If you chill fermions enough, they can pair up to form bosons and settle into a single collective ground state, a Bose–Einstein condensate. In the case of helium-3 atoms, the resulting BEC is a superfluid that flows without dissipation—provided the flow is not so energetic that it breaks the pairs apart or destroys the ground state's coherence. Until now, theorists could characterize placid flows in fermionic superfluids, but not the vigorous turbulence that results from shaking or stirring. Aurel Bulgac of the University of Washington in Seattle and his colleagues have adapted density functional theory—a computational approach originally devised to calculate molecular energy levels—and applied its time-dependent extension to model turbulent fermionic superfluids. Although the underlying quantum mechanical equations are straightforward, solving them required the use of one of the world's most powerful supercomputers, Jaguar at Oak Ridge National Laboratory in Tennessee. In their simulations, Bulgac and his colleagues agitated a fermionic superfluid by shooting spherical projectiles through it or by stirring it with a laser beam. Turbulent superfluids are known to harbor tubes of quantized vorticity. As the figure below shows, the simulation could track how two vortex tubes (marked a and b) joined to form a ring, which then opens in a manner reminiscent of the unzipping of a DNA molecule during transcription. Bulgac's model could help astronomers understand another agitated superfluid: the interior of a rapidly spinning neutron star. (A. Bulgac et al., Science 332, 1288, 2011.)—Charles Day
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Albert Einstein’s description of Bose–Einstein condensation is based on a statistical argument. In a gas of identical bosons, the statistical weighting of each state is such that the total occupation of the excited states is capped at an upper bound Nc. If the number of identical particles exceeds Nc, all additional particles must occupy the ground state. That textbook picture doesn’t include interparticle interactions, but it does assume thermal equilibrium, which cannot exist without some form of interaction, so it’s no surprise that the picture doesn’t describe real systems exactly. Now, Zoran Hadzibabic (Cambridge University, UK) and colleagues have taken a closer look at how the role of interactions in a Bose–Einstein condensate relates to the textbook picture. The researchers found, as shown in the figure, that for a gas of potassium-39 atoms in an optical trap, the thermal component (excited-state atoms) consistently exceeded the textbook upper bound. Qualitatively, they attribute the difference to repulsion between the thermal and condensed components, which turns the harmonic trap into a Mexican hat potential and thereby increases the number of thermally accessible excited states. Quantitatively, they found that when they tuned the interparticle interaction strength, by applying a magnetic field near a Feshbach resonance, an extrapolation to the zero-interaction limit recovered the textbook picture. (N. Tammuz et al., Phys. Rev. Lett., in press.)—Johanna Miller
To study the transport of spin in an ultracold gas, Martin Zwierlein and colleagues at MIT separated a cloud of spin-up atoms from a cloud of spin-down atoms and sent them traveling toward one another. Surprisingly, the two puffs, each a million times thinner than air, ricocheted off one another like billiard balls. The figure shows the spin-up (red) and spin-down (blue) clouds at 1-ms intervals during the first bounce. Such extreme behavior occurs when the atoms have the strongest possible interactions. The researchers cooled a trapped gas of fermionic atoms to nanokelvin temperatures, where the gas was degenerate, with atoms filling most of the lowest energy levels. Then they tuned an applied magnetic field to the so-called Feshbach resonance, where the atoms have their strongest possible interactions. The strong scattering between atoms with opposite spins, combined with the absence of interactions between those of the same spin, explains how the repulsion can be large enough for the clouds to rebound. After about six increasingly damped bounces, the clouds penetrated one another and the spins became homogeneously mixed. It took nearly a second for the spins to equalize. The diffusion time determines the spin diffusivity, which reached a minimum quantum value of ℏ /m, where m is the atomic mass. Zwierlein and his colleagues hope experiments like this can lend insight into the hydrodynamic properties of other strongly interacting systems, such as quark–gluon plasmas or neutron stars. (A. Sommer et al., Nature 472, 201, 2011.)—Barbara Goss Levi
