SET S&T-0810:00(Rev.1)( 8102025) : Quantum Tunnelling: Nobel Prize in Physics,2025 :
CIVIL SERVICES (PRELIMS), 2026
NOTES ON Science & Technology : Nobel Prize in Physics, 2025
Topic : Quantum Tunnelling,Physics (for G S
Papers) {Prepared on 8 .10.2025 }
For Study purpose only
NB: For any doubts clarification, please refer to the recommended text books
TOPIC : Nobel Prize in Physics,2025
(with questions already asked in previous
UPSC(CSE)(Prelims) Exams.
MULTIPLE CHOICE QUESTIONS
TOPIC : Nobel Prize in Physics ,2025
Question: Who are winners of Nobel Prize for Physics in 2025 ?
Ans : John Clarke, Michel Devoret and John Martinis
Question : Why John Clarke , Michel Devoret and John Martinis are selected for Nobel Prize for Physics in 2025 ?
Ans : “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”
The Nobel Prize Laureates in Physics for 2025, John Clarke, Michel H. Devoret and John M. Martinis, used a series of experiments to demonstrate that the bizarre properties of the quantum world can be made concrete in a system big enough to be held in the hand. Their superconducting electrical system could tunnel from one state to another, as if it were passing straight through a wall. They also showed that the system absorbed and emitted energy in doses of specific sizes, just as predicted by quantum mechanics.
Microscopic phenomena :
Quantum mechanics describes properties that are significant on a scale that involves single particles. In quantum physics, these phenomena are called microscopic, even when they are much smaller than can be seen using an optical microscope.
Macroscopic phenomena :
The above contrasts with macroscopic phenomena, which consist of a large number of particles. For example, an everyday ball is built up of an astronomical amount of molecules and displays no quantum mechanical effects. We know that the ball will bounce back every time it is thrown at a wall. A single particle, however, will sometimes pass straight through an equivalent barrier in its microscopic world and appear on the other side. This quantum mechanical phenomenon is called tunnelling.
This year’s Nobel Prize in Physics recognises experiments that demonstrated how quantum tunnelling can be observed on a macroscopic scale, involving many particles. In 1984 and 1985, John Clarke, Michel Devoret and John Martinis conducted a series of experiments at the University of California, Berkeley. They built an electrical circuit with two superconductors, components that can conduct a current without any electrical resistance. They separated these with a thin layer of material that did not conduct any current at all. In this experiment, they showed that they could control and investigate a phenomenon in which all the charged particles in the superconductor behave in unison, as if they are a single particle that fills the entire circuit.
Tunnelling is a quantum mechanical process, which entails that chance plays a role. Some types of atomic nuclei have a tall, wide barrier, so it can take a long while for a piece of the nucleus to appear outside it, while other types decay more easily. If we only look at a single
atom, we cannot predict when this will happen, but by watching the decay of a large number of nuclei of the same type, we can measure an expected time before tunnelling occurs. The most common way of describing this is through the concept of half-life, which is how long it takes for half the nuclei in a sample to decay
Cooper Pairs :
In an ordinary conductive material, current fows because there are electrons that are free to move through the entire material. In some materials, the individual electrons that push their way through the conductor may become organised, forming a synchronised dance that fows
without any resistance. The material has become a superconductor and the electrons are joined together as pairs. These are called Cooper pairs, after Leon Cooper who, along with John Bardeen and Robert Schriefer, provided a detailed description of how superconductors work (Nobel Prize in Physics 1972)
Cooper pairs behave completely differently to ordinary electrons. Electrons have a great deal of integrity and like to stay at a distance from each other – two electrons cannot be in the same place if they have the same properties. We can see this in an atom, for example, where the electrons divide themselves into different energy levels, called shells. However, when the electrons in a superconductor join up as pairs, they lose a bit of their individuality; while two separate electrons are always distinct, two Cooper pairs can be exactly the same. This means the Cooper pairs in a superconductor can be described as a single unit, one quantum mechanical system. In the language of quantum mechanics, they are then described as a single wave function. This wave function describes the probability of observing the system in a given state and with given properties.
