In chronological order during the year:

1. Slowed Light Handed Off

Several years ago, physicists gained the ability to slow a beam of light in a gas of atoms; by manipulating the atoms' + char(39)+ N' + char(39)+ N' + char(39)+ N' spins the energy of and information contained in the light could be transferred to the atoms in a coherent way (see PNU 521). By turning on additional laser beams, the original light signal, which we can think of as having been idling or temporarily stored in the atom cloud, could be reconstituted and sent on its way.

Now, one of the first researchers to slow light, Lene Hau of Harvard, has added an extra layer to this story. She and her colleagues, halting and storing a light signal in a gas of cold atoms-in this case a Bose-Einstein condensate (BEC) of sodium atoms-then transfer the signal, now in the form of a coherent pulse of atom waves rather than light waves, into a second BEC of sodium atoms some 160 microns away, from which, finally, the signal is revived as a conventional light pulse.

This feat, the sharing around of quantum information in light-form and in not just one but two atom-forms, offers great encouragement to those who hope to develop quantum computers.

Ginsberg et al., Nature, 8 February 2007

2. Electron Tunneling in Atoms Has Now Been Observed in Real Time

Electron tunneling in atoms has now been observed in real time by a German-Austrian-Dutch team (Ferenc Krausz, Max Planck Institute of Quantum Optics and Ludwig Maximilians University Munich, ferenc.krausz@mpq.mpg.de) using light pulses lasting only several hundred attoseconds (billionths of a billionth of a second), providing new glimpses into an important ultrafast process in nature.

An electron bound to an atom is at the bottom of a sort of energy hill. Escaping the atom usually requires the electron to get enough energy to roll over this hill. So for example, hitting an atom with a light pulse delivering photons of sufficient energy can allow the electron to escape.

However, if an atom is bathed in a shower of lower-energy photons, there is the chance that an electron, if located at the periphery of the atom, can escape even though it doesn' + char(39)+ N' + char(39)+ N' + char(39)+ N't have quite enough energy. This is through the phenomenon of quantum tunneling, in which there is a small chance that the electron can in effect burrow through the energy hill.

The tunneling process is responsible for the operation of certain electronic components, such as scanning tunneling microscopes, Esaki (tunneling) diodes, and quantum-cascade lasers. And in nuclear fission, alpha particles (two protons plus two neutrons) are believed to escape the fracturing nucleus through tunneling. Yet the tunneling process occurs so quickly, on the scale of attoseconds, that it has not been possible to observe directly. With the recent ability to create attosecond-scale light pulses--pioneered by Krausz and others--this is now possible.

In the new experiment, a gas of neon atoms is exposed to two light pulses. One is an intense pulse containing low-energy red photons. The second pulse is an attosecond-length pulse of ultraviolet light. This ultraviolet attosecond pulse delivers photons so energetic that they can rip off an electron and promote a second one to the periphery of the atom, into an excited quantum state.

Then, the intense red pulse, consisting of just a few wave cycles (peaks and valleys), has a chance to liberate the outlying electron via light-field-induced tunneling. Indeed, the researchers saw this phenomenon, predicted theoretically forty years ago but only verified now for the first time experimentally in a direct time-resolved study. As each wave crest in the few-cycle red pulse coursed through the atoms, the electrons each time upped their probability of escaping through tunneling until it reached about 100%.

The data indicate that, in this particular system, the electrons escape via tunneling in three discrete steps, synchronized with the three most intense wave crests at the center of the few-cycle laser wave. Each step lasts less than 400 attoseconds. (Uiberacker et al, Nature, 5 April 2007; also see press release with figures and more information at www.mpq.mpg.de)

3. Laser Cooling of Coin-sized Objects

Laser cooling of coin-sized objects down to one-kelvin temperatures is now possible. In a set of experiments performed last year, a variation on the laser-cooling technique used in chilling vapors of gases down to sub-kelvin temperatures had been used in macroscopic (but still tiny) samples in the nano- and micro-gram range.

Now, a collaboration of scientists from the LIGO Laboratory at MIT and Caltech and from the Max Planck Institutes in Potsdam and Hannover has used laser beams to cool a coin-sized mirror with a mass of 1 gram down to a temperature of 0.8 K. The goal of chilling such a comparatively large object (with more than 10^20 atoms) is to investigate the quantum properties of large ensembles of matter.