The Large Hadron Collider at CERN has reached a milestone in its quest to find—or lay to rest—the Higgs boson predicted by particle theory’s standard model. The LHC produces 7-TeV head-on collisions between protons in countercirculating beams stored in its 27-km-circumference ring, part of which is shown here. On 22 April, the colliding beams achieved a record luminosity of 4.7 × 1032/(cm2 s), surpassing for the first time the previous record held by the 2-TeV Tevatron collider at Fermilab. The new record luminosity (defined as the collision rate per unit scattering cross section) translates into 50 million proton–proton collisions per second at each of the LHC detectors. If the standard-model Higgs exists, it will be produced in only a tiny fraction of those collisions. But given the new record luminosity, the LHC detectors should accumulate enough data by the end of 2012 to provide a statistically robust sighting of the Higgs somewhere in the mass range (115–155 GeV) from which it has not already been excluded—or to demonstrate that Nature here parts company with the standard model. To allow that decisive accumulation of data, the start of the 18-month shutdown required to bring the LHC up to its full 14-TeV design energy has now been postponed from the end of this year to the end of 2012. (W. J. Murray et al., in Proceedings of the Chamonix 2011 Workshop on LHC Performance, https://espace.cern.ch/acc-tec-sector/Chamonix/Chamx2011/papers/BM_4_01.pdf.)—Bertram Schwarzschild
The double slit experiment, in which a coherent beam of particles diffracts through closely spaced slits to produce interference fringes, is perhaps the simplest and most famous demonstration of wave–particle duality. It has been shown to work with photons, electrons, and even the comparatively massive buckminsterfullerenes. According to a theory outlined by Howard Cohen and Ugo Fano nearly 50 years ago, interference fringes should also result from the photoionization of a diatomic molecule, provided electrons on each atom are ejected simultaneously and bestowed with enough kinetic energy that their wavelengths are similar to the interatomic distance, as illustrated here. Unfortunately, the direct approach—fixing molecules’ orientation in space, photoionizing them, and detecting fluctuations in the angular distribution of electron density—is, for the most part, impractical. Instead, an international team of scientists led by Fernando Martín (Autonomous University of Madrid) used an elaborate first-principles theory to predict the effect that interference should have on the relationship between a molecule’s photoionization cross section and its vibrational energy. Working at Lawrence Berkeley National Laboratory’s Advanced Light Source, they then used extreme UV light pulses to ionize hydrogen, nitrogen, and carbon monoxide gas. For H2, the simplest molecule of the bunch, the theory almost perfectly described the vibrationally resolved ionization spectra. For N2 and CO, the theory was less accurate, but still in qualitative agreement with the data. The results could help to resolve ambiguities surrounding previous attempts to demonstrate the phenomenon. (S. E. Canton et al., Proc. Natl. Acad. Sci. USA, in press.)—Ashley G. Smart
Quantum entanglement, by itself, cannot be used to communicate, but it can enhance the capacity, accuracy, or security of communication. In one example, researchers led by William Matthews (University of Waterloo, Canada) and Andreas Winter (University of Bristol, UK) showed last year that when two communicating parties—Alice and Bob—share pairs of entangled qubits, they can improve their chances of successfully transmitting a message over a complex noisy classical communication channel. Now, Robert Prevedel, Kevin Resch, Matthews, and other Waterloo colleagues have developed a similar protocol for a simpler channel, and they've implemented that protocol experimentally. The researchers used an electronic circuit to create a noisy classical channel that takes a pair of bits from Alice and tells Bob either the first bit, the second bit, or the parity of their sum—each with equal probability—and which of the three he received. Without the benefit of shared entanglement, Alice’s best strategy for transmitting a single bit to Bob succeeds 5/6, or 0.833, of the time. But if Alice and Bob each possess one of a pair of entangled qubits—polarization-entangled photons, in the experiment—then they can make better use of the channel and achieve a theoretical success rate of (2 + 2−1/2)/3, or 0.902. The experimental success rate was 0.891 ± 0.002, much higher than the classical limit and not much lower than the entanglement-enhanced maximum. (R. Prevedel et al., Phys. Rev. Lett., in press.)—Johanna Miller
