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2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team

(Read Full Article: "Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko"

Sunday, November 29, 2009

Bose-Einstein Condensation of Strontium

Figure 1: The SrBEC team. From left to right: Bo Huang, Meng Khoon Tey, Rudolf Grimm, Florian Schreck (Author), and Simon Stellmer

[This is an invited article based on the recently published work of the author and his collaborators -- 2Physics.com]

Author: Florian Schreck

Affiliation: Institut für Quantenoptik und Quanteninformation (IQOQI), Austria
Link to the 'Ultracold Atoms and Quantum Gases' Group >>

Atoms are particles as well as waves. The wave nature of atoms becomes evident when cooling a gas of bosonic atoms to extremely low temperatures. The de Broglie wavelength describing the atomic wave packets grows and as soon as it exceeds the interatomic spacing, the gas undergoes a phase-transition and enters a collective state of matter, known as Bose-Einstein condensate (BEC). This behavior was predicted in 1924 by Bose and Einstein and realized for the first time in 1995 in gases of alkali-metal atoms [1, 2, 3]. Research on these degenerate quantum gases has since grown strongly and has now connected to many other fields as, for example, condensed-matter physics, molecular physics and precision measurements.

Since then, ten elements have been Bose-condensed: all stable alkali-metal atoms and hydrogen, metastable helium, chromium, ytterbium [4] and very recently calcium [5]. Oftentimes new isotopes cooled to quantum degeneracy have, with their unique properties, opened the doors to the investigation of novel phenomena. Atoms with two electrons in their outer shell, as ytterbium and the alkaline-earth elements, have properties unlike any other of the condensed species: a non-magnetic ground-state (for bosonic isotopes), metastable states and a combination of broad and narrow linewidth optical transitions. This has made these elements, especially ytterbium and strontium, prime choices for neutral atom optical clocks. In addition, numerous proposals employ these properties to realize quantum simulation and computation schemes, mHz-linewidth lasers or to probe the time dependence of natural constants.

These applications either rely on or would benefit from the availability of quantum degenerate samples. Already ten years ago, strontium atoms have been cooled to near quantum degeneracy [6], but a BEC could not be reached. The problem resided in the last cooling stage used in all experiments that have produced degenerate quantum gases: evaporative cooling. This cooling process works by removing hot atoms from the sample and using elastic collisions to rethermalize the remaining gas at a lower temperature. For strontium evaporative cooling worked very badly: the most abundant isotope, 88Sr with 83% abundance, essentially does not collide. 86Sr with 10% abundance has the opposite problem: the atoms collide so strongly that often molecules are formed, releasing the molecular binding energy, which leads to heating and the loss of atoms. Sr has yet another bosonic isotope: 84Sr with only 0.56% natural abundance. Apparently for this reason nobody had undertaken experiments using this isotope.

However, we had experience working with low abundance isotopes and took a closer look at 84Sr. We asked Roman Ciuryło to calculate the scattering properties of 84Sr by scaling the known properties of the abundant isotopes with the mass. Based on measurements by Thomas Killians group he deduced an elastic scattering cross section in the Goldilocks zone: neither too small nor too big. This was shortly afterwards confirmed by two other groups [7, 8]. Therefore we decided to make 84Sr our main approach to Sr BEC.

To overcome the low natural abundance, a combination of Sr properties turned out to be very beneficial. To produce a sample of cold Sr atoms suitable for evaporative cooling, an atomic beam is slowed and then held and cooled in a magneto-optical trap (MOT) using laserlight near-resonant with a broad-linewidth transition. A small fraction of the atoms in the excited state of that transition will not decay back to the ground-state, but to a metastable state with a lifetime of several minutes. Atoms in this state are magnetic and can be trapped in the quadrupole magnetic field used for the MOT. Within 10 seconds we can accumulate about 100 million atoms in this state, overcoming the low natural abundance and giving us enough material for evaporative cooling.

Temperature and density achievable in a MOT depend on the linewidth of the transition used. Strontium has also a narrow-linewidth transition that is suitable for a MOT. It is too narrow to allow slowing of an atomic beam or capture of atoms from that beam, but it can be used to further cool the atoms accumulated in the metastable state (after optically pumping them back to the ground-state). The figure of merit for a cooling process with the goal to reach BEC is phase-space density, the product of density and the de Broglie wavelength cubed. The phase-transition to BEC will occur if the phase-space density exceeds 2.6. For alkali-metal atoms, only one MOT transition exists, which allows to obtain phase-space densities on the order of 10-6. The combination of broad-linewidth and just perfectly sized narrow-linewidth transition in strontium allows to achieve remarkably high phase-space densities of 10-2 already after the MOT stage. This means that only very little evaporative cooling is required to obtain quantum degeneracy.

