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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

Related links:

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

Related links:

Nanoscale Probes of Materials