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For almost 100 years, quantum ‘entanglement’ sparked a fierce but fascinating debate on the nature of reality among the sharpest minds of the 20th century. It is one of the main reasons why quantum theory is seen as strange and counterintuitive. It is this characteristic of quantum particles that Albert Einstein famously described as ‘terrifying’.
on tuesday, The Nobel Prize Committee decided to honor three scientists – the Frenchman Alain Aspect, the American John Clauser and the Austrian Anton Zeilinger – have tipped the balance of the debate in one direction over the last four decades. Their experiments conclusively prove that the phenomenon of ‘entanglement’ observed in quantum particles is real and not the result of any ‘hidden’ or unknown forces, and could be used to make technological advances in computing, hack-free communication and transformation. A science fiction-like concept of ‘teleportation’.
“Together, these three have contributed greatly to efforts that not only laid the foundations of quantum theory but now have wide-ranging applications. Urbasi Sinha, who works in the same field at the Raman Research Institute in Bengaluru, said the Nobel Prize for these three was long overdue and long overdue.
At the turn of the 20th century, when some began to believe that everything about the workings of nature had already been discovered, a few scientists noticed the behavior of tiny atomic particles such as protons or electrons. It does not conform to the classical Newtonian laws of physics. The more they investigated, the more puzzling they found.
In an effort to explain what they saw, a group of mostly young physicists made a series of remarkable discoveries over the next 30 years that completely changed our understanding of how nature works in subatomic space. Together they put together Quantum Theory, which explained the seemingly mysterious behavior of subatomic particles with astonishing accuracy.
But quantum theory is completely out of touch with everyday experiences. It allows a particle to exist in multiple locations simultaneously, a phenomenon known as superposition. The probability of finding a particle anywhere is dictated by probabilistic calculations, and once it is found or seen, in one place, it ceases to exist in all other places.
Entanglement was another of the many strange properties exhibited by these tiny particles. Two particles that ‘communicate’ with each other to some degree, are found to be ‘stuck’ in such a way that the behavior of one reacts instantaneously on the other, even though the two are not in any way connected and are separated by a great distance.
Einstein was particularly uncomfortable with this. His special theory of relativity prohibited any signal from traveling faster than the speed of light. Because of entanglement, the seemingly fast communication had a greater risk of upsetting the foundations of physics.
Einstein suggested that something was missing and that quantum theory was not yet complete. His objection, as expressed in his famous 1935 paper with Boris Podolsky and Nathan Rosen (often referred to as the EPR paradox), and counter-arguments by Niels Bohr and others are part of scientific lore and make it interesting and fun. An engrossing read even for the layman.
Experimentalists, meanwhile, find that almost every prediction made by quantum theory is being obeyed by subatomic particles. The concept was surprisingly accurate. The problem was that an attempt to test for such a phenomenon as entanglement did not seem feasible.
That is, until John Bell showed up on the scene. A relatively young physicist from Northern Ireland, in 1964, mathematically demonstrated what experimentalists had to do to create the phenomenon of entanglement. If the famous Bell’s equation holds true in the experiment, then Einstein was right. If violated, it confirms the predictions of quantum theory.
Clauser, 79, was the first to set up an experiment to test entanglement. In the year In 1972, his experiments produced results that violated Bell’s equation. But skeptics have pointed out some aspects of the experiment that may have influenced the good results.
Alain Aspek is credited with greatly improving Klauser’s organization and eliminating all the loopholes that critics have found. Aspect tests also yielded results that violated Bell’s equality.
Anton Zeilinger and his colleagues, meanwhile, began exploiting the hacked property to open up new technological possibilities. Zeilinger showed for the first time that it is possible to ‘teleport’ the quantum state of a particle to another location without the particle moving anywhere and without a medium.
These experiments by Clauser, Aspek and Zeilinger showed conclusively that entanglement is real and based on quantum theory and not driven by any hidden forces as suggested by Einstein and others. A satisfactory theoretical explanation of the phenomenon, however, continues to elude scientists. But as Rajamani Vijayaragava of the Tata Institute of Basic Research puts it, this doesn’t bother the current generation of scientists as much as it used to.
“There may be something that comes up later to explain this. However, at present, quantum theory does not seem to be incomplete or missing something. Interference can still seem overwhelming to some, and there is no easy solution to that, but most of us recognize that we have to work with it,” Vijayaraghavan said.
Vijayaraghavan says that Einstein’s one concern, that entanglement allows information to be transmitted faster than light, is not entirely correct. “When an operation is performed on one of the entangled particles, there is a rapid reaction in the other. But the observer at the other end has no way of knowing that the reaction has occurred. The only way for the observer to know that the operation has occurred is through classical communication channels limited by the speed of light,” he said.
The entanglement property is now being used to build computers called quantum computers, which use the quantum nature of particles to overcome seemingly insurmountable challenges. It is being used to create secure communication algorithms that will protect against hacking.
Sandeep Kumar Goyal, a theoretical physicist working at IISER Mohali, said that many such works are being done in Indian laboratories as well. “ISRO, DRDO, Department of Atomic Energy, Department of Science and Technology are interested in and supporting projects on quantum key distribution that build these secure communication channels. Scientific teams at Raman Research Institute and Physical Research Laboratory Ahmedabad have made significant progress in these areas, he said.
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