A quantum dot is a nanostructure that confines a single conduction-band electron in three spatial dimensions. Researchers have long sought to use the spin on that electron as a quantum bit—or qubit—to store binary information for a quantum computer. First, however, they must show that they have precise and rapid control over the quantum-dot qubit. Unfortunately, flipping the spins with oscillating magnetic fields requires high-frequency fields and very low temperatures, posing a challenge to experimenters. An alternative is to use purely electric fields and exploit the spin–orbit coupling of electrons: The orbital motion relative to the background of the semiconductor’s nuclear charges causes the electron to see a magnetic field, which couples to its spin. In traditional gallium arsenide quantum dots, the spin manipulation times obtained with spin–orbit coupling are too slow. Recently, Leo Kouwenhoven and his colleagues at Delft University of Technology and at Eindhoven University of Technology, both in the Netherlands, have turned to indium arsenide, which is known to have much larger spin–orbit coupling. Furthermore, the team formed the qubits in an InAs nanowire, which offers interesting possibilities for combining with other semiconductors. For example, one might make an optoelectronic device that converts the spin state to a photon for long-distance transportation. A more exotic prediction is that InAs nanowires might be useful for topological quantum computing. (S. Nadj-Perge, S. M. Frolov, E. P. A. M. Bakkers, L. P. Kouwenhoven, Nature 468, 1084, 2010.)—Barbara Goss Levi
Separating isotopes is difficult business: Since an element’s various isotopes share similar size and shape, separation methods such as thermal diffusion and centrifugation tend to be time and energy intensive. Now, however, a team led by Suresh Bhatia (University of Queensland, Australia) has shed new light on what may prove an attractive alternative—nanoporous materials known as molecular sieves. Normally, molecular sieves aren’t particularly effective isotope separators. Take diatomic hydrogen and its isotopic relative deuterium. As one would expect, the lighter H2 molecules diffuse through the porous molecular sieve faster than the heavier D2, but the difference is slight. That picture changes, though, if the temperature is low enough—and the pores small enough—for quantum effects to set in. If the pore size is on the order of the molecules’ de Broglie wavelength, the molecules' zero-point energy becomes the important barrier for pore diffusion and small mass becomes a disadvantage. Not only does D2 then diffuse faster than H2, it can do so by a substantial margin. Using quasi-elastic neutron scattering and carbon molecular sieves with 3-Å-diameter pores, Bhatia and company directly measured those diffusivities, which differed by nearly an order of magnitude at the coldest temperatures. Although quantum sieve effects—first predicted nearly 15 years ago—had been previously seen in equilibrium adsorption experiments, the research team’s findings represent the first microscopic observations of the kinetic phenomenon. (T. X. Nguyen, H. Jobic, S. K. Bhatia, Phys. Rev. Lett., in press.)—Ashley G. Smart
