It has been known for a century that electromagnetic fields can transport not
only energy and linear momentum but also angular momentum. However, it was not
until twenty years ago, with the discovery in laser optics of experimental
techniques for the generation, detection and manipulation of photons in
well-defined, pure orbital angular momentum (OAM) states, that twisted light
and its pertinent optical vorticity and phase singularities began to come into
widespread use in science and technology. We have now shown experimentally how
OAM and vorticity can be readily imparted onto radio beams. Our results extend
those of earlier experiments on angular momentum and vorticity in radio in that
we used a single antenna and reflector to directly generate twisted radio beams
and verified that their topological properties agree with theoretical
predictions. This opens the possibility to work with photon OAM at frequencies
low enough to allow the use of antennas and digital signal processing, thus
enabling software controlled experimentation also with first-order quantities,
and not only second (and higher) order quantities as in optics-type
experiments. Since the OAM state space is infinite, our findings provide new
tools for achieving high efficiency in radio communications and radar
technology.
LeopoldS on 23 Jan 14Progress in atomic, optical and quantum science1, 2 has led to rapid improvements in atomic clocks. At the same time, atomic clock research has helped to advance the frontiers of science, affecting both fundamental and applied research. The ability to control quantum states of individual atoms and photons is central to quantum information science and precision measurement, and optical clocks based on single ions have achieved the lowest systematic uncertainty of any frequency standard3, 4, 5. Although many-atom lattice clocks have shown advantages in measurement precision over trapped-ion clocks6, 7, their accuracy has remained 16 times worse8, 9, 10. Here we demonstrate a many-atom system that achieves an accuracy of 6.4 × 10−18, which is not only better than a single-ion-based clock, but also reduces the required measurement time by two orders of magnitude. By systematically evaluating all known sources of uncertainty, including in situ monitoring of the blackbody radiation environment, we improve the accuracy of optical lattice clocks by a factor of 22. This single clock has simultaneously achieved the best known performance in the key characteristics necessary for consideration as a primary standard-stability and accuracy. More stable and accurate atomic clocks will benefit a wide range of fields, such as the realization and distribution of SI units11, the search for time variation of fundamental constants12, clock-based geodesy13 and other precision tests of the fundamental laws of nature. This work also connects to the development of quantum sensors and many-body quantum state engineering14 (such as spin squeezing) to advance measurement precision beyond the standard quantum limit.
