Zapping zircons

Posted on January 17, 2013 by Charles Day

Fans of Physics Today's Facebook page occasionally send me messages, most of which are requests for more information about something to do with physics. The one I received on Monday was no exception. A fan from Jordan wanted to know about research in “gemstone treatment.”

Not knowing what he meant, I Googled the phrase, which led me to a website touting the value of untreated gemstones. Some gemstones, I found out, are routinely subjected to heat, chemicals, and even ionizing radiation to change or improve their appearance.

To achieve its so-called super blue color, this topaz has been bombarded with high-energy electrons from a linear accelerator.

Not having heard about the irradiation of gemstones, I investigated further. One of the first documents I came across, thanks to Wikipedia, was Charles Ashbaugh’s “Gemstone irradiation and radioactivity,” which appeared in the winter 1988 issue of Gems & Gemology.

When he wrote the article, Ashbaugh was an engineer at UCLA’s nuclear energy laboratory. His article is worth reading—not only for its review of how both natural and artificial radiation sources alter the optical properties of gemstone minerals, but also for its tutorial on radiation (the sidebar on the various radiation units, with its analogy to sun bathing, is exemplary!).

If you’re like me, you probably knew that amethysts, emeralds, and other gemstones owe their colors to the dilute presence of impurities. Ruby, for example, consists of an aluminum oxide (Al₂O₃ crystal) doped with chromium atoms. From Ashbaugh I learned that irradiating a gemstone with gamma rays, high-energy electrons, or neutrons transmutes the impurities, thereby changing the wavelengths absorbed by the crystal. Naturally pale blue topaz can be turned a deep “super blue.” Colorless zircon can be turned pink.

As you might expect, irradiation could make a gemstone radioactive. In 1988, when Ashbaugh wrote his article, the regulatory status of irradiated gemstones in the US was confusing, inconsistent, and subject to state and federal jurisdiction. It was easier for a US jeweler to legally obtain irradiated gemstones from abroad than from the US. The regulations are clearer now. In fact, now that there are more irradiated gemstones on the market, the Nuclear Regulatory Commission felt the need last year to issue a fact sheet, whose summary succinctly states (in bold font):

The NRC believes irradiated gemstones currently on the market are safe.
The NRC has not requested that jewelers take these stones off the market.

Does irradiation diminish the allure or value of gemstones? Not for me. For one thing, a perfect diamond crystal consists of identically arranged carbon atoms. If you could make one in the lab, it would be identical and indistinguishable from a perfect natural crystal. Structural perfection, not naturalness of origin, is a crystal’s paramount property.

What’s more, it doesn’t matter to me whether a tourmaline acquired its color through millions of years’ exposure to natural radiation emanating from the surrounding rock or through a few hours’ exposure to 1.17- and 1.33-MeV gamma rays from a cobalt-60 source.

Ashbaugh’s article is illustrated with several photographs of beautiful, gleaming gemstones in a variety of colors—which prompts another question: If you can make, say, a deep red gemstone by irradiating any one of several naturally transparent, colorless crystals, does it matter which crystal you start with?

The answer could be yes—if you care about how much a stone sparkles. Whereas a natural emerald’s refractive index is 1.6, an irradiated green diamond’s is 2.4. Until a crystal’s refractive index can be engineered, I suspect diamonds will remain the most prized gemstones.

As for the Jordanian Facebook fan who wanted to learn about gemstones, it turned out he was really interested in crystal healing. I couldn’t help him.

My Nobel wish list

Posted on October 2, 2012 by Charles Day

My record for predicting the winners of Nobel prizes is mixed. The last time I made a public prediction was two years ago. I correctly picked Konstantin Novoselov and Andre Geim as winners, but I thought their work on graphene—by analogy with the work of Robert Curl, Harold Kroto, and Richard Smalley on buckyballs—would earn the pair the chemistry prize. The only prize I got completely correct was Mario Vargas Llosa’s for literature.

This year, rather than make predictions, I’ve decided to identify who I hope will win the prizes I care about the most: physics, chemistry, physiology or medicine, and literature.

