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News Article | May 12, 2017
Site: phys.org

In contrast to the light emitted by a simple lamp, laser light has a very precisely defined frequency. This makes it ideally suited to spectroscopic investigations, in which the properties of substances are determined on the basis of the frequencies at which they absorb light. A complete spectroscopic analysis typically requires a little patience, as the frequency of the laser has to be gradually changed ("scanned") in order to obtain a full spectrogram. A group of physicists at ETH in Zurich led by Ursula Keller at the Institute for Quantum Electronics have now demonstrated a seminal method that could lead to simpler and faster spectroscopic investigations in the future. For that purpose, they developed a novel technique for creating so-called dual frequency combs. The results have now been published in the scientific journal Science. Whereas a normal laser emits light at one frequency, a frequency comb features a large number of frequencies at a constant distance from one another – just like the marks on a ruler. This is made possible by using lasers that create extremely short periodic light pulses. Such pulse trains have a comb-like frequency spectrum, which can be broadened further using particular optical materials. In 2005 the Nobel Prize was awarded for laser-based precision spectroscopy including the optical frequency comb technique, to which Ursula Keller in collaboration with Harald Telle from PTB Braunschweig invented the key enabling technology for the stabilization of the comb in 1999. In principle one could probe a substance simultaneously with many frequencies using such a frequency comb. In ordinary spectroscopy a part of the laser light is sent through the material to be studied, and the other part is used as a reference. The frequency of the laser is now steadily scanned, and at the same time the absorption of the laser light by the substance is measured relative to the reference beam using two photodetectors. From this frequency scan the characteristic spectrogram of the substance is obtained. Unfortunately, this procedure cannot be applied straightforwardly to a frequency comb. The different frequencies simultaneously contained in the comb would certainly be absorbed differently. The photodetector, however, would not be able to tell them apart. To do so, it would have to directly record the individual, superposed oscillations of the light, which, however, is impossible in practice because of their high frequency of several hundred Terahertz (a thousand billion oscillations per second). The technique developed by Keller and her co-workers "translates" these fast and not directly measureable oscillations into much slower ones that can be easily detected with conventional electronics. This procedure relies on a trick that is used in a similar form by piano tuners. In order to obtain an equal tuning of the different chords of the same tone a piano tuner uses the beat produced by the superposition of two different frequencies. The beat pulsates at a speed that corresponds to the difference of the two superposed frequencies. The researchers at ETH use a very similar method in which they create a second frequency comb, whose frequencies have a slightly different spacing than those of the first one. This creates pairs of frequencies, each of which results in a slightly different beat frequency. These beat frequencies are now in the Megahertz regime and can be easily measured using photodetectors. Two frequency combs for the price of one This kind of dual-comb spectroscopy has been around for a few years, but the technique now developed at ETH makes it considerably simpler and less expensive, as Sandro Link, PhD student and first author of the paper, explains: "The real novelty is that we create the two frequency combs with just one laser instead of two, which would have to be painstakingly stabilized with respect to each other." The trick they use consists in a birefringent crystal that is inserted into a laser, which causes the light to travel slightly different distances according to its polarisation (i.e., the direction of oscillation of the electromagnetic wave). As a consequence, the two laser beams thus produced have slightly different pulse periods, which in turn leads to frequency combs with different frequency spacings. As the two frequency combs are created by the same laser, stabilizing them against each other becomes redundant. A number of possible applications of the new technology present themselves. Since it allows one to produce a complete spectrogram in less than a thousandth of a second, it is ideally suited to measuring the concentration of substances in the environment or in the exhausts of factories. Fast flowing gases in petrochemical settings could also be analysed quickly, for example to monitor and control production processes. More information: S. M. Link et al. Dual-comb spectroscopy of water vapor with a free-running semiconductor disk laser, Science (2017). DOI: 10.1126/science.aam7424