For evaporative cooling, the atoms have to be held in a conservative potential. Atoms in the ground-state have no magnetic moment, so a magnetic trap can not be used. We confine them using a so-called optical dipole trap, which consists of two crossed infrared laser beams. About one million atoms are loaded into the crossing region after switching off the MOT. Evaporative cooling proceeds now by letting hot atoms escape from the trap and waiting for the remaining atoms to thermalize. To force evaporation to continue at ever lower temperatures, the potential depth is lowered over the course of a few seconds. We knew that the elastic scattering properties of 84Sr would be ideal for evaporative cooling, but it was impossible to predict the inelastic scattering properties that can lead to detrimental atom loss and heating. To our great pleasure we discovered that inelastic processes were very weak. Evaporative cooling worked the first time we tested it and only minutes later we created the first Bose-Einstein condensate of strontium.

Figure 2: Density distribution of strontium atoms released from a trap. To the left a thermal sample is shown. Further cooling results in the appearance of a dense and cold Bose-Einstein condensate in the middle of the cloud. Finally the thermal component is too small to be detectable.

Figure 3: Expansion of a 84Sr BEC from an elongated trap. The repulsion between the atoms leads to a faster expansion along the initially strongly confined directions. The sequence of images shows the temporal evolution in 5ms steps [Ref: Physical Review Letters, 103, 200401 (2009)]

Figure 2 shows images of the density distributions of clouds of 84Sr atoms across the phase transition from a thermal cloud to a pure BEC. As soon as the phase-transition is crossed, a dense central peak appears, the BEC. Figure 3 highlights another property of the BEC: after release from the trap it expands fastest along the direction in which it was initially strongest confined leading to a disk-shaped density distribution, shown here from the side. Thermal atoms would expand isotropically and show a spherical density distribution.

About two weeks after we had achieved BEC of Sr [9], the group of Thomas Killian arrived at the same goal [10]. It is clear from both experiments that BEC of Sr is very robust. Simple scaling up of the volume of the optical dipole trap should result in BECs in excess of one million atoms. The two other species with two electrons in the outer shell that have been Bose-condensed, Yb and Ca, have so far only produced relatively small BECs of up to 6 X 104 atoms. This puts Sr in a prime position for experiments with BECs of two-electron systems.

Using sympathetic cooling it should be possible to cool also the other Sr isotopes to quantum degeneracy. 88Sr is nearly non-interacting, which would be useful for precision sensors, for example force sensors. The fermionic isotope 87Sr has a nuclear magnetic moment, which can be used to store quantum information. It is at the heart of proposals for quantum computation and is the key to the study of a new class of many-body systems. Sr2 molecules can be created and used to measure the stability of fundamental constants. Cooling of the alkali-metal rubidium is compatible with the scheme employed for Sr. SrRb ground-state molecules would possess both, an electric and a magnetic dipole moment. This can be used to design many-body systems with spin-dependent long range interactions.

It will be exciting to explore all the new possibilities opened up by the Bose-Einstein condensation of strontium.

M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of bose-einstein condensation in dilute atomic vapor,” Science, vol. 269, pp. 198–201 (1995). Abstract.
[2] K. B. Davis, M. O. Mewes, M. R. Andrews, N. J. van Druten, D. S. Durfee, D. M. Kurn, and
W. Ketterle, “Bose-einstein condensation in a gas of sodium atoms,”
Phys. Rev. Lett., vol. 75, pp. 3969–3973 (1995). Abstract.
[3] C. C. Bradley, C. A. Sackett, J. J. Tollett, and R. G. Hulet, “Evidence of bose-einstein condensation in an atomic gas with attractive interactions,” Phys.Rev. Lett., vol. 75, pp. 1687–1690 (1995). Abstract.
[4] Y. Takasu, K. Maki, K. Komori, T. Takano, K. Honda, M. Kumakura, T. Yabuzaki, and Y. Takahashi, “Spin-singlet bose-einstein condensation of two-electron atoms,”
Phys. Rev. Lett., vol. 91, p.040404 (2003). Abstract.
[5] S. Kraft, F. Vogt, O. Appel, F. Riehle, and U. Sterr, “Bose-einstein condensation of alkaline earth atoms: 40Ca,” Phys. Rev. Lett., vol. 103, p. 130401, 2009. Abstract.
[6] H. Katori, T. Ido, Y. Isoya, and M. Kuwata-Gonokami, “Laser cooling of strontium atoms toward quantum degeneracy,” in Atomic Physics 17 (E. Arimondo, P. DeNatale, and M. Inguscio, eds.), pp. 382–396, American Institute of Physics, Woodbury (2001).
[7] A. Stein, H. Knöckel, and E. Tiemann, “Fourier-transform spectroscopy of Sr2 and revised groundstate potential,” Phys. Rev. A, vol. 78, p.042508 (2008). Abstract.
[8] Y. N. Martinez de Escobar, P. G. Mickelson, P. Pellegrini, S. B. Nagel, A. Traverso, M. Yan, R. Côté, and T. C. Killian, “Two-photon photoassociative spectroscopy of ultracold 88Sr,”
Phys.Rev. A, vol. 78, p.062708 (2008). Abstract.
[9] S. Stellmer, M. K. Tey, B. Huang, R. Grimm, and F. Schreck, “Bose-Einstein condensation of strontium,” Physical Review Letters, vol. 103, no. 20, p.200401 (2009). Abstract.
[10] Y. N. M. de Escobar, P. G. Mickelson, M. Yan, B. J. DeSalvo, S. B. Nagel, and T. C. Killian, “Bose-einstein condensation of 84Sr,” Physical Review Letters, vol. 103, no. 20, p.200402 (2009). Abstract.

