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The development of physics has reached an advanced stage. Analysis of phenomena has hit to a realm invisible to the human eye. Recently, one of the physics topics extensively discussed is Quantum Information. The topic also led the three physicists to win the Nobel Prize in Physics in 2022, namely John F. Clauser, Alain Aspect, and Anton Zeilinger.

They conducted experiments, proving the invalidity of the EPR Paradox proposed by Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) in 1935 regarding quantum entanglement. It has been a controversial issue among physicists for a long time, since its feature seems illogical through the view of classical physics. In fact, the subject has contradicted with the principles of locality and realism, the guideline in explaining natural phenomena in physics.

Basically, the phenomenon of quantum entanglement discusses about the two particles that are "linked", if the state of one particle is observed, the position of another one is instantly known. Simply put, as a black and white ball, if the two are put in two boxes and closed, then randomized so that the position of the couple balls cannot be identified. Logically, if one box is opened and found the black inside, then another box must be with the white one, and vice versa. It is certainly indisputable, even in classical physics.

However, the difference is, in quantum physics the two balls are in a state of superposition based on the wave function, which means they can be white and black at the same time. If the two balls are each put into different boxes then the first box can contain a white ball or a black ball and the second box does the same. The position of the first and the second ball would be ascertained if one box is opened. If one of the boxes is unlocked with a black ball inside, then it is eliminated possibility of the white ball in the first box as well as black ball in the second box.

The information about the color in the second box must go through the details in the first box. Nevertheless, what if the two boxes are separated by a distance, much greater than the speed of light. The facts regarding the ball in each box based on quantum physics occur instantly. Therefore, it is inconsistent with the principle of locality in classical physics.

This phenomenon was strongly opposed by Albert Einstein as if that happened, then information from this quantum entanglement (in quotation marks) could be transmitted faster than the speed of light. Furthermore, it contradicts the principles recognized in classical physics and the general theory of relativity. To support the phenomenon of quantum entanglement, he argued about hidden variables moving faster than light to maintain the entanglement. This approach is based on  principle of realism, the belief that objects have real properties that is independent, from measurement or observation.

Howbeit, after several years, John Bell, who read the paper submitted by Einstein-Podolsky-Rosen, then put forward a new idea showing that there were mathematical restrictions, known as Bell's inequalities. It provided mathematical constraints for testing interpretations combining the principles of realism and locality in the context of quantum entanglement.

Bell's inequality allows experiments to examine whether the interpretation of quantum mechanics or an interpretation involving hidden variables and the principle of classical realism is more consistent with experimental results.

Experiments to test Bell's inequality carried out by John F. Clauser and Stuart Freedman. They developed a new form of experiment known as the "Bell test". It was designed to measure the entanglement of photons (light particles) in many different angles using a polarizer and showed the results of measurement cannot be explained by classical physics. The Bell test offered stronger results in testing Bell inequality and proved that photon entanglement was indeed a quantum phenomenon that actually occured and that there were no hidden variables as predicted by Einstein.

In 1982, a French physicist, Alain Aspect, conducted a series of more sophisticated and famous experiments called the Aspect-Grangier-Roger (AGR) experiments. It used photons as particles entangled in an experimental configuration called Bell state measurement. Entangled photons were generated through a parametric down-conversion process, in which they were emitted as pairs from a nonlinear crystal.

Furthermore, the Aspect experiment showed that the results of measuring photon entanglement were immensely greater than what could be explained by classical theory. The result proved  that photon entanglement is a real quantum phenomenon. In other words, the results of the Aspect experiment  strengthen that quantum physics is better at explaining the phenomenon of photon entanglementcompared to classical physics.

The following experiment on quantum entanglement was carried out by Anton Zeilinger in 1997. He and his team succeeded in transferring quantum states from one photon to entangled photons using the phenomenon of quantum entanglement. It has made Anton Zeilinger well known in the field of quantum teleportation, because his experiment was one of the major breakthroughs in the field of quantum physics. This experiment strengthens the concept of quantum teleportation which previously only existed in theory.

In addition, Zeilinger is famous for his experiments using entangled photons to test Bell's inequality and verify the phenomenon of quantum entanglement. It involves measuring the polarization of entangled photons separated over large distances. The results of these experiments support the predictions of quantum mechanics and show incompatibility with interpretations involving hidden variables and the principle of local realism.

Based on the phenomenon of quantum entanglement, we know that if two particles separated by considerable distance could exchange information rapidly, then it would be unfeasible if Allah SWT could not do something as effortless as that. We can interpret it in the context of praying, even though we pray from a vast distance (in human terms), Allah SWT could hear it at that very moment. Although it cannot be validated through experiments, hopefully the existence of the quantum entanglement phenomenon deepens our faith in Allah SWT as well.

By : Ustadz Farhan Naufal Firdaus Al Fath, M.Si.