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The motion of an electron in a strong infrared laser field is tracked in real time by means of a novel method developed by MPIK physicists and applied to confirm quantum-dynamics theory by cooperating researchers at MPI-PKS. The experimental approach links the absorption spectrum of the ionizing extreme ultraviolet pulse to the free-electron motion driven by the subsequent near-infrared pulse. Their paper is published in the journal Physical Review Letters.

For this experimental scheme, the classical description of the electron motion is justified even though it is a quantum object. In the future, the new method demonstrated here for helium can be applied to more complex systems such as larger atoms or molecules for a broad range of intensities.

High-harmonic generation, namely the conversion of optical or near-infrared (NIR) light into the extreme-ultraviolet (XUV) regime, is fundamental to strong-field physics, since it is an extremely nonlinear process. In the famous three-step model the driving light field ionizes the electron by tunnel ionization, accelerates it away and back to the ionic core, where the electron re-collides and emits XUV light if it recombines.

The state of perfect stillness known as absolute zero is one of the Universe’s impossible achievements. As close as we can get, the laws of physics will always prevent us from hitting thermal rock bottom.

An international team of researchers has now identified a new theoretical route to reach the mythical mark of zero Kelvin, or-273.15 degrees Celsius (−459.67 degrees Fahrenheit). No, it’s not more likely to break any laws and remove every last shimmer of heat, but the framework could inspire new ways of exploring matter at low temperatures.

As a consequence of the third law of thermodynamics, the removal of increments of heat energy from a group of particles to cool them to absolute zero will always take an infinite number of steps. As such, it requires an infinite amount of energy to achieve. Quite the challenge.

Inside the proton are elementary particles called quarks. Quarks and protons have an intrinsic angular momentum called spin. Spin can point in different directions. When it is perpendicular to the proton’s momentum, it is called a transverse spin. Just like the proton carries an electric charge, it also has another fundamental charge called the tensor charge. The tensor charge is the net transverse spin of quarks in a proton with transverse spin.

The only way to obtain the tensor charge from experimental data is using the theory of quantum chromodynamics (QCD) to extract the “transversity” function. This universal function encodes the difference between the number of quarks with their spin aligned and anti-aligned to the proton’s spin when it is in a transverse direction. Using state-of-the-art data science techniques, researchers recently made the most precise empirical determination of the tensor charge.

Due to the phenomenon known as confinement, quarks are always bound in the proton or other hadrons (particles with multiple quarks). The challenge is to connect the theory of quark interactions (QCD) to experimental measurements of high-energy collisions involving hadrons.