A Century of Quantum Revolution

Dr. Sanjeev Kumar Varshney Advisor and Head, International Cooperation, Department of Science & Technology, Government of India. Dr. Punit Kumar Associate Professor, Department of Physics, University of Lucknow

2023-12-02 16:42:39

Credit: pixabay.com

Credit: pixabay.com

The natural phenomena have been a subject of interest for both the scientist and philosophers for the past many centuries. Till the first quarter of the 20th century, the thought was based on principles of Classical Physics, a concept tracing its origin to the French philosopher Rene Descartes, who considered the universe as a clockwork machine moving onwards to eternity.

As per this school of thought, the future is fully predictable and all phenomena can be described precisely by the laws of classical physics. The first decades of the 20th century witnessed the advent of a new approach to physics theory called the quantum mechanics. Classical Physics is successful in explaining the macroscopic phenomena, but fail at microscopic level or in other words the atomic scale. Alternatively, we can say that the reality of the world is not what we gather through our senses. In the backdrop of every object, we sense there is another shadow world of uncertainty and potentiality which is quite different in orientation. The transition between the macro and micro world and how the microscopic theory emerges from the microscopic is a puzzle for philosophers and physicists. The theory explaining the microworld is the quantum mechanics and is one of the most successful theories ever created and has revolutionarised the perception of the world. In spite of being successful in picturising the microworld, the theory has some baffling points as Feynman once remarked that nobody can understand the quantum mechanics completely. Similar remark was made by Steven Weinberg by saying that there is no satisfactory interpretation of quantum mechanics.

In classical theory or Newtonian physics, the properties of particle or system can be explained to a great precision and this uses position and momentum to describe the state of system. The evolution of the system is governed by its trajectory which can be traced if the initial conditions are known. From these initial conditions, the position and momentum of all the particles in the system or rather in the universe can be predicted to a high degree of accuracy. Thus, classical theory talks in terms of objective reality and is quite independent of human observers. This predictable nature of classical theory lead to determinism that alongwith the Newtonian mechanics and the European philosophers dominated the philosophy until the upcoming of the quantum concepts.

As per the common belief, there must be a cause to every phenomena observed and on this pillar the structure of classical physics was built. The classical physics predicts the experiment results and universe is like a mindless machine. The classical concept promoted the materialistic way of living with no room for emotions and aspirations. The Indian philosophy of Karma where every event is predetermined and destined closely resembles the classical concepts.

Where the classical concepts failed ?

  1. Black body radiation - classical physics was not successful in explaining the spectrum of blood body radiation which was commonly called as UV catastrophe. There was disagreement between the theory and the experiment. According to the classical theory the frequency of light increases as the radiant energy density approaches infinity, which is contradictory to the experimental observations where the radiant energy decreases as the frequency increases in the UV spectrum.

Max Planck in 1900 was successful in explaining the black body radiation through  his relation. While establishing his relation, Planck assumed that the energy of oscillators is quantized. This assumption of quantization of energy was a revolutionary idea that laid the foundation of the new concepts of quantum physics.

  1. Photoelectric effect : Classical physics, which considers light as a wave whose amplitude is related to intensity. The phenomena of photoelectric effect involving emission of photoelectrons when light falls on a metal surface was tried to be explained by the classical theory considering that metal’s electrons start or oscillating resonantly with the light wave and ultimately breeds up from the metal’s surface with kinetic energy depending on intensity of the incident radiation. On the contrary, the experimental studies showed that the kinetic energy of ejected electrons is independent of the intensity of the incident radiation. It was also noticed that the electrons were ejected only when the frequency of the incident radiation exceeds a threshold value.

Einstein applied the Planck’s hypothesis to explain the experimental observations of the photoelectric effect. He postulated that light travels in bundles of energy ‘photons’ rather than a classical wave, and showed that the energy of incident photon is equal to the sum of the kinetic energy of the ejected photoelectron and a part of energy (wave fuction) is spend on releasing the electron from the particular metal. This model successfully explained the experimental observations. Using the results of the photoelectric experiments, Einstein was able to establish the value of Planck's constant, which was in agreement to the value estimated by Planck himself. This supported the Planck’s hypothesis and supported the concept of quantum physics which still was at crossroads as the scientists were still divided in opinion.

The quantum concept

The quantum concept rejects the classical one and at the microscopic level, an electron behaves both like a wave as well as a particle denoting the dual nature. The trajectory of such a particle cannot be predicted exactly and hence the aspect of determinism turns out to be invalid in quantum scenario, wherein the particle behaviour is expressed in probabilities, a concept that flows from the Heisenberg’s principle. Thus follows that the universe is highly unpredictable and uncertain.