News Article | November 7, 2016
Site: www.sciencedaily.com

If one tries to understand weather phenomena, it's not much use looking at the behaviour of single water droplets or air molecules. Instead, meteorologists (and also laymen) speak of clouds, winds and precipitation -- objects that result from the complex interplay between small particles. Physicists dealing with the optical properties or the conductivity of solids use much the same approach. Again, tiny particles -- electrons and atoms -- are responsible for a multitude of phenomena, but an illuminating picture only emerges when many of them are grouped into "quasiparticles." However, finding out precisely what quasiparticles arise inside a material and how they influence one another is not a simple task, but more akin to a large puzzle whose pieces fit together, little by little, through arduous research. In a combination of experimental and theoretical studies, Ataç Imamoglu and his collaborators at the Institute for Quantum Electronics at the ETH in Zurich have now succeeded in finding a new piece of the puzzle, which also helps to put a previously misplaced piece in its correct position. In solids quasiparticles can be created, for instance, when a photon is absorbed. The motional energy of electrons teeming about in a solid can only take values within well-defined ranges known as bands. A photon can promote an electron from a low-lying to a high-lying energy band, thus leaving behind a "hole" in the lower band. The excited electron and the resulting hole attract each other through the electrostatic Coulomb force, and if that attraction is strong enough, the electron-hole pair can be viewed as a quasiparticle -- an "exciton" is born. Two electrons and a hole can bind together to form a trion. When excitons and a large number of free electrons are simultaneously present however, the description of the qualitatively new -- or "emergent" -- properties of the material requires the introduction of new type of quasiparticles called Fermi polarons. Imamoglu and his colleagues wanted to find out the nature of quasiparticles that appear in a certain type of semiconductors in which electrons can only move in two dimensions. To do so, they took a single layer of molybdenum diselenide that is thousand times thinner than a micrometer and sandwiched it between two disks of boron nitride. They then added a layer of graphene in order to apply an electric voltage with which the density of electrons in the material could be controlled. Finally, everything was placed between two mirrors that formed an optical cavity. With this complex experimental setup the physicists in Zurich could now study in detail how strongly the material absorbs light under different conditions. They found that when the semiconductor structure is optically excited, Fermi-polarons are formed -- and not, as previously thought, excitons or trions. "So far, researchers -- myself included -- have misinterpreted the data available at the time in that respect," admits Imamoglu. "With our new experiments we are now able to rectify that picture." "This was a team effort with essential contributions by Harvard professor Eugene Demler, who collaborated with us over several months when he was an ITS fellow," says Meinrad Sidler who is a doctoral student in Imamoglus group. Since 2013 the Institute for Theoretical Studies (ITS) of the ETH has endeavoured to foster interdisciplinary research at the intersection between mathematics, theoretical physics and computer science. In particular, it wants to facilitate curiosity-driven research with the aim of finding the best ideas in unexpected places. The study by Imamoglu and his colleagues, now published in "Nature Physics," is a good example for how this principle can be successful. In his own research, Eugene Demler deals with ultracold atoms, studying how mixtures of bosonic and fermionic atoms behave. "His insight into polarons in atomic gases and solids have given our research important and interesting impulses, which we may not have come up with on our own," says Imamoglu. The insights they have gathered will most likely keep Imamoglu and his collaborators busy for some time to come, as the interactions between bosonic (such as excitons) and fermionic (electrons) particles are the topic of a large research project for which Imamoglu won an Advanced Grant of the European Research Council (ERC) last year, and is also supported by the National Centre of Competence in Research Quantum Science and Technology (NCCR QSIT). A better understanding of such mixtures would have important implications for basic research, but exciting applications also beckon. For instance, a key goal of the ERC project is the demonstration of control of superconductivity using lasers.


