Recently in Metrology and fundamental constants Category

Avogadro’s number, N_A, links the microscopic and macroscopic worlds by specifying how many individuals make up a mole. Now an international team of metrologists has obtained N_A with an unprecedented precision of 30 parts per billion. The result: N_A = 6.02214078(18) × 10²³. The idea behind the new experiment is simple. A sample from a crystalline silicon-28 boule, shown in the figure, was subjected to x-ray interferometry, which yielded the volume of the 8-atom Si unit cell. Other bits of the boule were painstakingly fabricated into spheres whose volumes and masses were carefully measured. The spherical volume divided by the unit-cell volume gives the number of Si atoms; the mass gives the number of moles. Voilà, atoms per mole. The devil, of course, is in the details. The team needed to measure and account for such flaws as pointlike defects in the boule and surface oxidation on the spheres. Furthermore, uncertainty in the isotopic composition of the silicon translates into an uncertainty in the mass of a mole. Indeed, questions of isotopic composition plagued earlier, similar experiments. In their most recent determination, the researchers worked with a crystal that was highly enriched in ²⁸Si and applied an innovative suite of mass spectrometry techniques to measure the minute remainders of ²⁹Si and ³⁰Si. The new N_A does more than tweak the size of a mole; in combination with other precision experiments, it will be used by metrologists to refine the values of several other fundamental constants. (B. Andreas et al., Phys. Rev. Lett., in press.)—Steven K. Blau

There’s no reason to think that the lengthening of time predicted by the theory of relativity does not hold even for everyday speeds, but the effect is so minuscule that it has taken two of the world’s most accurate optical clocks to measure it. Trapped at the heart of each clock is an aluminum ion: Displacing the ion in one clock just slightly from the trap’s center induces a relative average speed difference between the two clocks’ ions of 10 m/s (22 mile/hr). The resulting fractional frequency difference is on the order of 10⁻¹⁶. That small shift was measured recently by James Chin-Wen Chou and his colleagues at NIST in Boulder, Colorado, using NIST’s newest optical clock, whose accuracy is 8.6 x 10⁻¹⁸, and a slightly less accurate older clock. The group also measured the frequency difference between the clocks’ ions when one clock was raised by 33 cm relative to the other. The measurements not only demonstrate the high performance reached by optical clocks but also show that they may play an important role in geodesy, the measurement of Earth’s gravitational potential. The time keepers might be sensitive to elevation changes as small as 1 cm if they can attain the current goal of 10⁻¹⁸ accuracy. Such measurements would complement those of satellite-borne instruments, which also have 1-cm sensitivity but average over large areas of Earth’s surface. (C. W. Chou et al., Science 329, 1630, 2010.)—Barbara Goss Levi

Overuse of antibiotics has spawned strains of bacteria whose cell walls are impervious to the crippling blows once delivered by penicillin and its derivatives. One such so-called superbug, methicillin-resistant staphylococcus aureus, although found primarily in prisons and hospitals, has now spread beyond those confines. Despite the controlled use of the drug vancomycin, a last line of defense against MRSA, the latest threat are vancomycin-resistant bacteria, which mutate by deleting a key hydrogen bond that allows the drug to bind and inhibit cell wall growth, thereby mechanically weakening the bacteria . Rachel McKendry at University College London and her collaborators recently demonstrated a nanoscale cantilever system that is sensitive enough to detect the difference between the native drug-sensitive bacteria and the mutated resistant form with the missing hydrogen bond. The researchers coated silicon cantilevers with vancomycin-resistant (DLac in the schematic) and vancomycin-sensitive (DAla ) bacterial cell-wall analogues, then immersed them in a solution containing free vancomycin molecules. As expected, the molecules preferentially bound to the cantilevers coated with the drug-sensitive analogue; those cantilevers experienced a marked deflection—as measured by an optical detector—that equated to an 800-fold difference in binding compared with the cantilevers coated with the drug-resistant analogue. The researchers believe their system will lead to sensitive, nondestructive, and rapid nanomechanical biosensors for high-throughput drug– target interaction studies and will aid in the design of more effective drugs. (J.W. Ndieyira et al., Nat. Nanotechnol., doi: 10.1038/nnano.2008.275 .) — Jermey N.A. Matthews

In the pursuit of a quantum computer, the photon is a leading candidate for the quantum bit, or qubit. Working models of photonic circuits, however, have been unscalable arrangements of bulky mirrors and beamsplitters sitting atop a square-meter-sized table. Now scientists at the Center for Quantum Photonics at the University of Bristol in the UK have printed several dozen photonic circuits onto a silicon wafer. The research team created waveguides by first depositing a doped layer of silica onto the wafer, then patterning 3.5-micron-wide ridges into the silica. Two waveguides are coupled when they approach each other and then diverge, as shown in the figure, allowing evanescent waves to overlap. Using such directional couplers, the researchers not only fabricated on-chip beamsplitters, interferometers, and even a controlled-NOT gate, but combined those devices into photonic circuits. Among their demonstrated results is a high-fidelity, path entangled state of two photons, an important element for quantum computation. The silica-on-silicon photonic circuits may also be applied to quantum metrology and communication technologies. (A. Politi et al., Science 320, 646, 2008 [MEDLINE].)   — Jermey N.A. Matthews