts tearing electrons away from the graphene due to its extremely strong electric field. By the time the ion has fully passed through the graphene layer, it has a positive charge of less than 10, compared to over 30 when it started out. The ion is able to extract more than 20 electrons from a tiny area of the graphene film. This means that electrons are now missing from the graphene layer, so the carbon atoms surrounding the point of impact of the xenon ions are positively charged. 'What you would expect to happen now is for these positively charged carbon ions to repel one another, flying off in what is called a Coulomb explosion and leaving a large gap in the material,' says Richard Wilhelm from the Helmholtz-Center Dresden-Rossendorf, who currently works at TU Wien as a postdoctoral assistant. 'But astoundingly, that is not the case. The positive charge in the graphene is neutralised almost instantaneously.' This is only possible because a sufficient number of electrons can be replaced in the graphene within an extremely short time frame of several femtoseconds (quadrillionths of a second). 'The electronic response of the material to the disruption caused by the xenon ion is extremely rapid. Strong currents from neighbouring regions of the graphene film promptly resupply electrons before an explosion is caused by the positive charges repelling one another,' explains Elisabeth Gruber. 'The current density is around 1000 times higher than that which would lead to the destruction of the material under normal circumstances - but over these distances and time scales, graphene can withstand such extreme currents without suffering any damage.' This extremely high electron mobility in graphene is of great significance for a number of potential applications: 'The hope is that for this very reason, it will be possible to use graphene to build ultra-fast electronics. Graphene also appears to be excellently suited for use in optics, for example in connecting optical and electronic components,' says Aumayr.


News Article | November 23, 2016
Site: www.eurekalert.org

Conventional memories used in today's computers only differentiate between the bit values 0 and 1. In quantum physics, however, arbitrary superpositions of these two states are possible. Most of the ideas for new quantum technology devices rely on this "Superposition Principle". One of the main challenges in using such states is that they are usually short-lived. Only for a short period of time can information be read out of quantum memories reliably, after that it is irrecoverable. A research team at TU Wien has now taken an important step forward in the development of new quantum storage concepts. In cooperation with the Japanese telecommunication giant NTT, the Viennese researchers lead by Johannes Majer are working on quantum memories based on nitrogen atoms and microwaves. The nitrogen atoms have slightly different properties, which quickly leads to the loss of the quantum state. By specifically changing a small portion of the atoms, one can bring the remaining atoms into a new quantum state, with a lifetime enhancement of more than a factor of ten. These results have now been published in the journal Nature Photonics. "We use synthetic diamonds in which individual nitrogen atoms are implanted", explains project leader Johannes Majer from the Institute of Atomic and Subatomic Physics of TU Wien. "The quantum state of these nitrogen atoms is coupled with microwaves, resulting in a quantum system in which we store and read information." However, the storage time in these systems is limited due to the inhomogeneous broadening of the microwave transition in the nitrogen atoms of the diamond crystal. After about half a microsecond, the quantum state can no longer be reliably read out, the actual signal is lost. Johannes Majer and his team used a concept known as "spectral hole burning", allowing data to be stored in the optical range of inhomogeneously broadened media, and adapted it for supra-conducting quantum circuits and spin quantum memories. Dmitry Krimer, Benedikt Hartl and Stefan Rotter (Institute of Theoretical Physics, TU Wien) have shown in their theoretical work that such states, which are largely decoupled from the disturbing noise, also exist in these systems. "The trick is to manoeuver the quantum system into these durable states through specific manipulation, with the aim to store information there," explains Dmitry Krimer. "The transitions areas in the nitrogen atoms have slightly different energy levels because of the local properties of the not quite perfect diamond crystal", explains Stefan Putz, the first author of the study, who has since moved from TU Wien to Princeton University. "If you use microwaves to selectively change a few nitrogen atoms that have very specific energies, you can create a "Spectral Hole". The remaining nitrogen atoms can then be brought into a new quantum state, a so-called "dark state", in the center of these holes. This state is much more stable and opens up completely new possibilities." "Our work is a 'proof of principle' - we present a new concept, show that it works, and we want to lay the foundations for further exploration of innovative operational protocols of quantum data," says Stefan Putz. With this new method, the lifetime of quantum states of the coupled system of microwaves and nitrogen atoms increased by more than one order of magnitude to about five microseconds. This is still not a great deal in the standard of everyday life, but in this case it is sufficient for important quantum-technological applications. "The advantage of our system is that one can write and read quantum information within nanoseconds," explains Johannes Majer. "A large number of working steps are therefore possible in microseconds, in which the system remains stable."