Light is a useful carrier of quantum information, but no method yet exists for storing and retrieving it without significantly corrupting its quantum state. Now, Matthew Sellars (Australian National University, Canberra) and colleagues have taken a step closer to a useful quantum memory by mapping a light pulse’s state onto an ensemble of praseodymium ions doped into a transparent crystal. Capitalizing on the Stark effect, the researchers used an electric-field gradient, as shown in the figure, to shift the ions’ transition energies. Absorption of an input light pulse created a coherent superposition of ground and excited states, whose phase in each ion evolved at a rate proportional to the transition energy. Flipping the electric field’s sign reversed the transition-energy distribution and brought the ions back into phase, at which point the pulse was re-emitted. The innovation was in how the researchers countered the spectral line’s natural broadening due to inhomogeneities in the crystal lattice: Applying a frequency-stabilized laser beam to the side of the long, thin crystal, they pumped atoms whose transition energies lay outside a narrow range into an out-of-the-way hyperfine state. Using the sharpened spectral line, which Sellars says may be “the sharpest optical filter ever demonstrated,” the researchers stored pulses of up to 500 photons more faithfully than could be done classically. When pulses were stored for 1.3 µs, the retrieved ones were 69% as intense—the previous record was 15%. (M. P. Hedges et al., Nature 465, 1052, 2010.)—Johanna Miller
In 1954 Princeton University’s Robert Dicke predicted a remarkable phenomenon: A dense cloud of excited atoms in a light field, he argued, could decay by spontaneously emitting coherent and highly polarized photons—an effect he termed superradiance. By subtly altering the Hamiltonian, researchers in the early 1970s realized that the phenomenon need not be restricted to transient pulses, and they made their own prediction: When light and matter interact strongly enough, even at zero temperature, they can exhibit a steady-state superradiant phase. By confining a Bose–Einstein condensate of some 105 rubidium atoms driven by a standing-wave laser beam in an optical cavity, Tilman Esslinger and his colleagues at ETH Zürich have now observed the predicted quantum phase transition. As shown in the sketch of their experiment, if the laser light exciting the BEC is intense enough to spawn superradiant photons along the cavity axis, the photons’ repeated reflections establish a field that interferes with the laser field to form a square-patterned potential. Because the condensate atoms produce the superradiant light, they are active participants, since atoms and photons dynamically influence each other’s motion through the coherent exchange of momentum. The upshot is that above some critical laser power, the atoms still exhibit superfluid behavior but become self-ordered into a crystalline lattice. Interestingly, although the Zürich group’s system is open, laser driven, and dissipative—far from the closed equilibrium system that Dicke considered—his Hamiltonian still captures the essential physics. (K. Baumann et al., Nature 464, 1301, 2010.)—R. Mark Wilson
Superfluid liquids—ultracold, zero-viscosity liquids that creep over vessel walls—manifest a kind of Bose–Einstein condensation. They are subject to quantum constraints that might be thought to thwart both the onset and dissipation of turbulence. Before superfluid turbulence was first seen in 1958, Richard Feynman posited the generation and reconnection of quantized vortex lines, tornadolike topological defects that would allow turbulent eddies to form and dissipate in superfluids. The quantized line integral of the fluid velocity around any loop enclosing a single vortex line would be h/m, where m is the mass of the relevant boson. For the next half century, the existence of such vortex lines was assumed but never directly seen. But techniques recently developed in Daniel Lathrop’s lab at the University of Maryland render the vortex lines and their reconnection events visible in superfluid helium-4. Injected hydrogen gas forms micron-sized hydrogen-ice particles that attach themselves to the vortex lines and scatter illuminating light, allowing the Maryland team to film reconnections. The close-ups show a reconnection event at 25-millisecond intervals in turbulent superfluid 4He. Lathrop and coworkers, having digitally filmed 40 000 such events at about 80 frames per second, report that the dynamics of reconnection is dominated by h/m, “the quantum of circulation.“ The tracer particles also make it possible to view the overall distribution of velocities in the turbulent superfluid at much finer spatial resolution than was previously possible. On that scale, a strong high-velocity tail in the velocity distribution makes turbulence in superfluids look quite different from classical turbulence. (M. S. Paoletti, M. E. Fisher, D. P. Lathrop, Physica D, in press, doi: 10.1016/j.physd.2009.03.006.)—Bertram Schwarzschild