An important caveat here is the fact that in all these experiments the "cooling" takes place in one dimension only. A temperature of 1 K applies to the motion of atoms along the direction of the laser beams, while the mirror is free to move (although not much) in other directions. Consequently, if you touched the sample it would not feel cryogenically cold. Beyond the record low temperature achieved for an object as large as 1 gram, another interesting feature of the experiment pertains to the strength of the force exerted by the laser beams.

In the chosen dimension, the beams fix the mirror so steadfastly that it' + char(39)+ N' + char(39)+ N' + char(39)+ N's as if it were being held in place by a spring that' + char(39)+ N' + char(39)+ N' + char(39)+ N's stiffer than a diamond with the same dimensions as the laser beam (long and thin). According to MIT researcher Nergis Mavalvala (nergis@ligo.mit.edu) the sample is held by a rigidity (if the laser beam were solid) characterized by a Young' + char(39)+ N' + char(39)+ N' + char(39)+ N's modulus (the parameter specifying stiffness) of 1.2 tera-pascals, some 20% stiffer than diamond. (Corbitt et al., Physical Review Letters, upcoming article; lab wiki at http://baikal.mit.edu/sqwiki/moin.cgi/Pictures

4. Newton' + char(39)+ N' + char(39)+ N' + char(39)+ N's Second Law of Motion

Newton' + char(39)+ N' + char(39)+ N' + char(39)+ N's second law of motion, that pillar of classical physics, the formula that says the force on an object is proportional to acceleration, has now been tested, and found to be valid, at the level of 5 x 10^-14 m/s^2. This is a thousandfold improvement in precision over the best previous test, one carried out 21 years ago (Physical Review D, vol 34, p 3240, 1986). The new test was performed by physicists at the University of Washington using a swiveling torsion pendulum, a special kind of pendulum in which the restoring force is not gravity (as you would have in a hanging pendulum) but is provided by a very thin torsion fiber.

One implication of Newton’s law is that the pendulum’s frequency (its tick-tock rate) should be independent of the amplitude of its swiveling (as long as the oscillation is small). Looking for a slight departure from this expected independence the Washington researchers watched the pendulum at very small amplitudes; in fact the observed swivel was kept so small that the Brownian excitation of the pendulum was a considerable factor in interpreting the results.

Newton’s second law is expected to break down for subatomic size scales, where quantum uncertainty frustrates any precise definition of velocity. But for this experiment, where the pendulum has a mass of 70 g and consists of 10^24 atoms, quantum considerations were not important. According to one of the scientists involved, Jens Gundlach (206-616-3012, jens@phys.washington.edu), this new affirmation that force is proportional to acceleration (at least for non-relativistic speeds), might influence further discussion of two anomalies:

(1) oddities in the rotation curves for galaxies---characterizing the velocity of stars as a function of their radii from the galactic center---suggest either that extra gravitational pull in the form of the presence of as-yet-undetected dark matter is at work or that some new form of Newton’s Second Law could be operating (referred to as Modified Newtonian Dynamics, or MOND); and

(2) the ongoing mystery surrounding the unaccounted-for accelerations apparently characterizing the trajectory of the Pioneer spacecraft (see http://www.aip.org/pnu/1998/split/pnu391-1.htm). (Gundlach et al., Physical Review Letters, upcoming article

5. Gravity Probe B

Gravity Probe B the orbiting observatory devoted to testing the general theory of relativity, has measured the geodetic effect-the warping of spacetime in the vicinity of and caused by Earth-with a precision of 1%. The basic approach to studying this subtle effect is to monitor the precession of gyroscopes onboard the craft in a polar orbit around the Earth. The observed precession rate, 6.6 arc-seconds per year, is close to that predicted by general relativity.

The geodetic effect can be measured in several ways, including the use of clocks, the deflection of light, and the perturbative influence of massive bodies on nearby gyroscopes. GP-B is of the latter type, and its current precision is as good as or better than previous measurements. And once certain unanticipated torques on the gyroscopes are better understood, GP-B scientists expect the precision of their geodetic measurement to improve to a level of 0.01%. These first GP-B results were reported at the APS meeting by Francis Everitt (Stanford).

The idea for using gyroscopes to observe the warping of spacetime was proposed almost 50 years ago, and Everitt has been an active proponent and then scientific overseer of the project for much of that subsequent time. A second major goal of GP-B is to measure frame dragging, a phenomenon which arises from the fact that space is, in the context of general relativity, a viscous fluid rather than the rigid scaffolding Isaac Newton took it to be. When the Earth rotates it partly takes spacetime around with it, and this imposes an additional torque on the gyroscopes.