Physics

One of the topics of enduring interest to physicists is the boundary between the realms of quantum and classical behavior. In 2004 I wrote a news story about an ingenious experiment that explored that boundary. Markus Arndt, Anton Zeilinger, and their colleagues at the University of Vienna sent buckyballs through a pair of closely spaced slits.

When the molecules were cold, they behaved like quantum objects and formed interference fringes after passing through the slits. But when the molecules were hot, the coherent fringes disappeared. Evidently, the molecules’ temperature and emission of thermal photons—not their size or mass—demarked the quantum–classical boundary.

That story was my first direct encounter with research on how the environment influences quantum behavior. The second came in 2009 when I wrote about a calculation that resolved a 82-year-old quantum paradox: Why is a chiral molecule found in either its left-handed or right-handed isomeric forms and not in a superposition of the two?

To reach their answer, Klaus Hornberger and Johannes Trost of Ludwig-Maximilians University calculated the most probable states of a deuterated dihydrogen dilsulfide molecule in the presence of helium atoms. At room temperature, once the pressure exerted by the He atoms exceeded 1.6 × 10⁻⁵ mbar, the He atoms would kick the D₂S₂ molecule out of a mix of superpositions and into either its left-handed or right-handed form.

As I noted in my story, that a calculation could precisely locate a quantum–classical boundary is both mundane and profound—mundane, because the calculation made use of standard, unadulterated quantum mechanics; profound, because it demystified the quantum–classical boundary.

The physicist who has done the most to advance the notion that the environment, when fully and properly accounted for, drives the quantum–classical boundary is Los Alamos National Laboratory’s Wojciech Zurek. I hope he’s awarded the physics prize.

Chemistry

The discovery, published in February 2008, of superconductivity in a compound that contains iron and arsenic touched off an explosion of research that continues to this day. Several branches of the family of iron-based superconductors have since been discovered.

Although no family member’s critical temperature can yet match the highest of the cuprates, the iron-based superconductors are significant because their superconductivty, like that of the cuprates, is mediated by electron–electron interactions. Evidence is building that the pairing symmetry is not d-wave, as in the case of the cuprates, but is a form of s-wave.

The iron-based superconductors, therefore, demonstrate that high-temperature superconductivity is not limited either to the cuprates or to the precise form it takes in the cuprates. Other chemical families, as yet undiscovered, could have still higher critical temperatures.

Hideo Hosono of the Tokyo Institute of Technology made the discovery. I hope he is awarded the chemistry prize.

Physiology or medicine

The last time pharmacology was honored with a Nobel Prize was in 1988, when James Black, Gertrude Elion, and George Hitchings shared the award “for their discoveries of important principles for drug treatment.” This year, I hope that Ravinder Maini and Marc Feldmann of Imperial College London are rewarded for identifying tumor necrosis factor as a potential (and now effective) drug target for treating inflammatory diseases, such as rheumatoid arthritis.

Literature

The Wikipedia entry on William Trevor, whose photo appears above, begins like this:

William Trevor, KBE (born 24 May 1928) is an Irish author and playwright. One of the elder statesmen of the Irish literary world, he is widely regarded as one of the greatest contemporary writers of short stories in the English language.

If the Swedish Academy can suspend its habitual political posturing and instead reward sensitivity, sympathy, and skill, then it might just bestow the literature prize on Trevor. Doing so would honor not just him, but two great writers whose work inspired him and who weren’t awarded Nobel prizes: Anton Chekhov and James Joyce.

Not enough peasants, not enough economics

Posted on July 24, 2012 by Charles Day

Many years ago I read an essay in which the author—I think it might have been Frederik Pohl—complained about historical inaccuracies in sword-and-sorcery novels. Of course, it’s hardly “wrong” in such works to have fire-breathing dragons, power-bestowing rings, or shape-shifting ravens. Rather, what so irked Pohl were errors in depicting a plausible world based on medieval technology.

In particular, he lambasted authors for getting the economics wrong. The lack of labor-saving devices in the Middle Ages meant that growing and raising food—the main economic activity—occupied most of the people most of the time. A tale of warring princes may have talking bears and magic cloaks but it must have plenty of peasants!

