How laureates showed quantum effects on a life-size scale

EVen people without a downplay in scientific discipline ar familiar spirit with Schrödinger’s true cat because of the sheer audacity of the idea: the cat in a box is both alive and dead at the same time until you actually observe it, at which time it is definitely dead. Although he was one of the pioneers of quantum physics, Erwin Schrödinger’s thought experiment showed that the ideas of quantum mechanics cannot easily be understood when viewed in terms of real-world objects.Harnessing the strange properties of the quantum world for real-world use is the key to the future. Quantum computers work at speeds several orders of magnitude higher than that of classical computers, but building them for mass use remains a challenge. The key area being explored — superconducting electrical circuits — was, however, shown as far back as 1984-85, by physicists John Clarke (currently with University of California Berkeley), Michel H Devoret (Yale University) and John M Martinis (UC Santa Barbara). Their work won them the Nobel Prize for Physics on Thursday.To understand the gap they helped bridge, consider a quantum particle, which is said to exist in multiple possible states simultaneously until an observation causes it to collapse into one definite state. Compare that with Schrödinger’s cat: how can it be dead and alive at the same time? The three laureates demonstrated quantum properties on a scale somewhere in between that of a particle and a cat — a system large enough to be held in the hand. They carried out an experiment in UC Berkeley with a superconducting electrical system that “tunnelled” from one state into another, with measurements matching the predictions of quantum mechanics.A cat, of course, is too large to enable scientists to demonstrate its quantum properties in the laboratory. Nevertheless, physicists such as Anthony Leggett (Nobel winner in 2003) have compared the work of Clarke, Devoret and Martinis with Schrödinger’s thought experiment. Legget has argued that the three scientists’ experiments showed that vast numbers of particles together can indeed behave just as quantum mechanics predicts.Micro vs macroImagine you are hitting a ball against a wall, a standard example of the scale at which quantum properties are not observable. The ball will bounce back every time. A single particle, on the other hand, can sometimes pass through an equivalent barrier in its microscopic world, and appear on the other side in a different state.“What bothered everyone was that a cricket ball follows the laws of classical physics but it is made up of atoms which obey quantum physics. So why does that happen?” said R Vijayaraghavan, who has worked separately with all three Nobel laureates and is currently a quantum physicist at the Tata Institute of Fundamental Research, Mumbai.Vijayaraghavan was Devoret’s first PhD student at Yale. “I have also shared publications with the other two laureates. I was a postdoc at UC Berkeley and collaborated with Clarke and his PhD student on a joint project. I also collaborated with Martinis on ultra-low noise parametric amplifiers,” he told HT.To return to the comparison between the ball and a particle, the phenomenon of the particle crossing a barrier and emerging in a different state is called tunnelling. To recreate this at a macroscopic level, the goal was to create a setup composed of multiple particles rather than a single one, and look for quantum mechanical effects there, Vijayaraghavan said.The answer lay in superconducting material, through which current passes with zero resistance. In such systems, the electrons synchronise themselves into pairs, called Cooper pairs (after the physicist Leon Cooper, Nobel winner in 1972) that behave as a single system. And if two superconductors are joined together with a thin insulating barrier between them, it creates a Josephson junction (after Brian Josephson, Nobel winner in 1973).The laureates exploited this existing knowledge.A tour de forceAt UC Berkeley, Clarke was leading a research group that included Devoret as a postdoc, and Martinis as a doctoral student. After precisely creating and refined the experimental setup, they fed a weak current into the Josephson junction and measured the voltage (initially zero) and the time it took for the system to tunnel out of this state, causing a voltage. Measurements showed that the energy levels matched the predictions of quantum mechanics.“This experiment showed that when you isolate a macroscopic collective degree of freedom composed of many particles by careful engineering, you can see quantum mechanical effects like tunnelling and quantised energy levels. This is possible to do in many different types of systems but the difficulty of engineering can vary from one to the other. The experiment was a tour de force in achieving the right conditions for observing quantum mechanical effects in an electrical circuit,” Vijayaraghavan said.It was a big step towards harnessing quantum mechanics for real-world applications. “This experimental result set the stage for making quantum bits using superconducting electrical circuits, which is now one of the leading platforms for building practical quantum computers,” Vijayaraghavan said.“And because these devices were made using techniques similar to those for making semiconductor electronics devices, one could imagine making them on a large enough scale to make practical devices.”