It was in 1930’s that witnessed the emergence of the first quantum revolution in the world. Neils Bohr, attempted to link the classical and quantum thoughts through his correspondence principle. The three main concepts that lead to the development of quantum physics are (i) Planck’s introduction of the concept of quantization of energy, (ii) Bohr’s theory with serves as a bridge between the two concepts and (iii) quantum theory as given by Heisenberg and Schrodinger. Neils Bohr, followed the idea of Planck’s quantum hypothesis and postulated the quantization of angular momentum of electrons. Although, his postulate was successful in explaining certain aspects of spectroscopy but could not describe the spectrum of Hydrogen atom exactly. Sommerfield, modified this model to remove the drawbacks.

 The atomic theory postulated by Bohr and the Corpuscular theory propounded by Einstein, got wild acceptance and they together laid the foundation of quantum revolution. The origin of the quantum revolution can be traced back to the mid 1920’s with the matrix formulation by Heisenberg, Max Born and Pascual Jordan. Later on, the wave mechanics was developed by Louis de Broglie and Erwin Schrodinger, followed by formulation of quantum statistics by Fermi-Dirac and Bose-Einstein. Paul Dirac formulated the relativistic quantum mechanics and the complete quantum theory was adopted in the Solvay congress of 1927 and this popularly known as Copenhagen interpretation of quantum mechanics. By the end of the first quarter of the 20th century, quantum mechanics was recognized as the right theory to explain matter and radiation at the microlevel. Gradually, quantum mechanics established as a science where the emphasis was on statistical knowledge of reality based on philosophical nature of observer. In the coming years, quantum mechanics widened its scope into quantum chemistry, quantum electronics, quantum optics, quantum information science, etc. More recent developments are in the quantum gravity theories.

As per the quantum hypothesis, the probabilistic nature is not a temporary feature that in future will be replaced by a deterministic theory. Einstein, who was one of the pioneers of the quantum theory was not happy with the absence of exactness and mentioned a possibility of a local hidden variable theory which was later on expressed as Einstein-Podolsky-Rosen (EPR) paradox.

Classical versus Quantum view

 We live in a world governed by the inertial laws of classical physics. Once we keep an object at a place, it remains there we can't imagine it to be found at a different place at a later time, until and unless somebody transfer it to the new position. If a football player aims at the goal, he can't expect that the ball will be in another ground at a distant place i.e., a certain degree of exactness is expected and we can estimate the behaviour of object knowing its initial coordinates.

The quantum world on the other hand is totally a different scenario, where estimation is less reliable and nature of behaviour is unpredictable. There are many paths possible and the prediction can only be done in terms of probability. In such a case, ISRO can't simulate the landing of Chandrayaan on surface of the Moon.

The postulates of quantum mechanics have proved to be experimentally correct to a high level of accuracy. As goes the correspondence principle, where all objects follow the quantum mechanics and classical mechanics is just an average approximation for large objects.

Philosophy of Quantum theory

            Aristotle the great philosopher, who is also regarded as the originator of modern science believed that there are two sets of laws in physics, one that are valid for terrestrial sphere and a separate set for celestial sphere. Later, Newton unified the two sets by postulating his universal law of gravitation. Gradually, it has been established that most of the Physics laws are universally invariant.

The Newtonian world was mechanical and nurtured the determinism philosophy advocated by Ren Destcartes, the famous French philosopher. The Newtonian cartesian notions created by dichotomies of mind-body, subject-object, facts-values, feeling-thought, poetry-prose and science-fiction. This philosophy contributed tremendously to the development of science and technology, thereby influencing the industrial revolutions and contributing to the establishment of the modern society. The dichotomies were not only limited to science, but also influenced the social sciences and governed the human behaviour. John Locke propounded the atomistic view of society, stressing the individual behaviour which result in a socio-economic order based on individualism. Ultimately, scientific rationalism and materialism formed the basis of a capitalistic society.

 The quantum theory rejects the view based on Newtonian cartesian system and thus a holistic approach is expected to emerge where unification concept will prevail in science and society replacing the atomistic view.