Pohl’s polemic came back to mind when I encountered an essay in the latest issue of Isis, one of the journals carried by the Niels Bohr Library and Archives. Under the title “Time, money, and history,” David Edgerton of Imperial College London urges his fellow historians of science to include economic factors more fully in how they regard and study science.

Edgerton presents several pieces of historical evidence to make his case. For example, in his view the R&D component of the Manhattan project is routinely overestimated and overemphasized by historians (or “oversized,” to use Edgerton’s word). Of the project’s $2 billion budget, only $70 million—3.5%—was spent on R&D. The lion’s share went to DuPont and other large corporations for building two nuclear factories at Oak Ridge in Tennessee and Hanford in Washington State (shown here under construction).

Addressing his fellow historians, Edgerton writes:

We need to follow all the money, not just that going to the university. Rough estimates of the comparative scale of industrial, government, and academic research through the century show that the usual implicit maps of the historians systematically oversize academic research by comparison with government and industrial research. Industry and the military (largely in industry) have been—nearly everywhere and nearly always—the main funders of research and development. Not only research within the academy but, indeed, those aspects of academic research least connected to industry are oversized—physics, particularly particle physics, and biology, particularly molecular biology—while chemistry, mathematics, and engineering are undersized.

Edgerton’s essay triggered another literary recollection—this time, of a paper I’d read in the March issue of the British Journal for the History of Science. In “The limits to ‘spin-off’: UK defence R&D and the development of gallium arsenide technology,” Graham Spinardi of Edinburgh University tells a fascinating story of government-sponsored R&D. And in doing so, he implicitly supports Edgerton’s case.

Gallium arsenide is a semiconductor whose properties make it better than silicon for certain applications. Thanks to its excellent electron mobility, GaAs can operate at the high frequencies used for mobile telephony. And thanks to its direct, as opposed to indirect, bandgap, GaAs beats Si as a material for making lasers, LEDs, and other optoelectronic devices.

But Si has offsetting advantages that continue to give it an edge over GaAs and other semiconductors in computational applications. Si is cheap, stable, and readily doped. It has an insulating phase, thanks to its native oxide. And its hole mobility, while an order of magnitude lower than the electron mobility of GaAs, is still high enough to support gigahertz clock rates.

By the mid 1950s, Si’s predominance in commercial electronics was clear. It was also clear that semiconductors would have military applications. Recognizing that market forces alone would likely propel Si-based technologies, the British defense establishment decided at that time to fund research into GaAs, which was less developed. The goal, to quote one of Spinardi’s sources, was “to leapfrog silicon technology.”

Fateful repercussions

That decision had fateful repercussions for Britain’s electronics industry. In a sense, the British government’s investment in GaAs paid off. By the 1990s, when the commercial applications of GaAs in LEDs and microwave telecommunications were taking off, British labs had already developed devices and manufacturing techniques that could support a consumer-focused GaAs industry.

But, as Spinardi explains, those companies, which included Plessey and Marconi, opted instead to continue developing devices for their original military sponsors. What’s more, their decades-long focus on GaAs had left them little room to develop Si-based technologies. Britain’s electronics industry missed out on booms in Si and GaAs.

To discover why British companies failed to achieve commercial success in GaAs in proportion to their expertise, Spinardi interviewed researchers and managers and studied documents from labs and government departments. Within the economic and industrial conditions of the time, those companies did not act foolishly. In fact, they developed successful products in three areas: defense, equipment for processing GaAs, and radar.

Those areas have one thing in common, notes Spinardi. They’re inhabited by a few big, commercial or government customers, rather than thousands or millions of individual customers. To quote Spinardi: “Unlike consumer products, where buyers are sought after production, made-to-order goods are only produced once a buyer has agreed terms. Investment is therefore far less risky because it can be based on, and costed into, procurement contracts.”

That aversion to risk partly reflected British corporate governance. The big British electronics companies were public and had to answer to their shareholders. In the short term, bidding on large contracts made commercial sense.