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Saturday, November 21, 2009

Testing the Foundation of Special Relativity

The Düsseldorf team in the laboratory. From left to right: Ch. Eisele, A. Yu. Nevsky and S. Schiller.

[This is an invited article based on recent work of the author and his collaborators -- 2Physics.com]

Author: Ch. Eisele

Affiliation: Institute for experimental physics, Heinrich-Heine-Universität Düsseldorf, Germany

Contact: christian.eisele@uni-duesseldorf.de

Since Albert Einstein developed the theory of special relativity (TSR) during his annus mirabilis in 1905 [1], we learned that all physical laws should be formulated invariant under a special class of transformations, the so called Lorentz transformations. This "Lorentz Invariance" (LI) follows from two postulates; these are general, but experimentally testable statements.

The first postulate is Einstein's principle of relativity. It states, that all laws describing the change of physical states do not depend on the choice of the inertial coordinate system used in the description. In this sense, all inertial coordinate systems (i.e. systems moving with constant velocity relative to each other) are equivalent.

The second postulate states the universality of the speed of light: in every inertial system light propagates with the same velocity, and this velocity does not depend on the state of motion of the source or on the state of motion of the observer. We may say that the observer’s clocks and rulers behave in such a way that (s)he cannot determine any change in the value of the velocity.

Fig. 1 : General Relativity, as well as the Standard Model, assume the validity of (local) Lorentz Invariance. Both theories might be only low-energy-limits of a more fundamental theory unifying all fundamental forces and exhibiting tiny violations of Lorentz Invariance.

Today, the laws of Special Relativity lie, either as local or global symmetry, at the very basis of our accepted theories of the fundamental forces (see fig.1), the theory of General Relativity, and the Standard Model of the electroweak and strong interactions. Physicists are trying hard to develop a single theory, a Grand Unified Theory (GUT), that describes all the fundamental interactions in a common way, i.e. including a quantum description of gravitation. Over the last few decades candidate theories for a unification of the fundamental forces have been developed, e.g. string theories and loop quantum gravity. These actually do not rule out the possibility of Lorentz Invariance violations. Thus it could be that Lorentz invariance may only be an approximate symmetry of nature. These theoretical developments stimulate experimentalists to test the basic principles, on which the theory of Special Relativity is built, with the highest possible precision allowed by technology.

To test Lorentz Invariance (LI) one needs a theory, which can be used to interpret experimental results with respect to the validity of LI, a so-called test theory. Two test theories are commonly used, the Robertson-Mansouri-Sexl (RMS) theory [2-5], a kinematical framework dealing with generalized transformation rules, and the Standard Model Extension (SME) [6], a dynamical framework based on a modified Lagrangian of the Standard Model with additional couplings.

Within the RMS framework possible effects of Lorentz Invariance violation are a modified time dilation factor, a dependency of the speed of light on the velocity of the source or the observer, and an anisotropy of the speed of light. Three experiments are sufficient to validate Lorentz invariance within this model: experiments of the Ives-Stillwell type [7], the Kennedy-Thorndike type [8] and of the Michelson-Morley-type [9].

The SME, in contrast, allows in principle hundreds of new effects, and for an interpretation of a single experiment one often has to restrict oneself to certain sectors of the theory. For the so-called minimal QED sector of the theory possible effects are, e.g., birefringence of the vacuum, a dispersive vacuum and an anisotropy of the speed of light.

Upper limits on a possible birefringence of the vacuum or for a possible dispersive character of the vacuum can be derived from astrophysical observations of the light of very distant galaxies [e.g. 10,11,12], respectively the light emitted during supernova explosions. These are extremly sensitive tests. However, one has to rely on the light made available by sources in the universe and to make certain assumptions on the source of the light.

Laboratory experiments, on the other hand, allow for a very good control of the experimental circumstances. Experiments include, e.g., measurements of the anomalous g-factor of the electron [13] or measurements of the time dilation factor using fast beams of 7Li+ ions [14]. Even data of the global positioning system (GPS) can be used to test Lorentz invariance [15].