A central tenet of quantum information processing asserts that an unknown qubit cannot be cloned (see Physics Today, February 2009, page 76). But the unknown state of one qubit can be transferred to another qubit in a process termed quantum teleportation. The first experimental demonstrations succeeded in teleporting a qubit state a meter or so (see Physics Today, February 1998, page 18). Subsequent experiments with photons, whose polarizations form a convenient basis for quantum information, have used fiber optics to achieve teleportation over hundreds of meters. But practical quantum communication will require teleportation over much greater distances. Jian-Wei Pan, Cheng-Zhi Peng, and coworkers at the University of Science and Technology of China and Tsinghua University have now transferred a qubit state through free space over a distance of 16 km, from "Alice" in the Beijing suburb of Badaling, across towns and roads, to "Bob" in Huailai, on the other side of Guanting Reservoir. The experiment employed a standard teleportation protocol: Alice and Bob each receive one of a pair of entangled photons; Alice measures hers in combination with an unknown qubit and sends the result, by classical means, to Bob; armed with that result, Bob projects his photon onto the state of the unknown qubit. The new work, though, adds many refinements, including novel telescope designs for open-air transmission, active feedback control for increased stability, and synchronized real-time information transfer. The resulting teleportation fidelity was nearly 90%. Such high-fidelity transmission, say the researchers, could help enable quantum teleportation to orbiting satellites. (X.-M. Jin et al., Nat. Photon., in press, doi:10.1038/nphoton.2010.87.)—Richard J. Fitzgerald
Neutral atoms held in optical traps are promising candidates for qubits in a quantum computer, with the atoms’ hyperfine states serving as the computer’s ones and zeros. But creating the necessary entangled states is a challenge, because atoms don't normally interact strongly at long distances. Two research groups, one at the University of Wisconsin and one at the Université Paris-Sud, the Institute d'Optique, and CNRS, recently demonstrated a long-range interaction called Rydberg blockade: When two atoms are separated by several microns, exciting one into a Rydberg state (an energetic state with a large, delocalized wavefunction) prevents the other from being similarly excited. (See Physics Today, February 2009, page 15.) Now, both groups have used Rydberg blockade to entangle the atoms in two hyperfine states. The Paris researchers irradiated both ground-state atoms with a laser pulse to create an entanglement with one atom in a Rydberg state and the other in the ground state. A second pulse coaxed the Rydberg atom back to the ground state, but into a different hyperfine level. The Wisconsin researchers constructed a quantum logic gate called a controlled NOT, or CNOT: a sequence of laser pulses, involving excitations to a Rydberg state, that changes the state of one atom if and only if the other, the control, is in a particular hyperfine state. Applying the CNOT gate when the control atom is in a superposition of states entangles the two atoms. (T. Wilk et al., Phys. Rev. Lett., in press; L. Isenhower et al., Phys. Rev. Lett., in press.) —Johanna Miller
Laser cooling of atoms enables a great deal of ultracold physics. In one of its forms, called Doppler cooling, a sample is irradiated from all sides with light tuned just below an atomic resonance. Photons that oppose an atom’s motion are shifted into resonance and absorbed, diminishing the atom’s momentum. The atom then reradiates the light in a random direction and returns to its ground state. Repeating that cycle tens of thousands of times can cool the sample below 1 mK. But applying the same technique to molecules is complicated, since rotational and vibrational degrees of freedom give them a multitude of low-lying states into which they can decay. Exciting each state with a separate laser is prohibitively difficult, but allowing molecules to accumulate in any state not excited by a laser breaks the cycle and ends the cooling. Now, Yale University’s David DeMille and colleagues have demonstrated a possible solution to that dilemma. Focusing on strontium monofluoride, one of several molecules that nearly always return to the vibrational level from which they were excited, and exploiting quantum mechanical selection rules to limit the rotational levels the molecules can access, the researchers achieved about 150 absorption–emission cycles per SrF molecule using just two lasers. They estimate that increasing the molecule–laser interaction time will be sufficient to demonstrate cooling of initially slow molecules, and adding a third laser will give them the 105 cycles needed for slowing and cooling their entire sample. (E. S. Shuman et al., Phys. Rev. Lett. 103, 223001, 2009.) —Johanna Miller