News Article | November 2, 2016
Site: phys.org

If one tries to understand weather phenomena, it's not much use looking at the behaviour of single water droplets or air molecules. Instead, meteorologists (and also laymen) speak of clouds, winds and precipitation - objects that result from the complex interplay between small particles. Physicists dealing with the optical properties or the conductivity of solids use much the same approach. Again, tiny particles - electrons and atoms - are responsible for a multitude of phenomena, but an illuminating picture only emerges when many of them are grouped into "quasiparticles". However, finding out precisely what quasiparticles arise inside a material and how they influence one another is not a simple task, but more akin to a large puzzle whose pieces fit together, little by little, through arduous research. In a combination of experimental and theoretical studies, Ataç Imamoglu and his collaborators at the Institute for Quantum Electronics at the ETH in Zurich have now succeeded in finding a new piece of the puzzle, which also helps to put a previously misplaced piece in its correct position. In solids quasiparticles can be created, for instance, when a photon is absorbed. The motional energy of electrons teeming about in a solid can only take values within well-defined ranges known as bands. A photon can promote an electron from a low-lying to a high-lying energy band, thus leaving behind a "hole" in the lower band. The excited electron and the resulting hole attract each other through the electrostatic Coulomb force, and if that attraction is strong enough, the electron-hole pair can be viewed as a quasiparticle - an "exciton" is born. Two electrons and a hole can bind together to form a trion. When excitons and a large number of free electrons are simultaneously present however, the description of the qualitatively new - or "emergent" - properties of the material requires the introduction of new type of quasiparticles called Fermi polarons. Imamoglu and his colleagues wanted to find out the nature of quasiparticles that appear in a certain type of semiconductors in which electrons can only move in two dimensions. To do so, they took a single layer of molybdenum diselenide that is thousand times thinner than a micrometer and sandwiched it between two disks of boron nitride. They then added a layer of graphene in order to apply an electric voltage with which the density of electrons in the material could be controlled. Finally, everything was placed between two mirrors that formed an optical cavity. With this complex experimental setup the physicists in Zurich could now study in detail how strongly the material absorbs light under different conditions. They found that when the semiconductor structure is optically excited, Fermi-polarons are formed - and not, as previously thought, excitons or trions. "So far, researchers - myself included - have misinterpreted the data available at the time in that respect", admits Imamoglu. "With our new experiments we are now able to rectify that picture." "This was a team effort with essential contributions by Harvard professor Eugene Demler, who collaborated with us over several months when he was an ITS fellow", says Meinrad Sidler who is a doctoral student in Imamoglus group. Since 2013 the Institute for Theoretical Studies (ITS) of the ETH has endeavoured to foster interdisciplinary research at the intersection between mathematics, theoretical physics and computer science. In particular, it wants to facilitate curiosity-driven research with the aim of finding the best ideas in unexpected places. The study by Imamoglu and his colleagues, now published in Nature Physics, is a good example for how this principle can be successful. In his own research, Eugene Demler deals with ultracold atoms, studying how mixtures of bosonic and fermionic atoms behave. "His insight into polarons in atomic gases and solids have given our research important and interesting impulses, which we may not have come up with on our own", says Imamoglu. The insights they have gathered will most likely keep Imamoglu and his collaborators busy for some time to come, as the interactions between bosonic (such as excitons) and fermionic (electrons) particles are the topic of a large research project for which Imamoglu won an Advanced Grant of the European Research Council (ERC) last year, and is also supported by the National Centre of Competence in Research Quantum Science and Technology (NCCR QSIT). A better understanding of such mixtures would have important implications for basic research, but exciting applications also beckon. For instance, a key goal of the ERC project is the demonstration of control of superconductivity using lasers. Explore further: Observing the birth of quasiparticles in real time More information: Meinrad Sidler et al, Fermi polaron-polaritons in charge-tunable atomically thin semiconductors, Nature Physics (2016). DOI: 10.1038/nphys3949