It might also have made sense in the long term. As British investment in GaAs was beginning to bear fruit in the 1980s, Sony, Matsushita and other deep-pocketed Japanese companies were moving into the consumer electronics industry. The US had begun investing in military applications of semiconductors. Competing against either the Japanese in the commercial sector or the Americans in their own military sector might have been a futile and costly mistake.

Whether Britain’s investment in GaAs was a success or a failure is a matter of perspective. On the one hand, the expected military applications were realized, to the gain of both Britain’s armed forces and their British suppliers. On the other hand, having made that investment, the final step to mass-market success was surely small enough that at least one company could have, and maybe should have, taken the financial risk and jumped. Spinardi concludes his paper with this observation:

The dominance of defense in the post-war UK innovation system helped provide a technology base with much spin-off potential, but ironically it also engendered industrial conditions that may have limited the capacity of UK industry to make the most of this.

From basic to applied in 83 years

Posted on May 12, 2012 by Charles Day

In June 1929 a paper by the 23-year-old Nevill Mott appeared in the Proceedings of the Royal Society of London. As Mott noted in his introduction, theoretical arguments and empirical evidence supported the notion that electrons have an intrinsic magnetic moment, or spin. “The question arises,” he wrote, “has the free electron ‘really’ got a magnetic moment, a magnetic moment that we can by any conceivable experiment observe?”

Mott’s question is subtler than it might first appear. If you turn to the paper’s appendix, you’ll find what Niels Bohr told Mott: The uncertainty principle forestalls any attempt to distinguish an electron’s intrinsic magnetic moment from the magnetic field that arises from its motion. But, as Mott (shown here) demonstrates in his paper, it is possible to quantify the intrinsic magnetic moment because it turns out the probability that an electron scatters off an atomic nucleus in a given direction depends on the orientation of the electron’s spin.

Forty-two years after Mott’s paper was published, Mikhail Dyakonov and Vladimir Perel of the Ioffe Institute in Leningrad found a similar effect in semiconductors. According to their theoretical analysis, an electric field applied along a strip of semiconductor drives electrons to scatter off impurities in a spin-dependent way: Those with up spins veer to one side, while those with down spins veer to the other side.

Dyakonov and Perel’s paper did not attract much attention. Indeed, in 1999 Jorge Hirsch reproduced the analysis without either himself or—one assumes—his paper’s reviewers being aware of its Russian antecedent. He called the phenomenon the spin Hall effect.

The effect that Dyakonov, Perel, and Hirsch predicted depends on the presence of extrinsic impurities. But in 2003 two groups of theorists—Shuichi Murakami, Naoto Nagaosa, and Shou-Cheng Zhang; Jairo Sinova, Allan MacDonald, and their collaborators—independently proposed that a spin Hall effect could arise intrinsically when spin–orbit coupling of electrons to the lattice atoms acts with the applied electric field to change the semiconductor’s band structure.

Controlling electron spins through electric fields is technologically enticing. Murakami, Nagaosa, and Zhang wrote in the 2003 Science paper:

Principles found here could enable quantum spintronic devices with integrated information processing and storage units, operating with low power consumption and performing reversible quantum computation.

What’s more, because electric fields ultimately arise from static charges, they constitute a finer, faster, and more convenient means to control spins than do magnetic fields, which ultimately arise from moving charges.

Weak then strong

I did not become acquainted with the spin Hall effect’s history through deep, broad study of 20th-century physics. Rather, in 2005 I wrote a news story about the effect’s experimental verification. David Awschalom, his graduate students Yuichiro Kato and Roberto Myers, and Art Gossard detected the effect through a spin-dependence in the polarization of reflected light. Independently, Jörg Wunderlich, Bernd Kästner, Sinova, and Tomas Jungwirth looked instead for the circularly polarized light emitted by an LED when spin-polarized electrons and holes recombine.

Both experiments were tours de force of ingenuity and execution. They needed to be. The spin Hall effect in gallium arsenide, the material the two groups used, is weak—too weak, perhaps, form the basis of a industrially significant technology. Nevertheless, the spin Hall effect has been one of the past decade’s most fruitful areas of research. There’s now a quantum spin Hall effect, a spin Hall effect for light, and an inverse spin Hall effect.

