Breakthroughs in quantum tech
Source: By Amitabh Sinha: The Indian Express
For about 100 years now, quantum ‘entanglement’ has triggered an intense but fascinating debate over the nature of reality among some of the sharpest brains of the 20th century. It’s one of the main reasons why Quantum Theory appears so strange and counterintuitive. It is also precisely this behaviour of quantum particles that Albert Einstein famously described as ‘spooky’.
On 4 October 2022, the Nobel Prize committee decided to honour three scientists — Alain Aspect of France, John Clauser of the US, and Anton Zeilinger of Austria — who, over the last four decades, have tilted the balance of the debate in one direction. Their experiments have conclusively established that the ‘entanglement’ phenomenon observed in quantum particles was real, not a result of any ‘hidden’ or unknown forces, and that it could be utilised to make transformative technological advances in computing, hack-free communications, and science fiction-like concept of ‘teleportation’.
“Together, these three have made seminal contributions to not just the foundations of quantum theory but also to efforts that have now enabled the possibility of a wide range of applications. In a way, the Nobel Prize for these three was long overdue, and quite expected,” said Urbasi Sinha, who works in a similar domain at the Raman Research Institute in Bengaluru.
By the start of the 20th century, just when some had begun believing that everything that was there to be discovered about the way nature worked had already been discovered, a few scientists observed that the behaviour of tiny sub-atomic particles like protons or electrons was not consistent with the classical Newtonion laws of physics. The more they probed, the more bewildering results they got.
In their endeavour to explain what they saw, a group of mostly young physicists made a series of astonishing discoveries over the next 30 years that completely altered our understanding of how nature worked in the sub-atomic space. Together they strung together the Quantum Theory, which described the seemingly bizarre behaviour of sub-atomic particles with remarkable accuracy.
But Quantum Theory went completely against everyday experiences. It allowed a particle to exist simultaneously at multiple locations, a phenomenon known as superposition. The chance of finding the particle at any given place was dictated by probabilistic calculations, and once it was found, or observed, at one location, it ceased to exist at all other places.
Entanglement was another of several weird properties exhibited by these tiny particles. Two particles, having ‘interacted’ with each other at some stage, were found to have got ‘entangled’ in a way that the behaviour of one produced an instantaneous reaction in the other even if the two were no longer connected in any way and were separated by very large distances.
Einstein, in particular, was extremely uncomfortable with this. His Special Theory of Relativity prohibited any signal from travelling faster than the speed of light. The seemingly instantaneous communication due to entanglement had the danger of further unravelling the foundations of physics.
Einstein proposed that there was something missing, and that Quantum Theory was not yet complete. His objections, as enunciated in a famous 1935 paper he wrote with Boris Podolsky and Nathan Rosen (often described as the EPR paradox), and the counter-arguments provided by Neils Bohr and others are part of scientific folklore, and make for a delightful and engrossing read even for a layperson.
Experimentalists, in the meanwhile, were discovering that almost every prediction made by Quantum Theory was being obeyed by the sub-atomic particles. The theory was remarkably accurate. The problem was that an experiment to test a phenomenon like entanglement did not appear feasible.
Until the appearance of John Bell on the scene, that is. The relatively young physicist from Northern Ireland, in 1964, showed mathematically what was required to be done by experimentalists to establish the phenomenon of entanglement. The famous Bell’s inequality, if maintained in the results of the experiment, would mean that Einstein was right. If violated, it would prove the predictions of Quantum Theory.
The 79-year-old Clauser was the first to set up an experiment to test entanglement. In 1972, his experiments produced results that were a clear violation of Bell’s inequality. But sceptics pointed to certain aspects of the experiment which could have influenced a favourable result.
Alain Aspect is credited with vastly improving the set-up of Clauser and removing all the loopholes critics had found. Aspect’s experiments also produced results that violated Bell’s inequality.
Anton Zeilinger, and his colleagues, in the meanwhile, had already started exploiting the entanglement property to open up new technological possibilities. Zeilinger demonstrated for the first time that it was possible to ‘teleport’ the quantum states of a particle to another location without the particle moving anywhere and without a medium.
These experiments conducted by Clauser, Aspect and Zeilinger have decisively demonstrated that entanglement was real and in accordance with the Quantum Theory, and that it was not being driven by any hidden forces as suggested by Einstein and others. The satisfactory theoretical explanation of the phenomenon, however, continues to elude scientists. But as Rajamani Vijayaraghavan of Tata Institute of Fundamental Research put it, this no longer troubles the current generation of scientists the way it used to previously.
“May be there is something that will come up at a later stage to explain this. But, at the moment, it doesn’t seem that Quantum Theory is incomplete or missing something. Entanglement may still seem spooky to some, and there is no easy resolution to that, but most of us realise that we have to work with this,” Vijayaraghavan said.
Vijayaraghavan said one major concern of Einstein that entanglement allows for transmission of information at speeds faster than light, was not entirely accurate. “When an operation is performed on one of the entangled particles, there is an instantaneous reaction in the other. But there is no way for the observer at the other end to know the reaction has happened. The observer has to be made aware of the operation having been performed, and this happens only through classical communication channels limited by the speed of light restriction,” he said.
The entanglement property is now being utilised to build the next generation of computers, called quantum computers, which exploit the quantum behaviour of particles to overcome challenges considered unsurmountable. It is being used to create secure communication algorithms that would be immune to hacking.
Sandeep Kumar Goyal, a theoretical physicist working at IISER Mohali, said a lot of such work was happening in Indian laboratories too. “ISRO, DRDO, Department of Atomic Energy, Department of Science and Technology have all been interested in, and supporting, projects on quantum key distribution that will build these secure communication channels. Scientific groups at the Raman Research Institute and Physical Research Laboratory, Ahmedabad, have made significant progress in these fields,” he said.
