Redefining the ampere; deuteron mystery; petahertz measurements.

**Redefining the ampere**

In 2014, an international group called the **BIPM** agreed to redefine four common units of measurements–the kilogram, the ampere, the kelvin and the mole.

These units of measurement make up the so-called International System of Units or SI. In total, there are seven SI base units—meter, kilogram, second, ampere, kelvin, mole, and the candela. Work is already under way to redefine the kilogram.

Now, the **National Institute of Standards and Technology (NIST)** is developing a technique that could help redefine the ampere. For years, the ampere (A), which is a base unit of electrical current, has been equivalent to one coulomb per second. The ampere, according to NIST, is a 70-year-old term that cannot be physically realized as written.

So by 2018, the ampere will be redefined in terms of a constant. It will be represented as an elementary electrical charge or “e”, which is exactly 1.602 17X x 10^{–19} coulomb.

Basically, the new measurement is a matter of counting the transit of individual electrons over time, according to NIST, which has devised a metrology solution for the problem.

For this, NIST is putting a new twist on a technique called single-electron transport (SET) pumping. In simple terms, a voltage is applied to a gate. This, in turn, causes an electron from a source to tunnel through a barrier. The electron moves onto an “island” made from a microscopic quantum dot, according to NIST. By repeating this process, researchers from NIST can generate a measurable current of single electrons.

Conventional SET systems are metallic, but it’s unclear if these products can produce enough current. So, NIST is developing a silicon-based SET, which, in turn, can produce a current 10,000 times larger than conventional units.

Meanwhile, in the other three SI base units–the kilogram, the kelvin and the mole – will be also redefined in terms of constants, according to BIPM. The new definitions will be based on fixed numerical values of the Planck constant (h), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively, according to BIPM. Moreover, the definitions of all seven base units of the SI will also be expressed using a constant formulation. Click here for more information.

**Deuteron mystery**

The **Paul Scherrer Institute, ETH Zurich**, the **Max Planck Institute** and others found that an atomic particle called deuteron is smaller than previously thought.

Deuterium is one of two stable isotopes of hydrogen. The nucleus of deuterium is called a deuteron. A deuteron contains one proton and one neutron. Deuterium has a number of applications. It could be used for spectroscopy as well as for heavy water fission nuclear reactors.

To determine the size of the deuteron, researchers used laser spectroscopy. First, they produced artificial atoms called muonic deuterium. The nucleus of this atom is deuteron, which is orbited by one muon. Muons are negatively charged elementary particles. They resemble electrons, but are around 200 times heavier.

In the lab, meanwhile, researchers injected around 300 muons per second into a chamber. The muons hit the deuterium, thereby forming muonic deuterium.

Still, there is a mystery surrounding deuteron. Paul Scherrer Institute’s results show one size for deuteron. Other researchers claim it’s a different size. One explanation for this is that there are new physical forces at work. Another explanation would be imprecise measurements by researchers.

**Petahertz measurements**

ETH Zurich is exploring how electrons react and can be controlled if or when systems reach speeds in the petahertz range.

Today’s chips run at frequencies in the gigahertz range. A gigahertz is one thousand million hertz. A petahertz is one quadrillion hertz, or 10 to the fifteenth power.

To devise a petahertz experiment in the lab, researchers first made a tiny diamond with a thickness of only 50nm. They exposed the diamond with an infrared laser pulse. The pulse lasted a few femtoseconds.

The electric field of that laser light had a frequency of about half a petahertz, according to researchers. The electric fields of the electrons were measured. This was conducted by sending light through the material and then observing how strongly the material absorbs it.

“The researchers were able to conclude that the dynamical Franz-Keldysh effect was responsible for the absorption in diamond under the influence of the infrared laser pulse,” according to ETH. “The fact that we could still see that effect even at petahertz excitation frequencies confirmed that the electrons could, indeed, be influenced at the speed limit of the laser field,” said Lukas Gallmann, a researcher at ETH.

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