News Article | November 22, 2016
Site: www.eurekalert.org

A Polish-British team of physicists has constructed and tested a compact, efficient converter capable of modifying the quantum properties of individual photons. The new device should facilitate the construction of complex quantum computers, and in the future may become an important element in global quantum networks, the successors of today's Internet. Quantum internet and hybrid quantum computers, built out of subsystems that operate by means of various physical phenomena, are now becoming more than just the stuff of imagination. In an article just published in the prestigious journal Nature Photonics, physicists from the University of Warsaw's Faculty of Physics (FUW) and the University of Oxford have unveiled a key element of such systems: an electro-optical device that enables the properties of individual photons to be modified. Unlike existing laboratory constructions, this new device works with previously unattainable efficiency and is at the same time stable, reliable, and compact. Building an efficient device for modifying the quantum state of individual photons was an exceptionally challenging task, given the fundamental differences between classical and quantum computing. Contemporary computing systems are based on the processing of groups of bits, each of which is in a specific, well-known state: either 0 or 1. Groups of such bits are continually being transferred both between different subcomponents within a single computer, and between different computers on the network. We can illustrate this figuratively by imagining a situation in which trays of coins are being moved from place to place, with each coin laying either with the heads side or the tails side facing upwards. Things are more complicated in quantum computing, which relies on the phenomenon of superposition of states. A quantum bit, known as a qubit, can be both in the 1 state and the 0 state at the same time. To continue the analogy described above, this would be like a situation in which each coin is spinning on its edge. Information processing can be described as "quantum" processing as long as this superposition of states can be retained during all operations - in other words, as long as none of the coins gets tipped out of the spinning state while the tray is being moved. "In recent years, physicists have figured out how to generate light pulses with a specific wavelength or polarization, consisting of a single quantum -- or excitation -- of the electromagnetic field. And so today we know how to generate precisely whatever kind of quantum 'spinning coins' we want," says Dr. Michal Karpinski from the Institute of Experimental Physics (FUW), one of the authors of the publication. "But achieving one thing always leaves you wanting more! If we now have individual light quanta with specific properties, it would be useful to modify those properties. The task is therefore more or less this: take a spinning silver coin and move it from one place to another, but along the way quickly and precisely turn it into a gold coin, naturally without tipping it over. You can easily see that the problem is nontrivial." Existing methods of modifying individual photons have utilized nonlinear optical techniques, in practice attempting to force an individual photon to interact with a very strong optical pump beam. Whether the photon so subjected actually gets modified is a matter of pure chance. Moreover, the scattering of the pump beam may contaminate the stream of individual photons. In constructing the new device, the group from the University of Warsaw and the University of Oxford decided to make use of a different physical phenomenon: the electro-optic effect occurring in certain crystals. It provides a way to alter the index of refraction for light in the crystal - by varying the intensity of an external magnetic force that is applied to it (in other words, without introducing any additional photons!). "It is quite astounding that in order to modify the quantum properties of individual photons, we can successfully apply techniques very similar to those used in standard fiber-optic telecommunications," Dr. Karpinski says. Using the new device, the researchers managed - without disrupting the quantum superposition! -- to achieve a six-fold lengthening of the duration of a single-photon pulse, which automatically means a narrowing of its spectrum. What is particularly important is that the whole