News Article | November 2, 2016
Site: www.eurekalert.org

If one tries to understand weather phenomena, it's not much use looking at the behaviour of single water droplets or air molecules. Instead, meteorologists (and also laymen) speak of clouds, winds and precipitation - objects that result from the complex interplay between small particles. Physicists dealing with the optical properties or the conductivity of solids use much the same approach. Again, tiny particles - electrons and atoms - are responsible for a multitude of phenomena, but an illuminating picture only emerges when many of them are grouped into "quasiparticles". However, finding out precisely what quasiparticles arise inside a material and how they influence one another is not a simple task, but more akin to a large puzzle whose pieces fit together, little by little, through arduous research. In a combination of experimental and theoretical studies, Ataç Imamoglu and his collaborators at the Institute for Quantum Electronics at the ETH in Zurich have now succeeded in finding a new piece of the puzzle, which also helps to put a previously misplaced piece in its correct position. In solids quasiparticles can be created, for instance, when a photon is absorbed. The motional energy of electrons teeming about in a solid can only take values within well-defined ranges known as bands. A photon can promote an electron from a low-lying to a high-lying energy band, thus leaving behind a "hole" in the lower band. The excited electron and the resulting hole attract each other through the electrostatic Coulomb force, and if that attraction is strong enough, the electron-hole pair can be viewed as a quasiparticle - an "exciton" is born. Two electrons and a hole can bind together to form a trion. When excitons and a large number of free electrons are simultaneously present however, the description of the qualitatively new - or "emergent" - properties of the material requires the introduction of new type of quasiparticles called Fermi polarons. Imamoglu and his colleagues wanted to find out the nature of quasiparticles that appear in a certain type of semiconductors in which electrons can only move in two dimensions. To do so, they took a single layer of molybdenum diselenide that is thousand times thinner than a micrometer and sandwiched it between two disks of boron nitride. They then added a layer of graphene in order to apply an electric voltage with which the density of electrons in the material could be controlled. Finally, everything was placed between two mirrors that formed an optical cavity. With this complex experimental setup the physicists in Zurich could now study in detail how strongly the material absorbs light under different conditions. They found that when the semiconductor structure is optically excited, Fermi-polarons are formed - and not, as previously thought, excitons or trions. "So far, researchers - myself included - have misinterpreted the data available at the time in that respect", admits Imamoglu. "With our new experiments we are now able to rectify that picture." "This was a team effort with essential contributions by Harvard professor Eugene Demler, who collaborated with us over several months when he was an ITS fellow", says Meinrad Sidler who is a doctoral student in Imamoglus group. Since 2013 the Institute for Theoretical Studies (ITS) of the ETH has endeavoured to foster interdisciplinary research at the intersection between mathematics, theoretical physics and computer science. In particular, it wants to facilitate curiosity-driven research with the aim of finding the best ideas in unexpected places. The study by Imamoglu and his colleagues, now published in "Nature Physics", is a good example for how this principle can be successful. In his own research, Eugene Demler deals with ultracold atoms, studying how mixtures of bosonic and fermionic atoms behave. "His insight into polarons in atomic gases and solids have given our research important and interesting impulses, which we may not have come up with on our own", says Imamoglu. The insights they have gathered will most likely keep Imamoglu and his collaborators busy for some time to come, as the interactions between bosonic (such as excitons) and fermionic (electrons) particles are the topic of a large research project for which Imamoglu won an Advanced Grant of the European Research Council (ERC) last year, and is also supported by the National Centre of Competence in Research Quantum Science and Technology (NCCR QSIT). A better understanding of such mixtures would have important implications for basic research, but exciting applications also beckon. For instance, a key goal of the ERC project is the demonstration of control of superconductivity using lasers.


Sergeyev A.,Institute for Quantum Electronics | Geiss R.,Friedrich - Schiller University of Jena | Solntsev A.S.,Australian National University | Sukhorukov A.A.,Australian National University | And 3 more authors.
ACS Photonics | Year: 2015

We experimentally demonstrate practical approaches to enhance second-harmonic (SH) generation in individual lithium niobate nanowires (NWs) with a sub-micrometer cross-section and length up to tens of micrometers. We establish that parametric interactions of guided modes propagating along the NW determine the SH output power, which can be therefore controlled by the NW length. We show that the SH power is increased by about 84 times at wavelengths corresponding to modal phase-matching. Importantly, at non-phase-matched wavelengths the SH power can be improved by a factor of up to 9.3 by adjusting the NW length with a focused ion beam. We also characterize SH emission directionality, which can be further tailored for applications in integrated optical circuits and nonlinear microscopy. © 2015 American Chemical Society.