Recently, at the Universität Düsseldorf, we have performed a test of the isotropy of the speed of light with a Michelson-Morley type experiment [16]. The main component of the setup is a block of ultra-low-expansion-coefficient glass (ULE), in which two optical resonators with high finesse (F = 180 000) are embedded under an angle of 90° (see fig.2). The resonance frequencies ν1, ν2 of the resonators are a function of the speed of light, c, and the length Li of the respective resonator, νi = nic/2Li, ni being the mode number of the light mode oscillating in the resonator. Thus, if one ensures the length of the resonator to be stable, one can derive limits on a possible anisotropy of the speed of light c = c(θ) by measuring the resonance frequencies of the resonators as a function of the orientation of the resonators. In our setup we measure the difference frequency (ν1 - ν2 ) between the two resonators by exciting their modes by laser waves. Compared to an arrangement with a single cavity and comparison with, say, an atomic reference, the hypothetical signal due to an anisotropy is doubled in size.

Fig 2: The ULE-block containing the resonators (left). For the measurement the block is actively rotated and the frequency difference ν1 – ν2 is measured as a function of the orientation (right).

A cross section of the complete experimental apparatus is shown in figure 3. Many different systems have been implemented to suppress systematic effects. For example, the ULE-block containing the resonators is fixed in a temperature stabilized vacuum chamber to ensure a high length stability. This is placed on breadboard I and is surrounded by a foam-padded wooden box for acoustical and thermal isolation. Furthermore, the laser and all the optics needed for the interrogation of the resonators, also on breadboard I, are shielded by another foam-padded box. To isolate the optical setup from vibrations, breadboard I is placed on two active vibration isolation supports (AVI), that suppress mechanical noise coming from the rotation table and the floor.

Fig.3 Cross section of the experimental setup. A: air bearings, B: piezo motors, C: voice coil actuators, D: tilt sensors, E: air springs system, AVI's: active vibration isolation supports

In addition, the AVI allows as well for active control of the tilt of breadboard I by means of voice coil actuators (C). This is necessary since a varying tilt leads to varying elastic deformations of the resonator block, and thus systematically shifts the resonance frequencies. The tilt is measured using electronic bubble-level sensors (D) with a resolution of 1 µrad. The described system and a rack carrying all the electronics used for the frequency stabilisation and other servo systems are standing on breadboard II, which is fixed to the rotor of a high precision air bearing (A) rotation table. This is used to actively change the orientation of the resonators with a rotation rate of ωrot = 2π / 90s. To minimize systematic effects due to tilt modulations, the rotation axis can be aligned in the direction of local gravitation using an air spring system (E). A tower around the complete setup with multilayered elements on the sides and ceiling plates containing thermo-electric coolers allows for thermal and acoustical isolation from the surrounding.

With this apparatus we have performed measurements over a period of more than 1 year, taken in 46 datasets longer than 1 day. From these datasets we have used 135 000 single rotations to extract upper limits for the parameters of the RMS and SME describing a potential anisotropy of the speed of light.

The mentioned models, the RMS and the SME, predict variations of the frequency difference Δν = ν12 on several timescales. Due to the symmetry of the resonator system and the active rotation θ(t) = ωrot·t, the models predict a variation of Δν with a frequency 2 ωrot , Δν(θ) = 2Bν0 sin2θ(t) + 2Cν0 cos2θ(t), where ν0 is the mean optical frequency (281 THz). Thus, for every single rotation we determine the modulation amplitudes 2Bν0 and 2Cν0 at this frequency (see fig.4). Due to the rotation and the revolution of the earth around the sun, these amplitudes will show, if Lorentz Invariance is violated, variations on the timescale of half a sidereal day, a sidereal day, and on an annual scale. The size of these modulations is directly connected to the parameters of the two test theories, which can be derived from the modulation amplitudes via fits.

Fig.4 Histograms of the determined modulation amplitudes due to active rotation. The mean values are (10 ± 1) mHz for 2Bν0 and (1± 1) mHz for 2Cν0, corresponding to (3.5 ± 0.4)·10-17 for 2B and (0.4 ± 0.4)·10-17 for 2C.

Within the RMS theory a single parameter combinaton (δ-β-1/2) describes the anisotropy. From our measurements we can deduce a value of (-1.6 ± 6.1)•10-12, thus yielding an upper 1σ limit of 7.7•10-12 . This means, that the anisotropy of the speed of light, defined as (1/2)•Δc(π/2)/c, is probably less than 6•10-18 in relative terms.

