The story so far: On October 4, The Nobel Committee announced the names of three physicists as Nobel Laureates for this year. They are Alain Aspect from the University of Paris-Saclay, France; John F. Clauser of John F Clauser and Associates, California, USA; 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 by the Royal Swedish Academy of Sciences, which awards the Nobel Prizes every year.
At the heart of the award is the concept of quantum entanglement, which Albert Einstein referred to as “spooky action at a distance.” The Prize has been given for experimental work in this area. Two of the laureates—John Clauser and Alain Aspect—worked on firming up this concept and developing more complex experiments that demonstrated this phenomenon, especially creating, processing and measuring what are called Bell pairs. The third laureate, Anton Zeilinger, has been chosen for his innovative use of entanglement and Bell pairs both in research and application such as quantum cryptography.
Mechanics is the branch of physics that deals with the movement and interaction of various bodies. Classical mechanics is the study of the dynamics of a system at the very basic level of Newton’s laws of motion.
When there are a few bodies or particles interacting, classical mechanics can be used in a straightforward manner. It can be extended to many particle systems like a box containing millions of molecules of a gas, by employing the powerful techniques of statistics. This is called statistical mechanics.
Breakdown of the classical
Newton’s laws were, of course, very successful in describing a lot of everyday activities, from playing tennis to sending a rocket to Mars. However, they broke down, or were of no use, when describing the behaviour of subatomic particles or light quanta, for example.
Despite all these innovations, there were phenomena that could not be explained by physicists. To understand these problems, in the early decades of the 20th century, postulates of quantum mechanics were brought in. The chief architects of this were Max Planck, Albert Einstein, Erwin Schrodinger, Werner Heisenberg and Niels Bohr to name a few.
Many of the concepts that were useful in visualising the movement of particles in the classical realm break down when you look at particles obeying quantum mechanics.
Trajectory and its absence
For example, when a tennis ball is struck, you can observe it and see that it traces out a definite path in space. This path called a trajectory, and it is eminently possible to theoretically calculate the trajectory of the ball to any given accuracy.
Look at particles that fall into the quantum regime – electrons or photons. They do not possess a definite trajectory because they are not little hard spheres that we initially imagined, but, weird quantum objects. Starting from this fundamental difference we end up with many other complexities.
One important difference in the behaviour of quantum systems, when compared to classical rigid bodies, is the concept of entanglement, which is at the heart of this year’s Nobel Prize for physics.
Weird quantum world
Quantum entanglement is a phenomenon by which a pair of particles, say photons, are 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 regardless of how far apart the two particles are transported.
There is an example of this from the classical domain. Imagine you have two balls, one black and one white. They are placed in identical boxes so that you do not know which box contains which coloured 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 can know not only the colour of the ball they have but also that of the one in Madurai and vice versa. This is somewhat trivial because that’s all there’s to it is.
If the ball is a quantum mechanical particle, its colour is not known to the observer until he or she makes an observation of the ball.
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.
Entanglement is when the two balls occupy a shared state. So, however far the two balls 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 may be in any colour.
But how is it possible to know that each ball did not have a set colour at the beginning? Perhaps there was some ‘hidden variable’ that instructed each ball which colour to take when the box was opened! How does one rule out such possibilities?
This is where the Bell’s inequalities come in. Bell’s inequalities are a theoretical insight that makes it possible to differentiate between two scenarios – one, that the indeterminacy of the colour of the balls is purely a quantum phenomenon, and two, that there are hidden variables that determine the colour when opened.
Nobel Prize 2022
This year’s Nobel Prize was awarded to John Clauser and Alain Aspect for devising sophisticated experiments to test the above cases and establish, through Bell’s inequality, the existence of entanglement.
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 the material.
The work of the three laureates can help in developing quantum technologies of the future, for example, quantum cryptography, quantum computation and precise timekeeping as is done in atomic clocks.