In conventional photography, photons bounce off an object and imprint its shape onto film. In ghost imaging, an object’s shape is revealed after interrogating two light beams, only one of which interacts with the object. Ghost imagers have been practicing their craft for more than 10 years. Some of their schemes are based on photon entanglement, but others use classical light sources. Now Barry Jack of the University of Glasgow and colleagues have reported experiments in which quantum entanglement is manifest; their ghost images violate a generalized Bell inequality—that is, a condition on correlations that can arise classically. The accompanying figure presents one of their runs. The object to be imaged introduces a π phase difference between a disk (turquoise) and the surroundings (red). A smaller reference object (inset) introduces the phase difference on either side of a diagonal. After bouncing entangled photons, separated in space, off the object and reference, Jack and company measured photon coincidence counts. The black-and-white ghost image shown here maps those counts, with brighter regions corresponding to a greater coincidence rate. The brightest sections appear along those portions of the disk’s bounding circle parallel to the reference bisector. The variation in the coincidence rate along the bounding circle violates the Bell inequality, thus demonstrating the quantum nature of Jack’s system. Evidently, the ghost imaging relies on spooky action at a distance, accepted nowadays but so troubling to Albert Einstein decades ago. (B. Jack et al., Phys. Rev. Lett., in press.) —Steven K. Blau
In 1927, during the formative years of quantum mechanics, Friedrich Hund posed a 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? After all, both isomers are equally likely. At first glance, the answer seems clear. If the tunneling time between the two isomers is long, their superposition is unlikely to arise. That answer might hold for a sugar, protein, or other large chiral molecule, whose tunneling time may exceed the age of the universe, but it fails for small molecules. Nor can it explain why the habitual states of a molecule, large or small, are its left-handed and right-handed isomers and not its energy or parity eigenstates. Now, Klaus Hornberger and Johannes Trost of Ludwig-Maximilians University in Munich have resolved Hund's venerable paradox. The two theoreticians analyzed the case of one of the smallest chiral molecules, deuterium disulphide (shown here), tumbling in and buffeted by a monoatomic gas. The calculation uncovered a surprisingly large phase-dependence in the scattering amplitude that distinguishes the two isomers. Thanks to the phase difference, the ambient gas atoms can pick out the states that correspond to the molecule's left-handed and right-handed isomers far more readily than the molecule’s other states. When the first few atoms strike a molecule, it's knocked into either its left-handed or right-handed configurational state. Further atomic bombardment acts on the molecule like repeated quantum measurements, keeping it in its chiral state. (J. Trost, K. Hornberger, Phys. Rev. Lett. 103, 023202, 2009.) —Charles Day
Quantum computing is a goal that both excites and challenges researchers, who are working on a wide variety of physical realizations of the basic building block: the quantum bit, or qubit.
One type is the superconducting qubit made from one or more Josephson junctions. The biggest advantage of superconducting qubits is their strong coupling to microwave signals, which can control the qubits and mediate their interactions. The greatest limitation is their short coherence lifetime.
Despite that limitation, recent experiments have demonstrated the kind of precise control that will be needed to make progress toward a viable quantum computer.