Within the SME test theory 8 different parameters can be derived using our measurements. For most of these coefficients we can place upper bounds on a level of few parts in 1017, except for one coefficient, κe-ZZ, which is determined from the mean values of the modulation amplitudes 2B and 2C, and is most seriously affected by systematic effects. For this coefficient we can only limit the value to below 1.3•10-16 (1σ). For all the coefficients our experiment improved the upper limits for a possible Lorentz Invariance violation by more than one order of magnitude compared to previous work [17-22] and no significant signature of an anisotropy of the speed of light is seen at the current sensitivity of the apparatus.

Currently, the institute is working on an improved version of the apparatus. The goal is a further significant improvement of the upper limits within the next years.

[1] "Zur elektrodynamik bewegter körper", A. Einstein, Annalen der Physik, 17:891 (1905). Article.
[2] "Postulate versus Observation in the Special Theory of Relativity",

H.P. Robertson, Rev. Mod. Phys., 21(3):378–382, (Jul 1949). Article.
[3] "A test theory of special relativity: I. Simultaneity and clock synchronization",

Reza Mansouri and Roman U. Sexl, Gen. Rel. Grav., 8(7):497 (1977). Abstract.
[4] "A test theory of special relativity: II. First order tests",

Reza Mansouri and Roman U. Sexl, Gen. Rel. Grav., 8(7):515, 1977. Abstract.
[5] "A test theory of special relativity: III. Second-order tests",

Reza Mansouri and Roman U. Sexl, Gen. Rel. Grav., 8(10):809 (1977). Abstract.
[6] "Lorentz-violating extension of the standard model",

D. Colladay and V. Alan Kostelecký, Phys. Rev D, 58(11):116002 (1998). Abstract.
[7] "An Experimental Study of the Rate of a Moving Atomic Clock. II",

Herbert E. Ives and G.R. Stilwell, J. Opt. Soc. Am., 31:369 (1941). Abstract.
[8] "Experimental Establishment of the Relativity of Time",

Roy J. Kennedy and Edward M. Thorndike, Phys. Rev., 42:400 (1932). Abstract.
[9] A.A. Michelson and E.W. Morley, American Journal of Science, III-34(203), (1887)
[10] "Limits on the Chirality of Interstellar and Intergalactic Space",

M. Goldhaber and V. Trimble, J. Astrophy. Astr., 17:17 (1996). Article.
[11] "Is There Evidence for Cosmic Anisotropy in the Polarization of Distant Radio Sources?",
S.M. Carroll and G.B. Field, Phys. Rev. Lett., 79(13):2394-2397 (1997). Abstract.
[12] "A limit on the variation of the speed of light arising from quantum gravity effects",

J. Granot , S. Guiriec, M. Ohno, V. Pelassa et al., Nature, 08574, doi:10.1038 (2009). Abstract.
[13] "A dynamical test of special relativity using the anomalous electron g-factor",

M. Kohandel, R. Golestanian, M. R. H. Khajehpour, Physics Letters A, 231, 5-6 (1997). Abstract.
[14] "Test of relativistic time dilation with fast optical atomic clocks at different velocities",

Sascha Reinhardt, Guido Saathoff, Henrik Buhr, Lars A. Carlson, Andreas Wolf, Dirk Schwalm, Sergei Karpuk, Christian Novotny, Gerhard Huber, Marcus Zimmermann, Ronald Holzwarth, Thomas Udem, Theodor W. Hänsch, Gerald Gwinner, Nature Physics, 3, 861-864 (2007). Abstract.
[15] "Satellite test of special relativity using the global positioning system",

P. Wolf and G. Petit, Phys. Rev. A, 56(6):4405-4409 (1997). Abstract.
[16] "Laboratory Test of the Isotropy of Light Propagation at the 10-17 Level",

Ch. Eisele, A. Yu. Nevsky and S. Schiller, Phys. Rev. Lett. 103, 090401 (2009). Abstract.
[17] "Tests of Relativity Using a Cryogenic Optical Resonator", C. Braxmaier, H. Müller, O. Pradl, J. Mlynek, A. Peters, S. Schiller,
Phys. Rev. Lett., 88(1):010401 (2001). Abstract.
[18] "Modern Michelson-Morley Experiment using Cryogenic Optical Resonators",

Holger Müller, Sven Herrmann, Claus Braxmaier, Stephan Schiller and Achim Peters, Phys. Rev. Lett., 91:020401 (2003). Abstract.
[19] "Test of the Isotropy of the Speed of Light Using a Continuously Rotating Optical Resonator",

Sven Herrmann, Alexander Senger, Evgeny Kovalchuk, Holger Müller and Achim Peters, Phys. Rev. Lett., 95(15):150401 (2005). Abstract.
[20] "Test of constancy of speed of light with rotating cryogenic optical resonators",

P. Antonini, M. Okhapkin, E. Göklü and S. Schiller, Phys. Rev. A, 71:050101 (2005). Abstract.
[21] "Improved test of Lorentz invariance in electrodynamics using rotating cryogenic sapphire oscillators", Paul L. Stanwix, Michael E. Tobar, Peter Wolf, Clayton R. Locke, and Eugene N. Ivanov
, Phys. Rev. D, 74(8):081101 (2006). Abstract.
[22] "Tests of Relativity by Complementary Rotating Michelson-Morley Experiments",
Holger Müller, Paul Louis Stanwix, Michael Edmund Tobar, Eugene Ivanov, Peter Wolf, Sven Herrmann, Alexander Senger, Evgeny Kovalchuk, and Achim Peters, Phys. Rev. Lett., 99(5):050401 (2007). Abstract.