operation was carried out while preserving very high conversion efficiency. Existing converters have operated only under laboratory conditions and were only able to modify one in several tens of photons. The new device works with efficiency in excess of 30%, up to even 200 times better than certain existing solutions, while retaining a low level of noise. "In essence we process every photon entering the crystal. The efficiency is less than 100% not because of the physics of the phenomenon, but on account of hard-to-avoid losses of a purely technical nature, appearing for instance when light enters of exits optical fibers," explains PhD student Michal Jachura (FUW). The new converter is not only efficient and low-noise, but also stable and compact: the device can be contained in a box with dimension not much larger than 10 cm (4 in.), easy to install in an optical fiber system channeling individual photons. Such a device enables us to think realistically about building, for instance, a hybrid quantum computer, the individual subcomponents of which would process information a quantum way using different physical platforms and phenomena. At present, attempts are being made to build quantum computers using, among others, trapped ions, electron spins in diamond, quantum dots, superconducting electric circuits, and atomic clouds. Each such system interacts with light of different properties, which in practice rules out optical transmission of quantum information between different systems. The new converter, on the other hand, can efficiently transform single-photon pulses of light compatible with one system into pulses compatible with another. Scientists are therefore gaining at a real pathway to building quantum networks, both small ones within a single quantum computer (or subcomponent thereof), and global ones providing a way to send data completely securely between quantum computers situated in different parts of the world. The experimental part of this work was carried out at the University of Oxford's Department of Physics, in the Optical Quantum Technologies Group led by Dr. Brian J. Smith, where Dr. Karpi?ski had held a postdoctoral fellowship under the prestigious Marie Sklodowska-Curie grants. On the Polish side, the work was funded by grants from Poland's National Science Centre and the 7th EU Framework Programme. Physics and Astronomy first appeared at the University of Warsaw in 1816, under the then Faculty of Philosophy. In 1825 the Astronomical Observatory was established. Currently, the Faculty of Physics' Institutes include Experimental Physics, Theoretical Physics, Geophysics, Department of Mathematical Methods and an Astronomical Observatory. Research covers almost all areas of modern physics, on scales from the quantum to the cosmological. The Faculty's research and teaching staff includes ca. 200 university teachers, of which 88 are employees with the title of professor. The Faculty of Physics, University of Warsaw, is attended by ca. 1000 students and more than 170 doctoral students. "Bandwidth manipulation of quantum light by an electro-optic time lens"; M. Karpinski, M. Jachura, L. J. Wright, B. J. Smith; Nature Photonics 2016; DOI: 10.1038/nphoton.2016.228 Dr. Michal Karpinski Institute of Experimental Physics, Faculty of Physics, University of Warsaw tel. 48-22-5532740, 48-22-5548872 email: mkarp@fuw.edu.pl M.Sc. Michal Jachura Institute of Theoretical Physics, Faculty of Physics, University of Warsaw tel. 48-22-5532969 email: michal.jachura@fuw.edu.pl Division of Optics, Institute of Experimental Physics, Faculty of Physics, University of Warsaw. Press office of the Faculty of Physics, University of Warsaw. A single photon -- a carrier of quantum information -- travels like a spinning coin, in a superposition of states. Modyfing its properties is extremely hard and should be done carefully, without destroying the superposition. (Source: FUW, Grzegorz Krzyzewski) A single photon converter (a yellow-orange box) installed on an optical fiber of the laboratory setup. (Source: FUW, Grzegorz Krzyzewski) Usually, due to the properties mismatch, the majority of single photons cannot be effectively stored e.g. in the quantum memory (represented as a white box). The new converter enables to modify the properties of photons so that virtually all of them can be stored inside the memory. (Source: FUW, Grzegorz Krzyzewski)


Pickl P.,Institute of Theoretical Physics
Journal of Statistical Physics | Year: 2010