Josephson Junction :
If two superconductors are joined together with a thin insulating barrier between them, it creates a Josephson junction. This component is named after Brian Josephson, who performed quantum mechanical calculations for the junction. He discovered that interesting phenomena arise when the wave functions on each side of the junction are considered (Nobel Prize in Physics 1973). The Josephson junction rapidly found areas of application, including in precise measurements of fundamental physical constants and magnetic felds. The construction also provided tools for exploring the fundamentals of quantum physics in a new way. One person who did so was Anthony Leggett (Nobel Prize in Physics 2003), whose theoretical work on macroscopic quantum tunnelling at a Josephson junction inspired new types of experiments
For macroscopic quantum tunnelling :
By the mid-1980s, Michel Devoret had joined John Clarke’s research group as a postdoc, after receiving his doctorate in Paris. This group also included the doctoral student John Martinis. Together, they took on the challenge of demonstrating macroscopic quantum tunnelling. Vast amounts of care and precision were necessary to screen the experimental setup from all the interference that could affect it. They succeeded in refining and measuring all the properties of their electrical circuit, allowing them to understand it in detail.
To measure the quantum phenomena, they fed a weak current into the Josephson junction and measured the voltage, which is related to the electrical resistance in the circuit. The voltage over the Josephson junction was initially zero, as expected. This is because the wave function for the system is enclosed in a state that does not allow a voltage to arise. Then they studied how long it took for the system to tunnel out of this state, causing a voltage. Because quantum mechanics entails an element of chance, they took numerous measurements and plotted their results as graphs, from which they could read the duration of the zero-voltage state. This is similar to how measurements of the halflives of atomic nuclei are based on statistics of numerous instances of decay.
Practical and theoretical benefits :
This experiment has consequences for the understanding of quantum mechanics. Other types of quantum mechanical effects that are demonstrated on the macroscopic scale are composed of many tiny individual pieces and their separate quantum properties. The microscopic components are then combined to cause macroscopic phenomena such as lasers, superconductors and superfluid liquids. However, this experiment instead created a macroscopic effect – a measurable voltage – from a state that is in itself macroscopic, in the form of a common wave function for vast numbers of particles.
Legget has argued that the series of experiments conducted by John Clarke, Michel Devoret and John Martinis showed that there are phenomena that involve vast numbers of particles which together behave just as quantum mechanics predicts. The macroscopic system that consists of many Cooper pairs is still many orders of magnitude smaller than a kitten – but because the experiment measures the quantum mechanical properties that apply to the system as a whole, for a quantum physicist it is fairly similar to Schrödinger’s imaginary cat
This type of macroscopic quantum state offers new potential for experiments using the phenomena that govern the microscopic world of particles. It can be regarded as a form of artificial atom on a large scale – an atom with cables and sockets that can be connected into new test set-ups or utilised in new quantum technology. For example, artifcial atoms are used to simulate other quantum systems and aid in understanding them.
Another example is the quantum computer experiment subsequently performed by Martinis, in which he utilised exactly the energy quantisation that he and the other two laureates had demonstrated. He used a circuit with quantised states as information-bearing units – a quantum bit. The lowest energy state and the frst step upward functioned as zero and one, respectively. Superconducting circuits are one of the techniques being explored in attempts to construct a future quantum computer.
==============================================================
Introduction :
Soon after the publication of Erwin Schrödinger’s equation in 1926 (awarded with a Nobel Prize in 1933), solutions were found where the wavefunction penetrates into classically forbidden regions, i.e. where the total energy of the particle was lower than its potential energy in the region. Although the wavefunction is exponentially decaying under the barrier, for finite length barriers, the wavefunction exists also on the other side of the barrier. Thus, there exists a finite probability for the particle to pass the barrier, although it does not have enough energy to do so classically.
What is Tunnelling ?
An early successful application of this theory was the explanation of alpha decay, where the alpha particle is confined in the nucleus by a potential barrier but has a finite probability to tunnel through this barrier. Tunnelling also explained why radioactive decay is a probabilistic process, where the half-life crucially depends on height and thickness of the potential barrier.
Why Tunnelling is necessary in Sun ?
Tunnelling is also necessary for fusion to occur in our Sun, where the temperature and pressure are actually too low to classically allow two protons to overcome the Coulomb repulsion and form a helium nucleus.