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Sunday, November 15, 2009

Science Begins At The World's Most Powerful X-Ray Laser

Joachim Stöhr, Director of LCLS (Photo courtesy: Stanford Linear Accelerator Laboratory)

The first experiments are now underway using the world's most powerful X-ray laser, the Linac Coherent Light Source (LCLS), located at the Department of Energy's SLAC National Accelerator Laboratory. LCLS produces ultrafast pulses of X-rays millions of times brighter than even the most powerful synchrotron sources — pulses powerful enough to make images of single molecules. Illuminating objects and processes at unprecedented speed and scale, the LCLS has embarked on groundbreaking research in physics, structural biology, energy science, chemistry and a host of other fields.

In early October, researchers from around the globe began traveling to SLAC to get an initial glimpse into how the X-ray laser interacts with atoms and molecules. The LCLS is unique, shining light that can resolve detail the size of atoms at ten billion times the brightness of any other manmade X-ray source.

"No one has ever had access to this kind of light before," said LCLS Director Jo Stöhr. "The realization of the LCLS isn't only a huge achievement for SLAC, but an achievement for the global science community. It will allow us to study the atomic world in ways never before possible."

The LCLS is a testament to SLAC's continued leadership in accelerator technology. Over 40 years ago, the laboratory's two-mile linear accelerator, or linac, was built to study the fundamental building blocks of the universe. Now, decades later, this same machine has been revitalized for frontier research underway at the LCLS.

After SLAC's linac accelerates very short pulses of electrons to 99.9999999 percent the speed of light, the LCLS takes them through a 100-meter stretch of alternating magnets that force the electrons to slalom back and forth. This motion causes the electrons to emit X-rays, which become synchronized as they interact with the electron pulses over this long slalom course, thereby creating the world's brightest X-ray laser pulse. Each of these laser pulses packs as many as 10 trillion X-ray photons into a bunch that's a mere 100 femtoseconds long—the time it takes light to travel the width of a human hair.

Atomic, Molecular and Optical Science (AMO) Instrument (Image courtesy: Stanford Linear Accelerator Laboratory)

Currently, user-assisted commissioning is underway, with researchers conducting experiments using the Atomic, Molecular and Optical science instrument, the first of six planned instruments for the LCLS. In these first AMO experiments, researchers are using X-rays from the LCLS to gain an in-depth understanding of how the ultra-bright beam interacts with matter.

Early experiments are already revealing new insights into the fundamentals of atomic physics and have successfully proven the machine's unique capabilities to control and manipulate the underlying properties of atoms and molecules. Earlier this month, researchers used the LCLS's strobe-like pulses to completely strip neon atoms of all their electrons. Researchers also watched for two-photon ionization—an event where two photons pool their energy to eject a single electron from an atom. Normally difficult to observe at X-ray facilities, researchers at the LCLS were able to study these events using the extreme brightness of the laser beam.

Future AMO experiments will create stop-action movies of molecules in motion. The LCLS's quick, short, repetitive X-ray bursts enable researchers to take individual photos as molecules move and interact. By stringing together many such images to make a movie, researchers will for the first time have the ability to watch the molecules of life in action, view chemical bonds forming and breaking in real time, and see how materials work on the quantum level.

By 2013, all six LCLS scientific instruments will be online and operational, giving researchers unprecedented tools for a broad range of research in material science, medicine, chemistry, energy science, physics, biology and environmental science.

"It's hard to overstate how successful these first experiments have been," said AMO Instrument Scientist John Bozek. "We look forward to even better things to come."

[We thank Stanford Linear Accelerator Laboratory for materials used in this posting.]