News Article | April 11, 2016
Site: www.rdmag.com

In phase transitions, for instance between water and water vapor, the motional energy competes with the attractive energy between neighboring molecules. Physicists at ETH Zurich have now studied quantum phase transitions in which distant particles also influence one another. When water in a pot is slowly heated to the boil, an exciting duel of energies takes place inside the liquid. On the one hand, there is the interaction energy that wants to keep the water molecules together because of their mutual attraction. On the other hand, however, the motional energy, which increases due to heating, tries to separate the molecules. Below the boiling point the interaction energy prevails, but as soon as the motional energy wins the water boils and turns into water vapor. This process is also known as a phase transition. In this scenario the interaction only involves water molecules that are in immediate proximity to one another. A team of researchers led by Tilman Esslinger at the Institute for Quantum Electronics at ETH Zurich, and Tobias Donner, a scientist in his group, have now shown that particles can be made to "feel" each other even over large distances. By adding such long-range interactions the physicists were able to observe novel phase transitions that result from energetic three-way battles. The physicists did not, of course, perform their experiments in a cooking pot, but rather in an artificially created quantum world called a "quantum simulator." To do so, the researchers cooled a tiny cloud of rubidium atoms to temperatures just above absolute zero and then caught them in a crystal-like lattice made of laser beams. The interaction energy stems from collisions between atoms that move back and forth between lattice sites. The motional energy of the atoms, on the other hand, can be controlled through the intensity of the laser beams, which determines how easily the atoms can move inside the lattice. Finally, in order to bring about an interaction between atoms that are far apart, Renate Landig, a PhD student in Esslinger's group, and her colleagues used a technical trick. Using two highly reflecting mirrors they built a resonator that ensured that light particles scattered by one of the atoms would fly through the rubidium cloud several times. In that way, sooner or later all the atoms in the cloud come into contact with the scattered photon. They thus "feel" the presence of the original atom that first deviated the photon. This feeling over a distance is tantamount to an effective long-range interaction. How strongly the atoms interact in this way can be exactly controlled through the frequency of the laser beams. "Using this trick we now have three competing energy scales in our system: besides the motional and interaction energies there is, in addition, the energy associated with the long-range interaction", explains Landig. "By varying the motional energy and the long-range interaction energy, we are able to study a number of novel quantum phase transitions." The researchers were already familiar with some of the possible phase transitions. For instance, when the long-range interaction is very small and the motional energy is increased little by little, the phase of the rubidium cloud changes from a Mott insulator, with one immobile atom sitting on each lattice site, to a superfluid, in which atoms can move completely freely. If, by contrast, the researchers increase the long range interaction energy, something completely different happens. At a particular strength of that interaction the atoms spontaneously arrange themselves in a checkerboard pattern, with one empty lattice site between two atoms. "The peculiarity of this phase transition, which is similar to that between water and water vapor, is that it's a first order transition", Donner emphasizes. In such phase transitions a particular property of a substance changes suddenly, whereas second order phase transitions, which are the type of transitions that have been detected in artificial quantum systems up to now, are characterized by a gradual change. The physicists were also able to induce another unusual phase transition by making both the motional energy and the long-range interaction energy very large. In that case, too, a checkerboard pattern appeared inside the lattice, but this time there was phase coherence between the atoms - in other words, their quantum mechanical wave functions were synchronized. Phase coherence is usually only observed when the atoms are relatively free to roam, as is the case, for instance, in the superfluid state. The coexistence of a checkerboard pattern and phase coherence at the same time indicates that one is dealing with a supersolid phase. The hybrid state of supersolidity was theoretically predicted as much as fifty years ago, but thus far unambiguously detecting it has proved difficult. In the future, Esslinger and his collaborators will use their quantum simulator to study such exotic effects more closely. The researchers' aim is to get a general idea of quantum phenomena in increasingly complex systems. This, in turn, goes hand in hand with the development and investigation of materials with special properties. The research was undertaken in conjunction with TherMiQ, a European research project examining the thermodynamics of mesoscopic open quantum systems.


