Down to the Quantum Dot

Using a single molecule as a sensor, scientists have successfully imaged electric potential fields with unrivalled precision. The ultrahigh-resolution images provide information on the distribution of charges in the electron shells of single molecules and even atoms. The 3D technique is also contact-free. The first results achieved using “scanning quantum dot microscopy” have been published in the current issue of Physical Review Letters.

Top: The scanning quantum dot micrograph of a PTCDA molecule reveals the negative partial charges at the ends of the molecule as well as the positive partial charges in the centre. Centre: Simulated electric potential above a PTCDA molecule with molecular structure. Bottom: Schematic of charge distribution in the PTCDA molecule. Source: Forschungszentrum Jülich

Experiment confirms quantum theory weirdness

The bizarre nature of reality as laid out by quantum theory has survived another test, with scientists performing a famous experiment and proving that reality does not exist until it is measured.

Schematics of Wheeler’s delayed-choice experiments. a, Optical version of Wheeler’s delayed-choice experiment. b, Atomic version of Wheeler’s experiment, where the physical beamsplitters and mirrors are now replaced with optical Bragg pulses. A quantum random number generator (QRNG) is used to decide whether the last beamsplitting pulse is either implemented or not. The random number is triggered and chosen after the π-pulse (mirror pulse), thereby ensuring that the atom has no prior knowledge of how it will be detected when it enters the interferometer.

Einstein saves the quantum cat

 

Illustration of a molecule in the presence of gravitational time dilation. The molecule is in a quantum superposition of being in several places at the same time, but time dilation destroys this quantum phenomenon. Source: Universität Wien

NASA Telescopes Set Limits On Spacetime Quantum “Foam”

Source: NASA

At the smallest scales of distance and duration that we can measure, spacetime – that is, the three dimensions of space plus time – appears to be smooth and structureless. However, certain aspects of quantum mechanics, the highly successful theory scientists have developed to explain the physics of atoms and subatomic particles, predict that spacetime would not be smooth. Rather, it would have a foamy, jittery nature and would consist of many small, ever-changing, regions for which space and time are no longer definite, but fluctuate.

The predicted scale of spacetime foam is about ten times a billionth of the diameter of a hydrogen atom’s nucleus, so it cannot be detected directly. However, If spacetime does have a foamy structure there are limitations on the accuracy with which distances can be measured because the size of the many quantum bubbles through which light travels will fluctuate. Depending on what model of spacetime is used, these distance uncertainties should accumulate at different rates as light travels over the large cosmic distances.

A team of researchers used observations of X-rays and gamma-rays from very distant quasars – luminous sources produced by matter falling towards supermassive black holes – to test models of spacetime foam. The authors predicted that the accumulation of distance uncertainties for light traveling across billions of light years would cause the image quality to degrade so much that the objects would become undetectable. The wavelength where the image disappears should depend on the model of space-time foam used.

Chandra’s X-ray detection of quasars at distances of billions of light years rules out one model, according to which photons diffuse randomly through space-time foam in a manner similar to light diffusing through fog. Detections of distant quasars at shorter, gamma-ray wavelengths with Fermi and even shorter wavelengths with VERITAS demonstrate that a second, so-called holographic model with less diffusion does not work.

The X-ray and gamma-ray data show that spacetime is smooth down to distances 1000 times smaller than the nucleus of a hydrogen atom.

An attosecond laser and the birth of an electron

Atomic processes take place on extremely short time scales. Measurements at the Vienna University of Technology (TU Vienna) can now visualize these processes.

Electron pairs (green) are produced in a laser field (red) by ionization of atoms (blue) with light oscillations synchronized on an attosecond time scale. Credit: Technische Universität Wien