Using a new method (Pickl in A simple derivation of mean field limits for quantum systems, 2010) it is possible to derive mean field equations from the microscopic N body Schrödinger evolution of interacting particles without using BBGKY hierarchies. In this paper we wish to analyze scalings which lead to the Gross-Pitaevskii equation which is usually derived assuming positivity of the interaction (Erdös et al. in Commun. Pure Appl. Math. 59(12):1659-1741, 2006; Invent. Math. 167:515-614, 2007). The new method for dealing with mean field limits presented in Pickl (2010) allows us to relax this condition. The price we have to pay for this relaxation is however that we have to restrict the scaling behavior of the interaction and that we have to assume fast convergence of the reduced one particle marginal density matrix of the initial wave function μΨ0 to a pure state {pipe}φ0〉〈φ0{pipe}. © 2010 Springer Science+Business Media, LLC.


News Article | December 21, 2016
Site: phys.org

Once again, graphene has proven itself to be a rather special material: an international research team led by Professor Fritz Aumayr from the Institute of Applied Physics at TU Wien was able to demonstrate that the electrons in graphene are extremely mobile and react very quickly. Impacting xenon ions with a particularly high electric charge on a graphene film causes a large number of electrons to be torn away from the graphene in a very precise spot. However, the material was able to replace the electrons within some femtoseconds. This resulted in extremely high currents, which would not be maintained under normal circumstances. Its extraordinary electronic properties make graphene a very promising candidate for future applications in the field of electronics. The Helmholtz-Center Dresden-Rossendorf and the University of Duisburg-Essen participated in the experiment alongside TU Wien. The international team received theoretical support from Paris and San Sebastian as well as from in-house staff (Institute of Theoretical Physics at TU Wien). 'We work with extremely highly-charged xenon ions,' explains Elisabeth Gruber, a PhD student from Professor Aumayr's research team. 'Up to 35 electrons are removed from the xenon atoms, meaning the atoms have a high positive electric charge.' These ions are then fired at a free-standing single layer of graphene, which is clamped between microscopically small brackets. 'The xenon ion penetrates the graphene film, thereby knocking a carbon atom out of the graphene – but that has very little effect, as the gap that has opened up in the graphene is then refilled with another carbon atom,' explains Elisabeth Gruber. 'For us, what is much more interesting is how the electrical field of the highly charged ion affects the electrons in the graphene film.' This happens even before the highly charged xenon ion collides with the graphene film. As the highly charged ion is approaching it starts tearing electrons away from the graphene due to its extremely strong electric field. By the time the ion has fully passed through the graphene layer, it has a positive charge of less than 10, compared to over 30 when it started out. The ion is able to extract more than 20 electrons from a tiny area of the graphene film. This means that electrons are now missing from the graphene layer, so the carbon atoms surrounding the point of impact of the xenon ions are positively charged. 'What you would expect to happen now is for these positively charged carbon ions to repel one another, flying off in what is called a Coulomb explosion and leaving a large gap in the material,' says Richard Wilhelm from the Helmholtz-Center Dresden-Rossendorf, who currently works at TU Wien as a postdoctoral assistant. 'But astoundingly, that is not the case. The positive charge in the graphene is neutralised almost instantaneously.' This is only possible because a sufficient number of electrons can be replaced in the graphene within an extremely short time frame of several femtoseconds (quadrillionths of a second). 'The electronic response of the material to the disruption caused by the xenon ion is extremely rapid. Strong currents from neighbouring regions of the graphene film promptly resupply electrons before an explosion is caused by the positive charges repelling one another,' explains Elisabeth Gruber. 'The current density is around 1000 times higher than that which would lead to the destruction of the material under normal circumstances – but over these distances and time scales, graphene can withstand such extreme currents without suffering any damage.' This extremely high electron mobility in graphene is of great significance for a number of potential applications: 'The hope is that for this very reason, it will be possible to use graphene to build ultra-fast electronics. Graphene also appears to be excellently suited for use in optics, for example in connecting optical and electronic components,' says Aumayr. Explore further: Graphene layer could allow solar cells to generate power when it rains More information: Elisabeth Gruber et al. Ultrafast electronic response of graphene to a strong and localized electric field, Nature Communications (2016). DOI: 10.1038/ncomms13948


