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
December 2009 Archives
The cosmologists’ widely accepted “concordance” model asserts that only about 15% of the mass of matter in the cosmos is baryonic—made of protons and neutrons. The “dark matter” that predominates is thought to consist of particles yet unknown. Particle theory provides an attractive candidate: weakly interacting massive particles (WIMPs) predicted by supersymmetric extensions of the theory’s standard model. Presumably created in the Big Bang, those stable neutral particles, about 100 times heavier than the proton, would be abundant enough to account for gravitational effects observed in the rotation and clustering of galaxies. The Cryogenic Dark Matter Search (CDMS) collaboration has now released its analysis of all the data taken by its CDMSII detector in three years of looking for WIMPs deep inside an old Minnesota iron mine. CDMSII is a 5-kg cryogenic array of germanium and silicon crystals micro-instrumented to detect the recoil of a nucleus in a rare collision with a WIMP as Earth sweeps through the halo of dark matter presumed to envelop the Milky Way. An instrument of CDMSII’s limited mass was predicted to find, at most, a statistically marginal handful of WIMP collisions in a three-year run. The community is working to decide which of several competing detector technologies can best be scaled up to provide a robust result. In its final year, the detector recorded two collision events that showed no evidence of coming from the enormous background of electron recoils or from a neutron collision. But the group calculates a 23% chance that both events were background imposters that squeezed past the analysis cuts that reduced backgrounds a millionfold. So the paper makes no claim of significant evidence for WIMP interactions. But it does present the most stringent upper limits to date on the WIMP-nucleon scattering cross section. The figure shows those limits as a function of the putative WIMP mass, together with a range of predictions from supersymmetric theories. (Z. Ahmed et al., CDMS collaboration, http://arxiv.org/abs/0912.3592. —Bertram Schwarzschild
Several methods exist for growing nanowires, whether attached to a substrate or dispersed in a liquid. Using those wires to make designated electrical connections in a circuit, however, has been difficult. Yves Galerne and his colleagues at the University of Strasbourg, France, now demonstrate a procedure that produces conducting wires across a gap between two electrodes. The chemical physicists first paint the electrodes with a polymer so as to create "anchors" in predetermined locations; when the gap is filled with nematic liquid crystals, an isolated defect line—a disclination—connects the anchors and therefore the electrodes. Next, silica particles coated with a conducting polymer are introduced and gather along the disclination like beads on a necklace. In the third step, a voltage across the electrodes welds the necklace beads together into a robust wire. Although ragged with extra polymer aggregates, the central region of a 150-micron-long wire, shown in the photo, demonstrates the team’s initial result. The researchers note that the wire’s size, smoothness, and conductivity can be improved—for example, by decreasing the silica particles’ size and concentration and by electroplating them. (J.-B. Fleury, D. Pires, Y. Galerne, Phys. Rev. Lett., in press.) —Stephen G. Benka
In recent years, notions of the ultrafast, the ultraintense, and the ultrasmall have been recurring themes in physics as those envelopes have been relentlessly pushed to reveal new phenomena. Caltech’s Brett Barwick, David Flannigan, and Ahmed Zewail have combined all three notions into a new technique they dub photon-induced near-field electron microscopy. PINEM exploits the fact that free–free interactions of electrons and photons are greatly enhanced when a third body, like a nanostructure, is present and when the electrons are more energetic than the photons. The physicists illuminated a carbon nanotube with an intense femtosecond laser pulse that generated an evanescent plasmonic field at the CNT’s surface. Simultaneously, a similar-duration pulse of 200-keV electrons from an electron microscope traversed the sample. During the few-hundred-attosecond interaction time, some of those electrons absorbed energy quanta from the 2.4-eV photon field. By selecting only those electrons that gained energy, the researchers could image the evanescent surface field with the spatial resolution of electron microscopy. That field extends about 50 nm into the vacuum from the dark surface of the roughly 150-nm-diameter CNT. As shown in the images, Zewail and colleagues also monitored the temporal decay of the surface field by varying the delay times between the exciting laser pulse and the probing electron pulse, from zero (top) to 400 fs (bottom) and beyond. With tunable and temporally controlled light pulses, PINEM enables visualization of dynamical optical responses of various nanostructures. (B. Barwick, D. J. Flannigan, A. H. Zewail, Nature 462, 902, 2009.) —Stephen Benka