In one experiment, Max Hofheinz, John Martinis, Andrew Cleland and colleagues from the University of California, Santa Barbara, showed that they could impose on a microwave resonator any desired superposition of photon-number states. (M. Hofheinz et al., Nature 459, 546, 2009.)—Barbara Goss Levi
Decades ago, theorists predicted that under some circumstances, solids could flow like superfluids. In 2004 Moses Chan and Eun-Seong Kim found evidence of such so-called supersolidity: When a torsion oscillator filled with solid helium was cooled below 200 mK, its resonant frequency increased. Some of the He’s mass appeared to have decoupled from the rest. But subsequent experiments revealed a more complicated picture with many aspects unexplained. For example, the oscillator’s dissipation (related to the damping strength) depended on temperature in a way that the theory hadn’t predicted. Now, Cornell University’s Séamus Davis and colleagues have developed a torsion oscillator, shown in the figure, whose position sensor is a superconducting quantum interference device rather than the usual capacitor. The SQUID allows them to measure the dissipation more accurately and to explore a broader range of frequencies and amplitudes than was previously possible. Among their results is the discovery that when the temperature is abruptly lowered, the oscillator’s resonant frequency and dissipation share the same response time constant, which increases steeply with decreasing temperature—much like the characteristic flow time of cooling molten window glass. Some theorists have postulated that He’s behavior results from an ordinary glass transition, not from supersolidity. The relative magnitudes of the changes in frequency and dissipation rule out that possibility. But the ultraslow low-temperature behavior suggests that a glasslike phase may be involved. (B. Hunt et al., Science 324, 632, 2009.) —Johanna Miller
In the late 1940s, Hendrik Casimir proposed that two perfectly conducting parallel plates should feel a feeble attractive force between them, due to the zero-point energy of the surrounding electromagnetic field and its dependence on the plates' positions. (See the article by Steve Lamoreaux, Physics Today, February 2007, page 40.) About a decade later, Evgeny Lifshitz and colleagues generalized Casimir's work to real conductors and dielectrics and found that the force persisted. In most cases the proposed force was still attractive, but for some configurations—a high-permittivity material and a low-permittivity material separated by a medium of intermediate permittivity—it could be repulsive. In fact, the repulsive Casimir-Lifshitz force is responsible for liquid helium's tendency to climb the walls of its container: The container repels the ambient vapor, and the liquid rises to fill the gap. Now, a group of researchers led by Harvard University's Federico Capasso have observed a repulsive Casimir-Lifshitz interaction between two solid objects, a silica surface and a 40-μm-diameter gold-coated sphere, immersed in bromobenzene. To monitor the force, they attached the sphere to an atomic force microscope cantilever and measured the cantilever's deflection using a light beam and a split-quadrant photodetector, as shown in the figure. A repulsive force of a few tens of piconewtons was measurable when the objects were brought within 40 nm of each other, and it increased as their separation decreased. The researchers suggest that the force they observed could levitate a solid within a liquid, which may lead to very low-friction sensors of force and torque. (J. N. Munday, F. Capasso, V. A. Parsegian, Nature 457, 170, 2009.) —Johanna L. Miller
Quantum communication networks and other quantum information processing will require coherent and efficient transfer of information between light and matter, and the realm of light-matter interfaces is an active area of research. Much of the activity has focused on the mapping of quantum information onto atomic systems (see, for instance, Physics Today, March 2001, page 17). Nicolas Gisin and colleagues at the University of Geneva in Switzerland have now demonstrated the coherent storage and retrieval of information using a solid-state system. The team's quantum memory was an ensemble of roughly 107 neodymium ions trapped in a crystal of yttrium vanadium oxide (YVO4). In such an environment, the resonant frequencies of the rare-earth atoms are inhomogeneously shifted, which broadens the absorption spectrum. That's normally undesirable, but the researchers turned it to their advantage. By optically pumping some of the Nd atoms out of the ground state, they sculpted the spectrum into a series of regularly spaced absorption peaks--an "atomic frequency comb." An incident weak light pulse, with on the order of one photon or less on average, will be uniformly absorbed by the comb and generate a coherent superposition of collective optical excitations, each at a slightly different frequency. The superposition will initially dephase but will get reestablished after a time determined by the comb spacing; once rephased, the atoms will collectively reemit a light pulse that conserves the coherence and phase of the original pulse. Gisin and company achieved storage times of up to a microsecond. Furthermore, they showed that the ensemble can simultaneously store multiple light fields, and they have proposed a means of on-demand retrieval. With such capability, the authors view solid-state systems as a promising contender for quantum storage. (H. de Riedmatten et al., Nature 456, 773, 2008; M. Afzelius et al., http://arxiv.org/abs/0805.4164.) — Richard J. Fitzgerald