News Article | December 16, 2016
Site: phys.org

Electrons embedded in the atomic lattice – the components of a solid. The mutual repulsion of the electrons prevents them from coming into close contact. This impedes the electron flow and the system can become an insulator. Credit: Dr. Ulrich Tutsch Whether water freezes to ice, iron is demagnetized or a material becomes superconducting – for physicists there is always a phase transition behind it. They endeavour to understand these different phenomena by searching for universal properties. Researchers at Goethe University Frankfurt and Technische Universität Dresden have now made a pioneering discovery during their study of a phase transition from an electrical conductor to an insulator (Mott metal-insulator transition). According to Sir Nevill Francis Mott's prediction in 1937, the mutual repulsion of charged electrons, which are responsible for carrying electrical current, can cause a metal-insulator transition. Yet, contrary to common textbook opinion, according to which the phase transition is determined solely by the electrons, it is the interaction of the electrons with the atomic lattice of the solid which is the determinant factor. The researchers have reported this in the latest issue of the Science Advances journal. The research group, led by Professor Michael Lang of the Physics Institute at Goethe University Frankfurt, succeeded in making the discovery with the help of a homemade apparatus which is unique worldwide. It allows the measurement of length changes at low temperatures under variable external pressure with extremely high resolution. In this way, it was possible to prove experimentally for the first time that it is not just the electrons which play a significant role in the phase transition but also the atomic lattice—the solid's scaffold. "These experimental results will herald in a paradigm shift in our understanding of one of the key phenomena of current condensed matter research," says Professor Lang. The Mott metal-insulator transition is namely linked to unusual phenomena, such as high-temperature superconductivity in copper oxide-based materials. These offer tremendous potential for future technical applications. The theoretical analysis of the experimental findings is based on the fundamental notion that the many particles in a system close to a phase transition not only interact with their immediate neighbours but also "communicate" over long distances with all other particles. As a consequence, only overarching aspects are important, such as the system's symmetry. The identification of such universal properties is thus the key to understanding phase transitions. "These new insights open up a whole new perspective on the Mott metal-insulator transition and permit more sophisticated theoretical modelling of the phase transition," explains Dr. Markus Garst, Senior Lecturer at the Institute of Theoretical Physics of Technische Universität Dresden. Explore further: The metal-insulator transition depends on the mass of the Dirac electrons More information: E. Gati et al. Breakdown of Hookes law of elasticity at the Mott critical endpoint in an organic conductor, Science Advances (2016). DOI: 10.1126/sciadv.1601646