Now comes a paper that reports a spin Hall effect of technologically interesting magnitude. In the 4 May issue of Science, Cornell University’s Robert Buhrman and his collaborators report the discovery of what they call a massive spin Hall effect in the brittle, semimetal β phase of tantalum. The effect, which works at room temperature, is strong enough to flip the spins in an adjacent ferromagnet.

That spin-flipping ability could form the basis of nonvolatile computer memory—that is, memory that isn’t wiped clean whenever you turn off the power. In fact, to demonstrate β-tantalum’s promise, Buhrman’s team built nanodevices whose active components consisted of few-nanometer-thick layers of β-tantalum, cobalt iron boron (a ferromagnet), and magnesium dioxide (to improve the ferromagnet’s performance).

The prototype device did indeed work, at room temperature, as a magnetization switch. The switching current I_c is higher than that of magnetic tunnel junctions (MTJs), which use spin-polarized currents to perform the switching. However, Buhrman anticipates that with routine optimization I_c could be reduced to the point that spin Hall devices would compete with MTJs.

Mott’s 1929 paper, his first, is characteristic of his early interest in atomic and nuclear physics. In 1933 he took up a professorship at the University of Bristol, and from then on he devoted himself to the field in which he received a Nobel prize: condensed matter physics. Mott died in 1996 at the age of 90—too soon, unfortunately, to learn what his theoretical paper in atomic physics had begotten 83 years later.

The hunt for supersolidity

Posted on May 17, 2011 by Charles Day

Superconductivity was discovered in 1911 when Heike Kamerlingh Onnes chilled a piece of solid mercury to 4.2 K and witnessed the resistivity of his sample vanish. Superfluidity was discovered in 1937 when Peter Kapitza and, independently, John Allen and Don Misener chilled helium-4 to 2.17 K and witnessed the viscosity of their respective samples vanish.

Both superconductivity and superfluidity owe their wondrous properties to the abrupt onset of a single collective ground state, a Bose–Einstein condensate (BEC). Bosonic helium-4 atoms form a BEC directly. Electrons, being fermions, must first pair up.

Although Satyendra Bose’s and Albert Einstein’s BEC papers appeared 12 years before superfluidity was discovered, the state wasn’t anticipated by theorists—at least as far as I can tell. That’s not the case for the third and still-elusive superstate, supersolidity.

In 1970 Geoffrey Chester speculated that a quantum crystal could harbor a mobile Bose–Einstein condensate. Chester’s quantum crystal was a generic lattice of particles, which, thanks to the uncertainty principle, never stop moving even at absolute zero. His BEC consisted of empty lattice sites, vacancies. Below a critical temperature Tc, the condensed vacancies would move without resistance, like paired electrons in a superconductor or superfluid helium in a capillary.

Because of their low mass and feeble interatomic interactions, helium atoms are so frisky at 0 K that low temperature alone is not enough to induce them to crystallize. Pressure must be applied, too. Even under pressure, helium-4 atoms might fail to line up to form a perfect solid. If that’s the case, Chester reasoned, the residual vacancies could form a BEC.

Four months after Chester’s paper appeared, Tony Leggett published a paper entitled “Can a Solid Be ‘Superfluid’?” Leggett proposed a direct test to find the BEC: Fill a bundt pan-like container with helium-4, apply the requisite temperature and pressure to solidify it, then rotate the pan about its axis. If, by lowering the temperature further, the experiment reaches Tc, the onset of a BEC sloshing through the otherwise rigid crystal would change the pan’s rotational inertia to a nonclassical value.

Leggett’s analysis also identified a critical angular velocity ωc above which the BEC, though still mobile, would start to dissipate energy.

In 1981 David Bishop, Mikko Paalanen, and John Reppy implemented Leggett’s test. Their vessel showed no signs of nonclassical rotational inertia (NCRI) from 2 K down to 25 mK. Either ωc or the fraction of atoms that formed the BEC was undetectably small.