Thus an extra precession, perpendicular to and 170 times weaker than for the geodetic effect, should be observed. Everitt said that GP-B saw “glimpses” of frame dragging in this early analysis of the data and expects to report an actual detection with a precision at the 1% level by the time of the final presentation of the data, now scheduled for December 2007. (An indirect measurement of frame dragging at the 10-15% uncertainty level was made earlier by the LAGEOS satellite.) Some of the GP-B equipment is unprecedented.

The onboard telescope used to orient the gyroscopes (by sighting toward a specific star) provided a star-tracking ability better by a factor of 1000 than previous telescopes. The gyroscopes themselves-four of them for redundancy-are the most nearly spherical things ever made: the ping-pong-ball-sized objects are out of round by no more than 10 nm. They are electrostatically held in a small case, spun up to speeds of 4000 rpm by puffs of gas. The gas is then removed, creating a vacuum of 10^-12 torr. Covered with niobium and reposing at a temperature of a few kelvin, the balls are rotating superconductors, and as such they develop a tiny magnetic signature which can be read out to fix the sphere’s instantaneous orientation. (For more information see einstein.stanford.edu)

6. One Neutrino Anomaly Has Been Resolved

One neutrino anomaly has been resolved while another has sprung up. A Fermilab experiment called MiniBooNE provides staunch new evidence for the idea that only three low-mass neutrino species exist. These results, reported over the past week at a Fermilab lecture and at the American Physical Society (APS) meeting in Jacksonville, Florida, seem to rule out two-way neutrino oscillations involving a hypothetical fourth type of low-mass neutrino.

Several experiments have previously shown that neutrinos, very light or even massless particles that only interact via gravity and the weak nuclear force, lead a schizoid life, regularly transforming from one species into another. These neutrino oscillations were presumably taking place among the three known types recognized by the standard model of particle physics: electron neutrinos, muon neutrinos, and tau neutrinos.

However, one experiment, the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos, provided a level of oscillation that implied the existence of a fourth neutrino species, a “sterile neutrino,” so-called because it would interact only through gravity, the weakest of physical forces. (For background see Physics Today, August 1995 and http://www.aip.org/pnu/1995/split/pnu239-1.htm and http://www.aip.org/pnu/1996/split/pnu269-1.htm)

From the start, this result stood apart from other investigations, especially since it suggested possible neutrino masses very different from those inferred from the study of solar or atomospheric neutrinos or from other accelerator-based neutrino experiments. MiniBooNE (whose name is short for Booster Neutrino Experiment; the “mini” refers to the fact that they use one detector rather than the originally proposed two) set out to resolve the mystery.

The experiment proceeds as follows: protons from Fermilab’s booster accelerator are smashed into a fixed target, creating a swarm of mesons which very quickly decay into secondary particles, among them a lot of muon neutrinos. Five hundred meters away is the MiniBooNE detector.Although muon neutrinos might well oscillate into electron neutrinos, over the short run from the fixed target to the detector one would expect very few oscillations to have occurred.

The Fermilab detector, and the LSND detector before it, looked for electron neutrinos. Seeking to address directly the LSND oscillation effect, Fermilab tried to approximate the same ratio of source-detector distance to neutrino energy. This ratio sets the amount of likely oscillation. The Los Alamos experiment used 30 MeV neutrinos observed after a 30 m distance; the Fermilab experiment used 500 MeV neutrinos detected after a distance of 500 m.

The trick of doing this kind of experiment is to discriminate between the few rare events in which an electron neutrino strikes a neutron in a huge bath of mineral oil, thereby creating a characteristic electron plus a slow moving proton, and the much more common event in which a muon neutrino strikes a proton to make a muon and proton. LSND saw a small (but, they argued, statistically significant) number of electron neutrino events. MiniBooNE, after taking into account expected background events, sees none. Thus they see no oscillation and therefore no evidence for a fourth neutrino. Actually it’s not exactly true that they see no electron neutrinos.

At low neutrino energy they do see events, and this tiny subset of the data remains a mystery, to be explored in further data taking now underway using a beam of anti-neutrinos. At the APS meeting, MiniBooNE co-spokesperson Janet Conrad (Columbia Univ) said that the low-energy data are robust (meaning that a shortage of statistical evidence or systematic problems with the apparatus should not be major factors) and that some new physical effect cannot be ruled out.

At the very least, the low-energy data do not undo the new assertion that the earlier LSND results cannot be explained by the existence of a fourth neutrino type. (Fermilab press release and figures, http://www.fnal.gov/pub/presspass/images/BooNE-images.html)

7. Tevatron' + char(39)+ N' + char(39)+ N' + char(39)+ N's Higgs Quest Quickens

Physicists from Fermilab’s Tevatron collider have just reported their most comprehensive summary yet of physics at the highest laboratory energies. At last week’s American Physical Society (APS) meeting in Jacksonville, Florida they delivered dozens of papers on a spectrum of topics, many of which are related in some way to the Higgs boson.