News Article | April 11, 2016
Site: www.nanotech-now.com

Abstract: When water in a pot is slowly heated to the boil, an exciting duel of energies takes place inside the liquid. On the one hand there is the interaction energy that wants to keep the water molecules together because of their mutual attraction. On the other hand, however, the motional energy, which increases due to heating, tries to separate the molecules. Below the boiling point the interaction energy prevails, but as soon as the motional energy wins the water boils and turns into water vapour. This process is also known as a phase transition. In this scenario the interaction only involves water molecules that are in immediate proximity to one another. A team of researchers led by Tilman Esslinger at the Institute for Quantum Electronics at ETH Zurich, and Tobias Donner, a scientist in his group, have now shown that particles can be made to "feel" each other even over large distances. By adding such long-range interactions the physicists were able to observe novel phase transitions that result from energetic three-way battles. Artificial quantum worlds The physicists did not, of course, perform their experiments in a cooking pot, but rather in an artificially created quantum world called a "quantum simulator". To do so, the researchers cooled a tiny cloud of rubidium atoms to temperatures just above absolute zero and then caught them in a crystal-like lattice made of laser beams. The interaction energy stems from collisions between atoms that move back and forth between lattice sites. The motional energy of the atoms, on the other hand, can be controlled through the intensity of the laser beams, which determines how easily the atoms can move inside the lattice. Finally, in order to bring about an interaction between atoms that are far apart, Renate Landig, a PhD student in Esslinger's group, and her colleagues used a technical trick. Using two highly reflecting mirrors they built a resonator that ensured that light particles scattered by one of the atoms would fly through the rubidium cloud several times. In that way, sooner or later all the atoms in the cloud come into contact with the scattered photon. They thus "feel" the presence of the original atom that first deviated the photon. This feeling over a distance is tantamount to an effective long-range interaction. How strongly the atoms interact in this way can be exactly controlled through the frequency of the laser beams. "Using this trick we now have three competing energy scales in our system: besides the motional and interaction energies there is, in addition, the energy associated with the long-range interaction", explains Landig. "By varying the motional energy and the long-range interaction energy, we are able to study a number of novel quantum phase transitions." First order phase transitions The researchers were already familiar with some of the possible phase transitions. For instance, when the long-range interaction is very small and the motional energy is increased little by little, the phase of the rubidium cloud changes from a Mott insulator, with one immobile atom sitting on each lattice site, to a superfluid, in which atoms can move completely freely. If, by contrast, the researchers increase the long range interaction energy, something completely different happens. At a particular strength of that interaction the atoms spontaneously arrange themselves in a checkerboard pattern, with one empty lattice site between two atoms. "The peculiarity of this phase transition, which is similar to that between water and water vapour, is that it's a first order transition", Donner emphasizes. In such phase transitions a particular property of a substance changes suddenly, whereas second order phase transitions, which are the type of transitions that have been detected in artificial quantum systems up to now, are characterized by a gradual change. Supersolidity detected The physicists were also able to induce another unusual phase transition by making both the motional energy and the long-range interaction energy very large. In that case, too, a checkerboard pattern appeared inside the lattice, but this time there was phase coherence between the atoms - in other words, their quantum mechanical wave functions were synchronized. Phase coherence is usually only observed when the atoms are relatively free to roam, as is the case, for instance, in the superfluid state. The coexistence of a checkerboard pattern and phase coherence at the same time indicates that one is dealing with a supersolid phase. The hybrid state of supersolidity was theoretically predicted as much as fifty years ago, but thus far unambiguously detecting it has proved difficult. In the future, Esslinger and his collaborators will use their quantum simulator to study such exotic effects more closely. The researchers' aim is to get a general idea of quantum phenomena in increasingly complex systems. This, in turn, goes hand in hand with the development and investigation of materials with special properties. The research was undertaken in conjunction with TherMiQ, a European research project examining the thermodynamics of mesoscopic open quantum systems. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | March 2, 2017
Site: phys.org