News Article | December 21, 2016
Site: www.cemag.us

Once again, graphene has proven itself to be a rather special material: an international research team led by Professor Fritz Aumayr from the Institute of Applied Physics at TU Wien was able to demonstrate that the electrons in graphene are extremely mobile and react very quickly. Impacting xenon ions with a particularly high electric charge on a graphene film causes a large number of electrons to be torn away from the graphene in a very precise spot. However, the material was able to replace the electrons within some femtoseconds. This resulted in extremely high currents, which would not be maintained under normal circumstances. Its extraordinary electronic properties make graphene a very promising candidate for future applications in the field of electronics. The Helmholtz-Center Dresden-Rossendorf and the University of Duisburg-Essen participated in the experiment alongside TU Wien. The international team received theoretical support from Paris and San Sebastian as well as from in-house staff (Institute of Theoretical Physics at TU Wien). “We work with extremely highly-charged xenon ions,” explains Elisabeth Gruber, a PhD student from Aumayr’s research team. “Up to 35 electrons are removed from the xenon atoms, meaning the atoms have a high positive electric charge.” These ions are then fired at a free-standing single layer of graphene, which is clamped between microscopically small brackets. “The xenon ion penetrates the graphene film, thereby knocking a carbon atom out of the graphene — but that has very little effect, as the gap that has opened up in the graphene is then refilled with another carbon atom,” says Gruber. “For us, what is much more interesting is how the electrical field of the highly charged ion affects the electrons in the graphene film.” This happens even before the highly charged xenon ion collides with the graphene film. As the highly charged ion is approaching it starts tearing electrons away from the graphene due to its extremely strong electric field. By the time the ion has fully passed through the graphene layer, it has a positive charge of less than 10, compared to over 30 when it started out. The ion is able to extract more than 20 electrons from a tiny area of the graphene film. This means that electrons are now missing from the graphene layer, so the carbon atoms surrounding the point of impact of the xenon ions are positively charged. “What you would expect to happen now is for these positively charged carbon ions to repel one another, flying off in what is called a Coulomb explosion and leaving a large gap in the material,” says Richard Wilhelm from the Helmholtz-Center Dresden-Rossendorf, who currently works at TU Wien as a postdoctoral assistant. “But astoundingly, that is not the case. The positive charge in the graphene is neutralized almost instantaneously.” This is only possible because a sufficient number of electrons can be replaced in the graphene within an extremely short time frame of several femtoseconds (quadrillionths of a second). “The electronic response of the material to the disruption caused by the xenon ion is extremely rapid. Strong currents from neighboring regions of the graphene film promptly resupply electrons before an explosion is caused by the positive charges repelling one another,” says Gruber. “The current density is around 1,000 times higher than that which would lead to the destruction of the material under normal circumstances — but over these distances and time scales, graphene can withstand such extreme currents without suffering any damage.” This extremely high electron mobility in graphene is of great significance for a number of potential applications: “The hope is that for this very reason, it will be possible to use graphene to build ultra-fast electronics. Graphene also appears to be excellently suited for use in optics, for example in connecting optical and electronic components,” says Aumayr.


News Article | November 9, 2016
Site: www.sciencedaily.com

For the first time ever, laser physicists have recorded an internal atomic event with an accuracy of a trillionth of a billionth of a second. When light strikes electrons in atoms, their states can change unimaginably quickly. Laser physicists at LMU Munich and the Max Planck Institute of Quantum Optics (MPQ) have now measured the duration of such a phenomenon – namely that of photoionization, in which an electron exits a helium atom after excitation by light – for the first time with zeptosecond precision. A zeptosecond is a trillionth of a billionth of a a second (10-21 s). This is the first absolute determination of the timescale of photoionization, and the degree of precision achieved is unprecedented for a direct measurement of the interaction of light and matter. When a light particle (photon) interacts with the two electrons in a helium atom, the changes take place not only on an ultra-short timescale, but quantum mechanics also comes into play. Its rules dictate that either the entire energy of the photon is absorbed by one of the electrons, or the energy is distributed between them. Regardless of the mode of energy transfer, one electron is ejected from the helium atom. This process is called photoemission, or the photoelectric effect, and was discovered by Albert Einstein at the beginning of the last century. In order to observe what occurs, you need a camera with an incredibly fast shutter speed: The whole process, from the point at which the photon interacts with the electrons to the instant when one of the electrons leaves the atom, takes between 5 and 15 attoseconds (1 as is 10-18 seconds) as physicists have worked out in recent years. Using an improved method of measurement, the Munich physicists can now accurately capture events that occur on timescales down to 850 zeptoseconds. The researchers directed an attosecond-long, extremely ultraviolet (XUV) light pulse onto a helium atom to excite the electrons. At the same time, they fired a second infrared laser pulse at the same target, lasting for about four femtoseconds (1 fs is 10-15 seconds). The ejected electron was detected by the infrared laser pulse as soon as it left the atom in response to the excitation by XUV light. Depending on the exact state of the oscillating electromagnetic field of this pulse at the time of detection, the electron was accelerated or decelerated. By measuring this change in speed, the researchers were able to establish the duration of the photoemission event with zeptosecond precision. In addition, the researchers were also able to determine, for the first time, how the energy of the incident photon is quantum mechanically distributed between the two electrons of the helium atom in the final few attoseconds before the emission of one of the particles. “Our understanding of these processes within the helium atom provides us with a tremendously reliable basis for future experiments,” explains Martin Schultze, a specialist in laser physics at LMU's Chair of Experimental Physics, who led the experiments at the MPQ. He and his team were able to correlate the zeptosecond precision of their experiments with theoretical predictions made by their colleagues in the Institute of Theoretical Physics at the Technical University of Vienna. With its two electrons, helium is the most complex system whose properties can be calculated completely from quantum theory. This makes it possible to reconcile theory and experiment. “We can now derive the complete wave mechanical description of the entangled system of electron and ionized helium parent atom from our measurements,” says Schultze.