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Sunday, November 08, 2009

Upcoming Physics Conferences

[To add an upcoming physics conference to this list, please send an email to 2Physics@gmail.com ]

Nov 29-Dec 04: Intl Conference on Hadron Spectroscopy (Tallahassee, FL, USA)
Nov 30-Dec 03: International Symposium on Computational Mechanics (Hong Kong and Macau, China)
Dec 01-04: Workshop on Quantum Chaos: Theory and Applications. Dedicated to the 65th birthday of Marcos Saraceno (Buenos Aires, Argentina)
Dec 15-20: Topical Conference on Elementary Particles, Astrophysics, and Cosmology (Fort Lauderdale, Florida, USA)
Jan 04-08: 3rd Intl Workshop On High Energy Physics in the LHC Era (Valparaiso, Chile)
Jan 05-08: 39th Winter Meeting on Statistical Physics (Taxco, Guerrero, Mexico)
Jan 11-15: Essential Cosmology for the Next Generation (Playa del Carmen, Mexico)
Jan 15-17: Axions 2010 (Gainesville, Florida)
Jan 26-29: GWDAW14: Gravitational Wave Data Analysis Workshop (Rome, Italy)
Feb 08-13: 46th Winter School of Theoretical Physics: Quantum Dynamics and Information: Theory and Experiment (Ladek Zdroj, Poland)
Feb 23-26: Gravitational Wave Symposium (John Hopkins U., USA)
Apr 05-09: PDEs, Relativity and Nonlinear Waves (Granada, Spain)
May 23-29: Workshop on Advances in Foundations of Quantum Mechanics and Quantum Information with atoms and photons ad memoriam of Carlo Novero (Turin, Italy)
Jun 14-17: Advances in Quantum Theory (Vaxjo, Sweden)
Jun 20-26: Theory Meets Data Analysis at Comparable and Extreme Mass Ratios (Waterloo, ON, Canada)
Jun 23-Jul 03: Quantum Gravity summer school (Morelia, Mexico)
Jun 28-Jul 02: LISA 8 (Stanford University, USA)
Jul 05-09: GR19 (Mexico City, Mexico)


Sunday, November 01, 2009

Observation of Magnetic Monopoles in Spin Ice

Hiroaki Kadowaki, Yuji Aoki and Naohiro Doi of Tokyo Metropolitan University

[This is an invited article based on recently published work of the authors -- 2Physics.com]

Authors: H. Kadowaki1, Y. Aoki1, T. J. Sato2, J. W. Lynn3

Affiliations: 1
Department of Physics, Tokyo Metropolitan University, Tokyo, Japan,
NSL, Institute for Solid State Physics, University of Tokyo, Tokai, Japan,
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA

From the symmetry of Maxwell's equations of electromagnetism, magnetic charges or monopoles would be expected to exist in parallel with electric charges. About 80 years ago, a quantum mechanical hypothesis of the existence of magnetic monopoles was proposed by Dirac [1]. Since then, many experimental searches have been performed, ranging from a monopole search in rocks of the moon to experiments using high energy accelerators [2]. But none of them was successful, and the monopole is an open question in experimental physics. Theoretically, monopoles are predicted in grand unified theories as topological defects in the energy range of the order 1016 GeV [2]. However these enormous energies preclude all hope of creating them in laboratory experiments.

Taku J. Sato of University of Tokyo

Alternatively, recent theories predict that tractable analogs of the magnetic monopole might be found in condensed matter systems [3,4,5]. One prediction [4] is for an emergent elementary excitation in the spin ice compound Dy2Ti2O7 [6], where the strongly competing magnetic interactions exhibit the same type of frustration as water ice [7]. In addition to macroscopically degenerate ground states [6], the excitations from these states are topological in nature and mathematically equivalent to the Dirac monopoles [1,4]. We have successfully observed [8] the signature of magnetic monopoles in the spin ice Dy2Ti2O7 using neutron scattering, and find that they interact via the magnetic inverse-square Coulomb force. In addition, specific heat measurements show that the density of monopoles can be controlled by temperature and magnetic field, with the density following the expected Arrhenius law.

Jeffrey W. Lynn of NIST, USA

In Fig. 1 we illustrate creation of a magnetic monopole and antimonopole pair in spin ice under applied magnetic field along a [111] direction. This excitation is generated by flipping a spin, which results in ice-rule-breaking "3-in, 1-out" and "1-in, 3-out" tetrahedral neighbors, simulating magnetic monopoles, with net positive and negative charges sitting on the centers of tetrahedra. The monopoles can move and separate by consecutively flipping spins in the kagome lattice.

Fig. 1. Spins of Dy2Ti2O7 occupy a cubic pyrochlore lattice, which is a corner -sharing network of tetrahedra, and consists of a stacking of triangular and kagome lattices. The competing magnetic interaction brings about a geometrical constraint where the lowest energy spin configurations on each tetrahedron follow the ice rule, in which two spins point inward and two point outward on each tetrahedron. (A) By applying a small magnetic field along a [111] direction, the spins on the triangular lattices are parallel to the field, while those on the kagome lattices retain disorder under the same ice rules. This is referred to as the kagome ice state [9]. (B) Creation of a magnetic monopole (blue sphere) and antimonopole (red sphere) pair in the kagome ice state.