The first hints of supersolidity

The hunt for what has become known as supersolidity got a boost in 2004 when Moses Chan and Eun-Seong Kim performed a version of Leggett’s test. Like Bishop, Paalanen, and Reppy, Chan and Kim suspended their vessel by a torsion bar and looked for changes in the resonant oscillation frequency. Their experiment was different in that the helium-4 occupied the interconnected pores of a foamy glass called Vycor.

Why Vycor? Chan and Kim thought that the pores’ extra surface area would promote the formation of defects and therefore increase the fraction of helium-4 atoms in the mobile BEC phase.

Chan and Kim detected a seemingly abrupt change in their vessel’s oscillating frequency at a temperature of 175 mK. The experiment did not, however, constitute the discovery of supersolidity. A change in frequency could indeed arise because of the appearance of a superfluid, although not necessarily of the kind envisioned by Chester. It could also arise because the crystal gains access to structural excitations that don’t move in lockstep with the bulk.

Those structural excitations would leave a telltale fingerprint—ωτ = 1 behavior—in the torsion oscillator data. The behavior is hardly super. Even Jello exhibits ωτ = 1 behavior. If the gelatin–water mixture is hot, a spoon passes through it easily, whereas if it’s cold, the spoon sticks to the surrounding Jello. Neither case presents much friction. But at some intermediate temperature, the mobile proteins, which embody Jello’s structural excitations, have a natural reaction time τ that perfectly balances the stirring frequency ω. The resulting resonance briefly makes stirring the Jello much harder.

Chan and Kim repeated their experiment on bulk helium-4 without the Vycor and found more or less the same result. Other groups confirmed that some sort of structural loosening takes place in solid helium-4, but the evidence did not converge on a single explanation, let alone on a BEC supersolid.

Indeed, helium-4′s apparent and partial loss of solidity at low temperature was looking less like the other BEC superstates, superconductivity and superfluidity, and more like a phenomenon, such as conductivity, which is caused by contaminants, dislocations, grain boundaries, phonons, and other features of crystalline materials.

In 2006 John Beamish and James Day performed an experiment that, so far, most closely resembles Kapitza’s and Allen and Misener’s discovery of superfluidity. Beamish and Day filled capillaries with helium-4, chilled and pressurized the material to solidify it, then lowered the temperature while applying additional pressure at one end of the capillaries. They didn’t find evidence of viscosity-free flow.

A SQUID and a magnet

The latest development in the hunt for supersolidity came last week in the form of a paper in Science by Séamus Davis and his collaborators. Like Chan and Kim, they filled a torsional oscillator with solid helium-4. Their goal was not to find or rule out a BEC supsersolid, but to measure the oscillator’s response to changes in temperature and rim speed in an unbiased way.

They also hoped to find a single consistent explanation for Chan and Kim’s rotational anomaly, Beamish and Day’s shear modulus anomaly, and ωτ = 1 behavior.

Mapping the oscillator’s resonant frequency with enough accuracy to characterize the onset of NCRI required boosting experimental sensitivity. Chan and Kim had measured their oscillator’s change in resonant frequency Δf using a capacitive method. Each twist of their oscillator brought one plate of a capacitor close to the other plate, changing its capacitance as a function of time.

Davis and his team used a magnetic method. Each twist of their oscillator brought a samarium–cobalt magnet close to a SQUID magnetometer, which measured the resulting change in magnetic field. SQUIDs are exquisitely sensitive. Davis and his team could determine the oscillator’s rotational characteristics with four orders of magnitude more precision (per unit time) than Chan and Kim could.

These figures taken from Davis’s Science paper show Δf and the dissipation factor D as functions of the speed of the cylindrical vessel and its temperature. Each figure is made up of 98 smoothly interpolated curves derived at different values of temperature. Two things stand out.