The Higgs is the cornerstone ingredient in the standard model of high energy physics. It is the particle manifestation of the curious mechanism that kicked in at an early moment in the life of the universe: the W and Z bosons (the carriers of the weak force) became endowed with mass while the photon (the carrier of the electromagnetic force) did not. This asymmetry makes the two forces very different in the way they operate in the universe.

Validating this grand hypothesis by actually making Higgs particles in the lab has always been a supreme reason for banging protons and antiprotons together with a combined energy of 2 TeV. Nature is prodigal in its creativity, however, and the search for Higgs is expected to be shadowed by the production of other rare scattering scenarios, some of them nearly as interesting as the Higgs itself.

The Tevatron labors can be compared to work at the Burgess Shale, the fossilbed in the Canadian Rockies where archeologists uncovered impressions of organisms that hadn’t been seen in 600 million years, including some new phyla. No new phyla (no new particles) were reported at the Florida meeting, but much preparatory work-the necessary chipping away of outer layers at the physicists’ equivalent of a high-energy “rockface”-was accomplished. According to Jacobo Konigsberg (Univ Florida), co-spokesperson for the CDF collaboration (one of the two big detector groups operating at the Tevatron, the search for the Higgs is speeding up owing to a number of factors, including the achievement of more intense beams and increasingly sophisticated algorithms for discriminating between meaningful and mundane events, a bread-and-butter issue when sifting through billions of events.

Here is a catalog of some of the freshest results from the Tevatron. Kevin Lannon (Ohio State) reported a new best figure (170.9 GeV, with at uncertainty of 1%) for the mass of the top quark. Lannon also described the class of event in which a proton-antiproton smashup resulted in the production of a single top quark via a weak-force interaction, a much rarer event topology than the one in which a top-antitop pair is made via the strong force.

Moreover, observing these single-top events allows a first rudimentary measurement of Vtb, a parameter (one in a spreadsheet of numbers, called the CKM matrix, that characterize the weak force) proportional to the likelihood of a top quark decaying into a bottom quark. Gerald Blazey (Northern Illinois Univ), former co-spokesperson of the D0 collaboration, reported on the first observations of equally exotic collision scenarios, those that feature the simultaneous production of an observed W and Z boson, and those in which two Z bosons are observed.

Furthermore, he said that when the new top mass is combined with the new mass for the W boson, 80.4 GeV, one calculates a new likely upper limit on the mass of the Higgs. This value, 144 GeV, is a bit lower than before, making it just that much easier to create energetically. Ulrich Heintz (Boston Univ) reported on the search for exotic particles not prescribed by the standard model.

Again, no major new particles were found, but further experience in handling myriad background phenomena will help prepare the way for what Tevatron scientists hope will be their main accomplishment: digging evidence for the Higgs out from a rich seam of other particles. To start with, Heintz broached but then dismissed rumors of pseudo-Higgs “bumps” in the data. The artefacts in question-the presumed exotic particle decaying into a pair of tau leptons-were of too low a statistical stature to take seriously, he said, at least for now.

Other exotic particles not found, but for which there are now new lower mass limits, include such things as excited (extra heavy) electrons or Z and W bosons, extra dimensions, so-called leptoquarks (which turn bosons into leptons and vice-versa), and supersymmetric particles, a whole hypothetical family of particles for which all known bosons would have fermion counterparts and vice versa. Besides the consideration of having enough energy in the collision to create the Higgs and other interesting particles, a vital requirement in producing rare eventualities is possessing a large statistical sample.

All the results above are based on a data-recording sample of one inverse-femtobarn (fb^-1), a unit denoting the integrated amount of scattering events up till now. By the end of the summer, the amount of data analyzed will be two fb^-1. By the end of 2007, the amount will have doubled again, and by 2009 doubled once again (8 fb^-1). For finding the Higgs, energy and statistics will tell.

8. The Shortest Light Pulse Ever

Researchers in Italy have created the shortest light pulse yet-a single isolated burst of extreme-ultraviolet light that lasts for only 130 attoseconds (billionths of a billionth of a second). Shining this ultrashort light pulse on atoms and molecules can reveal new details of their inner workings--providing benefits to fundamental science as well as potential industrial applications such as better controlling chemical reactions.