In an emptying bathtub, water forms a whirlpool around the drain. But circular flow can’t persist to the very center of the vortex; there must be a water-free funnel. In 1985 Wojciech Zurek, following on work of Tom Kibble, suggested that “topological defects” analogous to the whirlpool could be generated spontaneously in a system undergoing a second-order phase transition. For a fast enough process in a large enough system, small regions independently change state, being unable to communicate with other, relatively far off regions. That independence allows parameters such as the quantum-mechanical phase angle to arrange themselves in vortex structures. Researchers have seen spontaneous vortex formation in, for example, superfluid helium-3, nonlinear optical systems, and superconductors (see the article by Kibble, PHYSICS TODAY, September 2007, page 47). Now a new system can be added to the list: the Bose–Einstein condensate. Deliberately inducing a vortex in a BEC is nothing new, but recent joint experimental work at the University of Arizona and numerical work at the University of Queensland in Australia represents the first study of spontaneous vortex formation in that particularly clean system. In the experiment, Chad Weiler and colleagues tweaked standard procedures to maximize the chance of their observing spontaneously formed vortices. After a trapped atomic gas transitioned to a BEC over the course of a few seconds, the group removed the trapping potential and imaged the escaping condensate. The vortices are revealed by dark, zero-density spots in the figure; the rightmost image shows two vortices, the others a single vortex. Continuing experiment and simulation together, Weiler and colleagues hope, will shed light on the universality of spontaneous topological defect formation in phase transitions. (C. N. Weiler et al., Nature 455, 948, 2008.) — Steven K. Blau
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Bose Einstein Condensation Lab at the University of Arizona College of Optical Sciences
Centre for Quantum-Atom Optics at the University of Queensland
The dynamics of a quantum system with four charged particles can be a tough nut to crack, and competing theoretical models often differ qualitatively in their predictions. Helium provides a good four-body system to study when an electron collides with the atom to knock out both native electrons and leave the doubly charged bare nucleus behind. Theorists have disagreed about the directions the three escaping electrons would take when the incoming projectile is near the threshold energy for such an electron-impact double ionization process. Alexander Dorn, Joachim Ullrich, and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have now measured the momenta of the three electrons in that very energy regime and have found that the electrons tend to emerge in an equilateral triangle shape, separated by angles of 120 degrees, as predicted by some theories. Interestingly, one of the successful theories predicts that the escape pattern depends on the initial bound-state configuration and that the electron paths for triply photoionized lithium would take a T-shape, with two electrons emerging back-to-back. (X. Ren, A. Dorn, J. Ullrich, Phys. Rev. Lett., in press; A. Emmanouilidou, P. Wang, J. M. Rost, Phys. Rev. Lett. 100, 063002, 2008.) — Stephen G. Benka
At the May Conference on Lasers and Electro-Optics in San Jose, California, University of Colorado graduate student Mark Siemens reported on studying how tiny parcels of heat, called phonons, spread in a crystal. He and his colleagues used a near-IR laser to heat a grating of nickel lines—each 20 nm high and 1 µm
wide—grown on a sapphire substrate that acted as a heat sink. Then, by recording the transient diffraction of 10-fs pulses of coherent soft x rays from the sample, the researchers could monitor with picometer (10-12 m) precision the displacement of the heated nickel nanostructure. The transport of heat is considered "ballistic" if the characteristic distance over which a phonon moves—about a micron in this case—is smaller than its mean free path before scattering off another phonon. At room temperature a typical phonon's mean free path in sapphire is a mere 150 nm but grows to more than a micron when the sample is cooled below 130 K. At that temperature the data show a clear transition from thermally diffusive to ballistic behavior. One reason for trying to understand how heat moves away from a nanoscale interface, says Siemens, is to manage the thermal environment of future advanced high-speed transistors. — Phillip F. Schewe
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