News Article | November 10, 2016
Site: www.rdmag.com

Laser physicists in Munich have measured a photoionization -- in which an electron exits a helium atom after excitation by light -- for the first time with zeptosecond precision. A zeptosecond is a trillionth of a billionth of a second (10^-21 seconds). This is the greatest accuracy of time determination ever achieved, as well as the first absolute determination of the timescale of photoionization. If light hits the two electrons of a helium atom, one must be incredibly fast to observe what occurs. Besides the ultra-short periods in which changes take place, quantum mechanics also comes into play. Laser physicists at the Max Planck Institute of Quantum Optics (MPQ), the Technical University of Munich (TUM) and the Ludwig Maximilians University (LMU) Munich have now measured such an event for the first time with zeptosecond precision. Either the entire energy of a light particle (photon) can be absorbed by one of the electrons or a division can take place, if a photon hits the two electrons of a helium atom. Regardless of the energy transfer, one electron leaves the atom. This process is called photoemission, or photoelectric effect, and was explained by Albert Einstein at the beginning of last century. It takes between five and fifteen attoseconds (1 as is 10^-18 second) from the time a photon interacts with the electrons to the time one of the electrons leaves the atom, as physicists already discovered in recent years. First glance into the world of Zeptoseconds With their improved measurement method, laser physicists can accurately measure events at a rate of up to 850 zeptoseconds. The researchers shone an attosecond-long, extremely ultraviolet (XUV) light pulse onto a helium atom to excite the electrons. At the same time, they fired a second infrared laser pulse, lasting about four femtoseconds (1 fs is 10^-15 seconds). The electron was detected by the infrared laser pulse as soon as it left the atom following excitation by XUV light. Depending on the exact electromagnetic field of this pulse at the time of detection, the electron was accelerated or decelerated. Through this change in speed, the physicists were able to measure photoemission with zeptosecond precision. The researchers were also able to determine for the first time how the energy of the incident photon is quantum-mechanically divided between the two electrons of the helium atom in a few attoseconds before the emission of one of the particles. "With the measurement of the electronic correlation, our experiments solved a promise of attosecond physics, namely the temporal resolution of a process which is inaccessible with other methods," says Reinhard Kienberger, professor of the Chair of Laser- and X-Ray Physics at TU Munich. The physicists were also able to correlate the zeptosecond precision of their experiments with the theoretical predictions of their peers from the Institute of Theoretical Physics at the Technical University of Vienna. With its two electrons, helium is the only multi electron system that can be calculated completely quantum mechanically. This makes it possible to reconcile theory and experiment. "We can now derive the complete wave mechanic description of the interconnected systems of electron and ionized helium mother atoms from our measurements," says Martin Schultze, project leader at the Max Planck Institute of Quantum Optics in Garching (Germany). With their metrology experiments in zeptosecond time dimensions, the laser physicists have maneuvered another important puzzle piece in the quantum mechanics of the helium atom into position, and thus advanced measuring accuracy in the microcosm to a whole new dimension.

Loading Institute of Theoretical Physics collaborators
Loading Institute of Theoretical Physics collaborators