Solid, liquid or gas are the three clearly defined states of matter. It is difficult to imagine that substances could simultaneously exhibit properties of two of these states. Yet, precisely such a phenomenon is possible in the realm of quantum physics, where matter can display behaviours that seem mutually exclusive. Supersolidity is one example of such a paradoxical state. In a supersolid, atoms are arranged in a crystalline pattern while at the same time behaving like a superfluid, in which particles move without friction. Until now, supersolidity was merely a theoretical construct. But in the latest issue of Nature, a group of researchers led by Tilman Esslinger, professor of quantum optics at the Institute for Quantum Electronics, and Tobias Donner, senior scientist at the same institute, report the successful production of a supersolid state. The researchers introduced a small amount of rubidium gas into a vacuum chamber and cooled it to a temperature of a few billionths of a kelvin above absolute zero, such that the atoms condensed into what is known as a Bose-Einstein condensate. This is a peculiar quantum-mechanical state that behaves like a superfluid. The researchers placed this condensate in a device with two intersecting optical resonance chambers, each consisting of two tiny opposing mirrors. The condensate was then illuminated with laser light, which was scattered into both of these two chambers. The combination of these two light fields in the resonance chambers caused the atoms in the condensate to adopt a regular, crystal-like structure. The condensate retained its superfluid properties – the atoms in the condensate were still able to flow without any energy input, at least in one direction, which is not possible in a "normal" solid. "We were able to produce this special state in the lab thanks to a sophisticated setup that allowed us to make the two resonance chambers identical for the atoms," explains Esslinger. With their experiment, the physicists in the team of Esslinger and Donner realised a concept theorised by scientists including British physicist David Thouless. In 1969, he postulated that a superfluid could also have a crystalline structure. Theoretical considerations led to the conclusion that this phenomenon could be most easily demonstrated with helium cooled to just a few kelvins above absolute zero. In 2004, a U.S. group reported that they had found experimental evidence for such a state, but later attributed their findings to surface effects of helium. "Our work has now successfully implemented Thouless's ideas," explains Donner. "We didn't use helium, however, but a Bose–Einstein condensate." A second, independent study on the same topic appears in the same issue of Nature: a group of researchers led by Wolfgang Ketterle at MIT announced last autumn – shortly after the researchers at ETH – that they had also succeeded in finding evidence of supersolidity, using a different experimental approach. More information: Julian Léonard et al. Supersolid formation in a quantum gas breaking a continuous translational symmetry, Nature (2017). DOI: 10.1038/nature21067


Sieber O.D.,Institute for Quantum Electronics | Wittwer V.J.,Institute for Quantum Electronics | Mangold M.,Institute for Quantum Electronics | Hoffmann M.,Institute for Quantum Electronics | And 3 more authors.
Optics Express | Year: 2011

We present a femtosecond vertical external cavity surface emitting laser (VECSEL) that is continuously tunable in repetition rate from 6.5 GHz up to 11.3 GHz. The use of a low-saturation fluence semiconductor saturable absorber mirror (SESAM) enables stable cw modelocking with a simple cavity design, for which the laser mode area on SESAM and VECSEL are similar and do not significantly change for a variation in cavity length. Without any realignment of the cavity for the full tuning range, the pulse duration remained nearly constant around 625 fs with less than 3.5% standard deviation. The center wavelength only changed ±0.2 nm around 963.8 nm, while the output power was 169 mW with less than 6% standard deviation. Such a tunable repetition rate is interesting for various metrology applications such as optical sampling by laser cavity tuning (OSCAT). © 2011 Optical Society of America.


PubMed | Institute for Quantum Electronics
Type: Journal Article | Journal: Optics express | Year: 2011

We present a femtosecond vertical external cavity surface emitting laser (VECSEL) that is continuously tunable in repetition rate from 6.5 GHz up to 11.3 GHz. The use of a low-saturation fluence semiconductor saturable absorber mirror (SESAM) enables stable cw modelocking with a simple cavity design, for which the laser mode area on SESAM and VECSEL are similar and do not significantly change for a variation in cavity length. Without any realignment of the cavity for the full tuning range, the pulse duration remained nearly constant around 625 fs with less than 3.5% standard deviation. The center wavelength only changed 0.2 nm around 963.8 nm, while the output power was 169 mW with less than 6% standard deviation. Such a tunable repetition rate is interesting for various metrology applications such as optical sampling by laser cavity tuning (OSCAT).

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