Δf and D together bear the fingerprints of ωτ = 1 behavior, which varies smoothly. The increased resolution of the magnetic detection method appears to rule out an abrupt phase change of the sort that would characterize the formation of a BEC.
In both plots, the variation along the temperature axis looks just like the variation along the rim speed axis. The similarity would arise if lowering either the temperature or the rim speed extended the lifetime of structural excitations that mimic a fluid component. A BEC component would not give rise to similar behavior with respect to temperature and rim speed.

The findings of Davis and his team rule out a BEC as the source of NCRI. Instead, they demonstrate that crystal defects that display ωτ = 1 can provide a single explanation for rotational and shear anomalies. But, as Davis is careful to point out, the findings don’t exclude the possibility that a BEC supersolid could form. Moreover, although the findings constrain theories for what causes NCRI, they don’t identify its microscopic cause.

Superconductivity and superfluidity are dramatic effects that were discovered by people who weren’t looking for them. Having calculated that the fraction of superfluid BEC in a supersolid is small, Leggett concluded his 1970 paper by observing

it seems highly unlikely that these effects would have been discovered by accident even if “superfluid solids” do exist at attained temperatures.

After three decades, supersolidity remains undiscovered, by accident or otherwise.

Charles Day

Yes, cold-atom condensates are interesting and useful

Posted on September 9, 2010 by Charles Day

When Eric Cornell and Carl Wieman made the first Bose–Einstein condensate from a gas of cold rubidium-87 atoms in 1995, I was excited and impressed. Conceptually, BECs are simple enough to have been included in my undergraduate quantum mechanics course. But making one—what a feat!

Two years later, I joined the editorial staff at Physics Today. It became my job to follow developments in all of physics, not just in my former field of high-energy astrophysics. As more groups began working on BECs and publishing results, my initial enthusiasm for condensates waned. My (no doubt mistaken) impression was that people made BECs to study them and studied them to make them.

My interest perked up when physicists began recreating in BECs phenomena that had already been observed in condensed-matter systems. Two results stand out for me. In 2002, Immanuel Bloch and his collaborators filled the egg-carton-like pockets of an optical lattice with the atoms of a BEC. By making the pockets deeper, they could observe a transition in the way each atom interacted with its nearest neighbors—just like the metal–insulator transition predicted by Nevill Mott in 1949.

The other personal standout was the observation by Zoran Hadzibabic and his collaborators of another transition: a Berezinskii-Kosterlitz-Thouless crossover in BECs that had been squashed to near two-dimensionality.

Both Bloch’s and Hadzibabic’s papers were experimental tours de force, as is the more recent work shown in the figure. Last week in Science, David Hall and his collaborators published a new method to observe quantized vortices in a BEC. The figure shows a sequence of direct, real-time images of a vortex pair in rubidium-87 condensate.

Mott transitions, Berezinskii-Kosterlitz-Thouless crossovers, vortex pairs, and some other phenomena predicted and seen in BECs are rediscoveries. But BECs have the potential for testing theories that are beyond the reach of other systems. If experimenters can cool condensates to the point that particleparticle interactions extend robustly beyond nearest neighbors, then several important models in condensed-matter physics can be vindicated or refuted. The models, like John Hubbard’s two-term Hamiltonian for electrons in solids, are outwardly simple, yet remain too difficult either to isolate in doped crystals or to fully realize in a computer simulation.

Proving that high-temperature superconductivity emerges from the Hubbard model would be a major coup. But the feat would be unlikely to excite the congressional paymasters who fund condensate research in the US. Fortunately—and surprisingly—applications for BECs are already appearing. On that topic, it’s perhaps fitting to end this entry with the words of one of the BEC discoverers, Eric Cornell. When he appeared in 2006 before the House of Representatives’ subcommittee on environment, technology, and standards, Cornell testified:

Where has Bose-Einstein condensation led us, in the ten years since we first created it? What, in particular has it been good for? BEC has found several direct applications, and in particular we and other research groups around the country are trying to develop precision accelerometers, gravitometers, and gyroscopes, to be used for remote sensing and navigation by dead reckoning. In the long run, BEC is likely to be still more important because of its role as a scientific building block, a tool to help us understand and tame quantum mechanics, and to put quantum mechanics to use on problems with relevance to our economy, our health, and our national security.