The strangeness of the quantum world is epitomized by entangled states, whose nonintuitive correlations cannot be mimicked by any classical system. These days experimenters routinely create two-photon states in which the photons’ polarization is entangled. Now, starting with such a state, Sven Ramelow and Lothar Ratschbacher (Institute for Quantum Optics and Quantum Information and University of Vienna) and colleagues have entangled the frequencies of two photons. It’s not the first demonstration of frequency entanglement, but earlier protocols relied on frequency filtering. In the Vienna work, only the two frequencies to be entangled are present in the initial state. The accompanying figure depicts the technique. Initially, the “red” photon in fiber 1 has a definite frequency, as does the “green” photon in fiber 2. The two photons have entangled polarizations—both are either horizontal or vertical. The key step is implemented by a polarizing beamsplitter that shunts the red photon into fiber 3 if it is horizontally polarized and into fiber 4 if it is vertically polarized. The PBS performs a similar operation on the green photon. The resulting intermediate state is passed through diagonal polarizers and, voila, the output has entangled frequencies. With a suitable initial state, report the Vienna researchers, their technique can transfer polarization entanglement onto any desired photon degree of freedom. (S. Ramelow et al., Phys. Rev. Lett., in press.) —Steven K. Blau
An ultracold gas of atoms known as a Bose–Einstein condensate (BEC) is a nearly ideal system for creating new states of matter or studying many-body quantum phenomena at macroscopic scales. (For one example, see the article on Anderson localization by Alain Aspect and Massimo Inguscio in Physics Today, August 2009, page 30.) The BEC’s charge neutrality, though, hinders its use as a probe of phenomena that arise from Lorentz forces on electrons in a magnetic field; magnetic fields produce only Zeeman shifts. Researchers at the Joint Quantum Institute, a collaboration of NIST and the University of Maryland, have now removed that limitation. The researchers, led by Ian Spielman, began with a BEC of roughly 250 000 rubidium-87 atoms held at 100 nK. By illuminating the atoms with a suitable pair of laser beams close to resonance, they imprinted an effective vector potential A* on the system. In the presence of a detuning gradient, the vector potential depends on position in the trap. The spatial dependence can thus be engineered to give a nearly uniform synthetic magnetic field B* = ∇ × A* that does couple to neutral atoms. A signature of that field is the formation of vortices—the spots shown in this time-of-flight image of the BEC—that mark points about which the atoms swirl. Spielman and colleagues plan to add to their system a two-dimensional optical lattice, which may allow them to create, for example, exotic quantum Hall states of bosons. (Y.-J Lin, R. L. Compton, K. Jiménez, J. V. Porto, I. B. Spielman, Nature 462, 628, 2009.)—R. Mark Wilson
The fractional quantum Hall effect with conductance plateaus at fractional rather than integer multiples of the conductance quantum e2/h values took experimenters aback when it first appeared at two-dimensional semiconductor interfaces nearly 30 years ago. The FQHE was soon explained by invoking strong correlations among the electrons that led to the formation of a collective state with an effective charge of 1/3 (see PHYSICS TODAY, October 1997, page 42). Recently, rather than being surprised by the FQHE, researchers had become frustrated by their inability to see it in a new 2D system—graphene. Consisting of a single layer of carbon atoms in a hexagonal lattice, graphene is expected to have intriguing electronic properties, produced by electrons that behave as massless relativistic particles (see PHYSICS TODAY, August, 2007, page 35). Researchers have been eager to see manifestations of those exotic particles, starting with the FQHE as proof of their expected strong electron interactions. Alas, the intensive search for the FQHE had come up empty. Now, two experiments have finally succeeded, as reported by Eva Andrei and her colleagues at Rutgers University and by Philip Kim and his coworkers from Columbia University. Two steps seem to have been key. One was to free graphene from disruptive perturbations of the substrate by suspending it in vacuum. The second was to measure the Hall-effect voltages using just two terminals rather than the conventional four. Experimenters hope to find ways to return to four-terminal measurements, which give more complete information. (X. Du et al., Nature 462, 192, 2009; K. I. Bolotin et al., Nature 462, 196, 2009.)—Barbara Goss Levi
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