Working at Italy' + char(39)+ N' + char(39)+ N' + char(39)+ N's National Laboratory for Ultrafast and Ultraintense Optical Science in Milan (as well as laboratories in Padua and Naples), the researchers believe that their current technique will allow them to create even shorter pulses well below 100 attoseconds. In previous experiments, longer pulses, in the higher hundreds of attoseconds, have been created.

The general process for this experiment is the same. An intense infrared laser strikes a jet of gas (usually argon or neon). The laser' + char(39)+ N' + char(39)+ N' + char(39)+ N's powerful electric fields rock the electrons back and forth, causing them to release a train of attosecond pulses consisting of high-energy photons (extreme ultraviolet in this experiment).

Creating a single isolated attosecond pulse, rather than a train of them, is more complex. To do this, the researchers employ their previously developed technique for delivering intense short (5 femtosecond) laser pulses to an argon gas target. They use additional optical techniques (including the frequency comb that was a subject of the 2005 Nobel Prize in Physics) for creating and shaping a single attosecond pulse.

The results are being presented this week (paper JThA5) at the Conference on Lasers and Electro-Optics and the Quantum Electronics and Laser Science Conference (CLEO/QELS). Expanded story at: http://www.cleoconference.org/media_center/lightpulse.aspx ; also see Sansone et al., Science, 20 October 2006.

9. The Highest- Energy Cosmic Rays.

Probably come from the cores of active galactic nuclei (AGN), where supermassive black holes are thought to supply vast energy for flinging the rays across the cosmos. This is the conclusion reached by scientists who operate the Pierre Auger Observatory in Argentina. This gigantic array of detectors spread across 3000 sq. km of terrain, looks for one thing: cosmic ray showers.

These arise when extremely energetic particles strike our atmosphere, spawning a gush of secondary particles. Many of the rays come from inside our own Milky Way, especially from our sun, but many others come from far away. Of most interest are the highest-energy showers, with energies above 10^19 electron volts, far higher than any particle energy that can be produced in terrestrial accelerators. The origin of such potent physical artifacts offers physicists a tool for studying the most violent events in the universe.

To arrive at Earth most cosmic rays will have crossed a great deal of intergalactic space, where magnetic fields can deflect them from their starting trajectories. But for the highest-energy rays, the magnetic fields can’t exert as much influence, and consequently the starting point for the cosmic rays can be traced with some confidence.

This allowed the Auger scientists to assert that the premier cosmic rays were not coming uniformly from all directions but rather preferentially from galaxies with active cores, where the engine for particle acceleration was probably black holes of enormous size. The very largest of cosmic ray showers, those with an energy higher than 57 EeV (1EeV equals 10^18 eV), correlated pretty well with known AGN’s. (Science, 9 November 2007)

10. Cooper Pairs in Insulators

Cooper pairs are the extraordinary link-up of like-charged electrons through the subtle flexings of a crystal. They act as the backbone of the superconducting phenomenon, but have also now been observed in a material that is not only non-superconducting but actually an insulator. An experiment at Brown University measures electrical resistance in a Swiss-cheese-like plank of bismuth atoms made by spritzing a cloud of atoms onto a substrate with 27-nm-wide holes spaced 100 nm apart. Bismuth films made this way are superconducting if the sample is many atom-layers thick but is insulating if the film is only a few atoms thick, owing to subtle effects which arise from the restrictive geometry.

The superconducting and insulating states are easily distinguished; as the temperature is lowered below the transition temperature (2 K) the resistance goes to zero for bismuth-as-superconductor, whereas for the insulating bismuth the resistance becomes extremely high. Cooper pairs are certainly present in the superconducting sample; they team up to form a non-resistive supercurrent. But how do the researchers know that pairs are present in the insulator too? Because of an additional test. By seeing what happens to resistance as an external magnetic field is increased.

The resistance should vary periodically, with a period proportional to the charge of the electrical objects in question. From the periodicity, proportional in this case to two times the charge of the electron, the Brown physicists could deduce that they were seeing doubly-charged objects moving through the sample. In other words, Cooper pairs are present in the insulator. This is true only at the lowest temperatures. One of the researchers, James Valles (james_valles_jr@brown.edu), says that there have been previous hints of Cooper pairs in some films related to superconductors, but that in those cases the evidence for pairs in the insulating state was ambiguous, and not as direct as the observation recorded in the Brown lab. He asserts that the realization of a boson insulator (in which the charge carriers are electron pairs) will help to further explore the odd kinship between insulators and superconductors. (Stewart et al., Science 23 November 2007)