Question

what is quantum physics

Answer 1

Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles. Classical physics, the physics existing before quantum mechanics, describes nature at ordinary scale.

OR
the branch of physics concerned with quantum theory.

Answer 2

Light that has travelled billions of light-years from now-ancient quasars has been used by physicists to close the “freedom-of-choice” loophole in Bell tests of quantum entanglement. Done on Spain’s Canary Islands, the experiment is a significant improvement over a similar test done in 2017 that used light from nearby stars. Meanwhile in China, an independent team has done a similar experiment using starlight that also closes the “fair sampling” loophole.

Entanglement is a curious consequence of quantum mechanics that allows two particles to be connected in a way that cannot be described by classical physics. It is observed as correlations between measurements made on two particles (such as their polarizations). In 1964 the Northern Irish physicist John Bell described his famous test of whether such correlations are stronger than those allowed by classical physics – as defined by a violation of what is now called Bell’s inequality.

Many Bell test experiments have since been done to confirm entanglement. However, no experiment is perfect and there are number of experimental “loopholes” that could allow purely classical phenomena such as faulty detectors to affect the outcome. In 2015, physicistssimultaneously closed two important loopholescalled “fair sampling” and “locality”.

Unknown correlations

Freedom of choice is another important loophole that involves how the measurements are done. In a Bell test on entangled photons, a large number of measurements are made on different entangled pairs in which the direction of the polarization measurement is selected at random. If, for some reason, the polarization selection is not random but correlated to other aspects of the experiment, then the outcome of the Bell test could be affected.

In 2017 Johannes Handsteiner and Anton Zeilinger of the University of Vienna and an international team used the random nature of starlight to close this loophole. Two telescopes at two locations in Austria separated by nearly 2 km were pointed at two different stars. The colour of the starlight changes in a random manner and this was used to decide how to set Bell test polarization detectors. The stars were chosen so that their light arrives at their respective telescopes first, before reaching other parts of the experiment. This, and the fact that the starlight light was created hundreds of years ago, very far away from Earth and in stars separated by a great distance allowed the physicists to conclude that there is no correlation between the choices of polarization measurement and the rest of the Bell test experiment.