Physics Nobel Prize - 2022

These experiments have laid the foundations of a new era of quantum computers and information technology, which will usher in the fourth industrial revolution.
What it means is that using the classical laws of physics, we can exactly predict the evolution of a physical system.
What it means is that using the classical laws of physics, we can exactly predict the evolution of a physical system. The Nobel Prize/Twitter

The Royal Swedish Academy of Sciences has announced to award the Nobel Prize in Physics, 2020jointly to Alain Aspect, John F. Clauser and Anton Zeilinger for “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. The novel experiments performed by the three laureates have demonstrated the existence of entangled pairs even though the individual particles are too far apart to affect each other.

These experiments have laid the foundations of a new era of quantum computers and information technology, which will usher in the fourth industrial revolution.

Quantum mechanics is probably the most counterintuitive scientific theories of modern science. It was developed at the beginning of the twentieth century to explain the puzzling phenomena observed in the microscopic or atomic world. 

The world we live in is what is called the macroscopic or classical world, and all the phenomena occurring in our world are deterministic. What it means is that using the classical laws of physics, we can exactly predict the evolution of a physical system.

This is not only true for terrestrial objects, but also for celestial systems. For instance, we can quite accurately predict the motion of the planets in our solar system using the classical or Newtonian laws of physics. In the microscopic world of quantum mechanics, on the other hand, the predictions are completely probabilistic, and a system can exist in two different states at the same time.

The most outstanding problem that could not be explained using the classical laws of physics was the stability of the atomic model proposed by Rutherford.  In an effort to understand the experimental results of alpha-particle scattering, Rutherford proposed that electrons are revolving around the positively charged nucleus in circular orbits.

The problem is that charged particles, like electrons, revolving in an orbit will emit radiation as per classical electrodynamics. Since the electrons in an atom have a finite amount of energy stored in them, they will lose all the energy at some point and will fall into the nucleus.

The classical mechanics thus predicts that atoms will collapse, which is clearly in contradiction to the reality.  The very fact that the universe exists, and is made-up of atoms, points to the failure of classical physics when applied to the atomic system.

Bohr in 1913 proposed that energy of the atomic system is quantized, and the electron will gain or lose energy only when it transitions from one orbit to another. The quantization of the energy was earlier hypothesized by Planck to explain the radiation spectrum of a blackbody system.

The electron revolving in a single orbit will not shred energy within the tenet of the new quantum theory of matter, and the atom will remain stable. 

There is not a single experiment performed till date in the microscopic world that has disproved the predictions of the quantum theory of matter.  This theory is the working principle behind all the electronic and telecommunication systems around us.

There are some puzzling aspects of quantum mechanics which are beyond our common perception. The counterintuitive features of the quantum world that don’t occur in the classical world are superposition of states, entanglement and uncertainty. The principle of superposition states that if a system can be in a state “A” and in a state “B”, then it can also be in a mixture of the two states.

What it means is that if a system has several possible accessible states, then it can exist in any one of those states and we don’t have any prior knowledge of it until we perform a measurement.

For a quantum mechanical system, the measurement process is central to unravel the characteristic properties of a physical system. For a classical system, the properties are independent of the measurement process.

The other most intriguing aspect of quantum theory is the entanglement phenomenon which was discovered by Einstein, Podolsky and Rosen in their seminal work in 1935. This work, referred to as EPR paradox,demonstrated that two particles which are emitted together and move away from each other are entangled.

What it means is that if we perform a measurement on one particle, then the other particle is also disturbed instantaneously, although there is no physical interaction between the two particles. Einstein called this bizarre prediction of quantum mechanics as “the spooky action at a distance”.

This new insight into quantum mechanical description of a physical system violates the principle of causality and the nature of reality as there is no need of sending signals to communicate between the entangled subsystems.

Although there were many unanswered questions on the foundation and interpretation of quantum mechanics, nevertheless, the theory blossomed as it was found to be amazingly successful in explaining all the physical processes occurring in the microscopic world.

David Bohm in 1951 rekindled the EPR paradox that had raised the fundamental question whether quantum mechanics provides a complete description of reality and proposed the idea of hidden variables. In this alternative proposal, the particles carry a hidden information as to which characteristic property to depict as a result of a particular measurement.

There were other formulations of quantum mechanics, for instance, Hugh Evertt in 1957 proposed a many-world model in which performing a measurement means transitioning to a different world. In this version of quantum mechanics, Schrodinger’s cat will be dead in one world, and alive in the other world.

To decipher as to which version of quantum mechanics provides the correct description of reality, John Bell in 1964 proposed an experimental set-up that transformed the philosophical thinking into an empirical science.

The experiment is repeated several times to obtain the correlations among the results and he proved that for any theory obeying local realism like hidden variable theory of Bohm, this correlation must be less than or equal to a specific limit.

This is what is referred to as Bell’s inequality, and quantum mechanics will violate this inequality as it doesn’t obey local realism.

John Cluaser became interested in the foundations of quantum mechanics by accidently reading the paper of John Bell on inequality.

In collaboration with three other scientists, he proposed a realistic experimental set-up that can test Bell’s inequality. In 1972, Clauseralong with a doctoral student, Stuart Freedman, at the University of California, Berkeley set-up an apparatus that emitted two entangled photons from calcium atoms at a time and moved towards filters that measure their polarizations.

The results obtained were in clear violation of Bell’s inequality and agreed with the results expected from quantum mechanics.

There were several loopholes in the experimental set-up of Clauser’s group that had to be plagued before a final conclusion can be drawn regarding the validity of the quantum mechanical theory. These loopholes were removed by the research group led by Alain Aspect in a series of pioneering and novel experiments during 1981-82.

In the first experimental set-up, the group increased the statistics of the events by many-fold so that experimental results became more reliable.

In the second series of experiments that garnered world attention, the research group was able to switch the directions of the photons after they had been emitted from the source.

This experiment was a triumph of human ingenuity as it required the switching to occur in a few billionth of a second with the detectors just six meters away.

This experiment once for all disproved the hidden variable theory as the switching of the photons, the hidden information, if any, regarding which detector a particular photon should strike is completely erased.

The experiments proved that Bell’s inequality is violated by tens of standard deviations, and unambiguously proved the correctness of the quantum theory.

Further, it was elucidated by the research group led by Anton Zeilingerin 1997 that quite interesting things are possible with entangled states.

For example, if particles in the entangled pair move in opposite directions, and one of them interacts with a third particle and forms a new entangled pair.

The third particle loses its identity as it has become a part of the new entangled pair.

However, what is quite amazing is that the properties of this third particle are now transferred to the particle of the first entangled pair that was left alone.

This new way of transferring information from one system to another system is what is called quantum teleportation and is destined to revolutionize every aspect of our lives.

The author is Professor of Physics at University of Kashmir

DISCLAIMER: The views and opinions expressed in this article are the personal opinions of the author.

The facts, analysis, assumptions and perspective appearing in the article do not reflect the views of GK

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