A straightforward signature of monopole-pair creation is an Arrhenius law in the temperature (T) dependence of the specific heat (C). This Arrhenius law of C(T) is clearly seen in Fig. 2 at low temperatures, indicating that monopole-antimonopole pairs are thermally activated from the ground state, and that the number of monopoles can be tuned by changing temperature and magnetic field.

Fig. 2. Specific heat of Dy2Ti2O7 under [111] magnetic fields is plotted as a function of 1/T. In intermediate temperature ranges these data are well represented by the Arrhenius law denoted by solid lines.

A microscopic experimental method of observing monopoles is to perform magnetic neutron scattering using the neutron's dipole moment as the probe. One challenge to the experiments is to distinguish the relatively weak scattering from the monopoles from the very strong magnetic scattering of the ground state. By choosing appropriate field-temperature values, we have successfully observed scattering by magnetic monopoles, diffuse scattering close to the (2,-2,0) reflections, and that by the ground state (Fig. 3) [8].

Fig. 3. Intensity maps of neutron scattering at T = Tc + 0.05 K in the scattering plane perpendicular to the [111] field are shown for H = 0.5 T and H = Hc. The kagome ice state at H = 0.5 T (A) compared with the MC simulation (C). The weakened kagome-ice state scattering plus the diffuse monopole scattering (B) at H = Hc agree with the MC simulation (D).

Typical elementary excitations in condensed matter, such as acoustic phonons and (gapless) magnons, are Nambu-Goldstone modes where a continuous symmetry is spontaneously broken when the ordered state is formed. This contrasts with the monopoles in spin ice, which are point defects that can be fractionalized in the frustrated ground states. Such excitations are unprecedented in condensed matter, and now enable conceptually new emergent phenomena to be explored experimentally [10].

[1] "Quantised singularities in the electromagnetic field",
P. A. M. Dirac, Proc. R. Soc. A 133, 60 (1931). Article.
[2] "Theoretical and experimental status of magnetic monopoles",
K. A. Milton, Rep. Prog. Phys. 69, 1637 (2006).
[3] "The anomalous Hall effect and magnetic monopoles in momentum space", Zhong Fang, Naoto Nagaosa, Kei S. Takahashi, Atsushi Asamitsu, Roland Mathieu, Takeshi Ogasawara, Hiroyuki Yamada, Masashi Kawasaki, Yoshinori Tokura, Kiyoyuki Terakura, Science 302, 92 (2003).
[4] "Magnetic monopoles in spin ice"
C. Castelnovo, R. Moessner, S. L. Sondhi, Nature 451, 42 (2008).
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X-L. Qi, R. Li, J. Zang, S-C. Zhang, Science 323, 1184 (2009).
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S. T. Bramwell, M. J. P. Gingras, Science 294, 1495 (2001).
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[8] "Observation of Magnetic Monopoles in Spin Ice", H. Kadowaki, N. Doi, Y. Aoki, Y. Tabata, T. J. Sato, J. W. Lynn, K. Matsuhira, Z. Hiroi, J. Phys. Soc. Jpn. 78, 103706 (2009).
[9] "A new macroscopically degenerate ground state in the spin ice compound Dy2Ti2O7 under a magnetic field" K. Matsuhira, Z. Hiroi, T. Tayama, S. Takagi and T. Sakakibara, J. Phys. Condens. Matter 14, L559 (2002).
Article; "Kagome ice State in the dipolar spin ice Dy2Ti2O7" Y. Tabata, H. Kadowaki, K. Matsuhira, Z. Hiroi, N. Aso, E. Ressouche, and B. Fåk, Phys. Rev. Lett. 97, 257205 (2006). Abstract.
[10] In Oct. 2009, in addition to [8], three experimental papers on the magnetic monopoles in spin ice have been published: "Measurement of the charge and current of magnetic monopoles in spin ice" S. T. Bramwell, S. R. Giblin, S. Calder, R. Aldus, D. Prabhakaran & T. Fennell
, Nature 461, 956 (2009), Abstract; "Dirac Strings and Magnetic Monopoles in the Spin Ice Dy2Ti2O7" D. J. P. Morris, D. A. Tennant, S. A. Grigera, B. Klemke, C. Castelnovo, R. Moessner, C. Czternasty, M. Meissner, K. C. Rule, J.-U. Hoffmann, K. Kiefer, S. Gerischer, D. Slobinsky, R. S. Perry, Science 326, 411 (2009) Abstract; "Magnetic Coulomb Phase in the Spin Ice Ho2Ti2O7" T. Fennell, P. P. Deen, A. R. Wildes, K. Schmalzl, D. Prabhakaran, A. T. Boothroyd, R. J. Aldus, D. F. McMorrow, S. T. Bramwell, Science 326, 415 (2009). Abstract.

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