Quantum Tunnelling :
Quantum tunnelling is important not only for radioactive decay: the 1973 Nobel Prize in Physics was awarded with one half to Leo Esaki and Ivar Giaever for their experimental discoveries regarding electron tunnelling phenomena in semiconductors and superconductors, respectively. Giaver’s 1960 experiments confirmed the existence of an energy gap in superconductors, something predicted by John Bardeen, Leon N. Cooper and Robert Schrieffer in 1957. Their BCS theory was awarded with the Nobel Prize in Physics 1972. The other half of the 1973 Nobel Prize in Physics was awarded to Brian Josephson, whose theoretical predictions are essential also for this year’s Prize
Cooper Pairs :
The BCS theory posits that electrons, being fermions, pair up into so-called Cooper pairs, which are composite bosons. To a very good approximation, the BCS ground state can be understood as a macroscopic Bose-Einstein condensate of these bosonic Cooper pairs. This state is described by a complex order parameter, which in many respects can be thought of as an effective wavefunction of the center of mass of the condensed Cooper pairs
Josephson effect :
In 1962 Brian Josephson predicted that Cooper pairs can tunnel without resistance across an insulating barrier giving rise to a zero-voltage current across a tunnel barrier between two superconductors. This so-called Josephson effect was experimentally confirmed at Bell Labs as early as 1963, and in 1964, a very sensitive magnetometer called the Superconducting Quantum Interference Device (SQUID) was developed at Ford Research Labs
Macroscopic Quantum Tunnelling (MQT) :
Tunnelling also explained why radioactive decay is a probabilistic process, where the half-life crucially depends on height and thickness of the potential barrier.
Tunnelling is also necessary for fusion to occur in our Sun, where the temperature and pressure are actually too low to classically allow two protons to overcome the Coulomb repulsion and form a helium nucleus
Quantum tunnelling is important not only for radioactive decay: the 1973 Nobel Prize in Physics was awarded with one half to Leo Esaki and Ivar Giaever for their experimental
discoveries regarding electron tunnelling phenomena in semiconductors and superconductors, respectively
The macroscopic quantum state is much more sensitive to interactions with its environment.
Such states are sometimes called “cat states”, named after Schrödinger’s famous thought experiment. Upon decay, a radioactive substance triggers a hammer to break a bottle of poison.
The microscopic superposition inherent in radioactive decay, e.g. due to tunnelling of an alpha particle, is thereby coherently connected to the superposition of a living and dead cat.
Schrödinger’s cat illustrated the absurdity of quantum physics at the macroscopic scale. In practice, the cat superposition would decay extremely fast due to interaction with the environment.
What if a smaller version of the Schrödinger’s cat thought experiment could actually be performed in superconducting or superfluid systems? This was one of the questions formulated by Leggett in 1978 . He suggested that quantum tunnelling between the two macroscopically distinct components of such a cat state could be realized in superconducting circuits, at milliKelvin temperatures. One reason that such Macroscopic Quantum Tunneling (MQT) might be found in superconducting circuits is that their very low resistance indicates that they have very weak coupling to dissipative degrees of freedom in the environment
Uses :
1)Today, a qubit design called “Transmon” is insensitive to charge noise and used in a number of efforts around the world, aiming to realize a large-scale quantum computer. Here, we note that superconducting circuits is only one among a number of promising technologies used in this global effort.
2)Beyond qubits (quantum bits ) , superconducting quantum circuits have impacted the field of quantum optics which traditionally studies the interplay between atoms and the electromagnetic field. Using superconducting circuits, new artificial atoms based on Josephson junctions are designed, allowing for the study of quantum optics in parameter regimes not accessible to atomic physics . Superconducting circuits are also used to probe the quantum nature of other macroscopic solid-state systems , such as micromechanical resonators and large spin ensembles.
3)Recently, superconducting circuits placed in a 30-m-long cryostat were used for a loophole-free violation of Bell’s inequality .
These are only a few of the numerous examples of how macroscopic quantum physics with superconducting circuits has impacted quantum science and played an important role in the formation of a diverse and expanding field of quantum engineering.
No comments:
Post a Comment