**The story so far: **On October 4, the Nobel Committee of The Royal Swedish Academy of Sciences announced the names of three physicists as the laureates for the Nobel Prize in physics. They are Alain Aspect from the University of Paris-Saclay, France; John F. Clauser of John F. Clauser and Associates, California, U.S.; and Anton Zeilinger, University of Vienna, Austria. They have been awarded “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”, according to a press release given out by the Academy of Sciences, which is based in Stockholm, Sweden.

#### Why were these three physicists chosen for the award?

The prize has been given for experimental work in quantum entanglement, which Einstein referred to as ‘spooky action at a distance’. John Clauser and Alain Aspect firmed up this concept, developing more and more complex experiments that demonstrated and established that entanglement was indeed a true characteristic of quantum mechanics. They did this by creating, processing and measuring what are called Bell pairs. Anton Zeilinger innovatively used entanglement and Bell pairs, both in research and in applications. These include quantum computation and quantum cryptography.

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#### Why is the word quantum so important here?

Classical mechanics is the study of the dynamics of a system which uses Newton’s laws of motion at the very basic level. The dynamics of a few bodies or particles interacting with each other can be described using classical mechanics itself. This can be extended to many particle systems, such as a box containing millions of molecules of a gas, by employing the powerful technique of statistics, leading to statistical mechanics.

The success of Newton’s laws, classical mechanics, and classical statistical mechanics is not to be sneezed at. From describing a tennis match to sending a rocket to Mars this encompasses a whole lot of everyday activities. However, this approach breaks down when one wishes to describe subatomic particles such as light quanta.

To understand these problems that could not be explained using classical mechanics, postulates of quantum mechanics were invoked. Some of the chief architects of this were Max Planck, Albert Einstein, Erwin Schrodinger, Werner Heisenberg and Niels Bohr.

#### What is at the centre of the quantum revolution?

Many of the concepts that were useful in visualising the movement of particles in the classical realm break down when applied to particles obeying quantum mechanics. For example, when a tennis ball is struck, we see that it traces out a definite path in space. The path it traces out is called a trajectory, and it is eminently possible to theoretically calculate the trajectory to any given accuracy. Simultaneously, there is no restriction on measuring the speed, or momentum of the ball at every point on the trajectory. Particles that fall into the quantum regime on the other hand — electrons or photons, for example — do not even possess a definite trajectory because they are not little hard spheres that we initially imagined them to be, but are weird, wavelike quantum objects. Because of this, there is a limit to how precisely you can measure the position and momentum of these particles simultaneously. Many differences arise, starting from this fundamental difference.

One important difference in the behaviour of quantum systems, when compared to classical bodies, is the concept of entanglement, which is at the heart of this year’s Nobel Prize for physics.

#### What is the practical use of quantum mechanics?

Electronic devices that we employ today use transistors that apply quantum mechanical ideas. Lasers have been built that apply the quantum properties of light.

#### What is quantum entanglement? Does it have a classical counterpart?

Quantum entanglement is a phenomenon by which a pair of particles, say photons, is allowed to exist in a shared state where they have complementary properties, such that by measuring the properties of one particle, you automatically know the properties of the other particle. This is true however far apart the two particles are, provided the entanglement is not broken.

There is a trivial example of this from the classical domain. Take two coloured balls, one black and one white, and put them in identical boxes so that no one other than you know which box contains the black ball. One of the boxes is sent to Vienna and the other to Madurai. Just by opening the box they have received, the person in Vienna (or Madurai) can know not only the colour of the ball they have received but also that of the one in Madurai (or Vienna). This is a classical example and is somewhat trivial because nothing more can be made of it.

If the ball obeys quantum mechanics, its colour is not known to the observer until he or she makes an observation. So, until the box is opened, the state of the ball inside is a superposition of black and white states. Like the absence of a well-defined trajectory described earlier, this is one of the features of quantum mechanics. If the two balls occupy a shared state to start with, which is possible in quantum mechanics, however far the two may be transported, because of entanglement, opening one box can tell the user what the other ball’s colour is. Until one box is opened, the two balls exist in a superposition of colours.

But how is it possible to know that each ball did not have a set colour at the beginning? Was there a ‘hidden variable’ that instructed each ball which colour to take when the box was opened?

This is where the theory of Bell’s inequalities come in. Bell’s inequalities are theoretical insights that make it possible to differentiate between two scenarios. One, that the indeterminacy of the colour of the balls is purely a quantum phenomenon, and the other, that there are hidden variables that determine the colour when opened.

#### What was the work done by the laureates?

John Clauser and Alain Aspect devised sophisticated experiments to test the above cases and establish through Bell’s inequality, that entanglement was indeed a consequence of quantum physics. The third laureate Anton Zeilinger and his group used the phenomenon of entanglement to perform what is called quantum teleportation. This is a way of conveying information from one place to another without the actual transport of material.

#### Where does the work find use in practical applications?

The work of the three laureates can help in developing quantum technologies of the future, for example, quantum cryptography, and precise timekeeping as is done in atomic clocks.

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