The 2nd quantum revolution

Quantum technology is the application of the concepts of quantum physics to the development of new techniques to design and control the complex systems. There are mainly two motivating factors towards quantum technology. The first one concerns with the trend for ultrasmall devices, making physics reach the nano or atomic scales, where the systems to be developed ought to be based on quantum principles. The second concern is the thirst for optimum performance which doesn't seem viable in the classical framework. In the first quantum revolution, the postulates of quantum mechanics were applied in understanding what already existed. The second quantum revolution is concerned with the development of technology on the laid principles of quantum mechanics encompassing quantum entanglement, quantum superposition and quantum tunnelling. The emerging fields of quantum dots and excitons, quantum computing, sensors, cryptography, simulation, measurement and imaging, constitute what is popularly called the second quantum revolution

Quantum technologies

Nuclear power constitutes the major portion of our energy requirement. We must not forget that it is the quantum mechanics through which we get a clear understanding of the nuclear structure and how the constituent particles of the nucleus behave. It is only by the knowledge of quantum physics that we can harness the power of the nucleus for securing nuclear energy. Time is crucial for efficient working of any machine or a network. Accurate time is monitored by using the atomic clocks where time in is measured in terms of electronic transition from one state to another. Laser which have countless applications in science and  technology are based on the Einstein’s  Quantum Theory of Radiation. These days doctors rely on MRI scan for diagnosis which has been possible only by the knowledge of nuclear magnetic resonance and the quantum theory of spin. Semiconductors which are the backbone of the electronics industry exhibit semiconductivity which is the phenomena based on quantum mechanics. Monitors including a television use liquid crystals and light emitting diodes based on quantum technology to display high resolution images. Quantum computers are the talk of the day due to there immense potential in terms of speed and efficiency. Cyber keys are applied for cyber security are also based on the quantum principles. Quantum technology is also being used in research to study the molecular behaviour and designing novel materials for devices. Recently, scientists have also been successful in demonstrating quantum teleportation by teleporting photon from Earth to a satellite at a distance of 1400 km from the Earth. Although the technological innovations in quantum mechanics are helping us in leading a better life, but there is still plenty of work to be done in developing newer technologies that will rock our daily lives.

Quantum information technology - The earliest attempt in this area was done by Einstein, Podolsky and Rosen (EPR) in 1935, wherein they commented on the possible correlations in quantum systems. These nonlocal EPR effects were experimentally studied in 1980s in a table-top quantum optical experiment conducted by Clauser. The experiments started gaining maturity in 1994, when a group of scientist from a British defence agency established that nonlocal photon correlations could be used to make an unbreakable quantum cryptographic key distribution through optical fibre upto a distance of 4 km. Thus, quantum entanglement turned into a technological tool from a concept. There is the worldwide effort going on to build simple quantum processors that could serve as powerful machines for code breaking.

Quantum algorithms are outline of computer programs designed to run on a quantum computer. These are based on nonlocal, quantum entanglement and some of such programs are exponentially faster than the corresponding classical programs. A well known program is the Grover’s algorithm that is manyfold faster in searching for a random database with application ranging from data mining, optimisation to code breaking.

Quantum cryptography constitutes the development of quantum key distribution systems using the optical fibre, thereby allowing the transmission of quantum entangled photons over large distances. The Quantum cryptographic keys applied in this are expected to be nonviable to the uncertainties as propounded by Heisenberg’s principle.

Quantum information theory deals with the analysis of information processing using the quantum principles. The classical information theory needs to be modified and new quantum analogs of theories like Shannon’s theorem have already been developed. Efforts are going on to achieve new targets in quantum data compression, which will ensure an error free operation of a quantum computer. Studies concerning quantum decoherence and noise to achieve optimum results from communication channels are being conducted.

Single spin magneto resonance force microscopy - The quantum electromechanical system is an ultrasmall system with action corresponding to the order of Planck’s constant and incorporating transducers operating at quantum limit. These devices are sensitive enough to sense the magnetic moment of a single spin and apply in force microscopy.

Coherent quantum electronics - The past decade has witnessed the development of microfabricated electronic devices which operate at low temperatures and the transport is mainly through elastic scattering. Such devices constitute what is called mesoscopic electronics. These are nonohmic circuits and the conduction is quantized in terms of quantum of resistance. A two dimensional motion constitute a quantum wire which show quantum conductance. Similarly, a three dimension restricted device is called the quantum dot.

Superconducting quantum circuits are a type of quantum coherent electronic devices and include one or two Josephson junctions which is like an insulating barrier between two superconductors. Such devices are most sensitive in detecting magnetic fields to the limit that they can be employed to detect magnetic signature of electric pulses flowing in human brain.

Quantum photonics - When an electron - hole pair (exciton) is confined in a quantum dot, it behaves like an artificial atom. Quantum dot excitons are significant in quantum electronics where conduction of photons occur through principles of quantum optics thereby leading to what is being termed as quantum optoelectronics. Such devices are significant in developing a single photon source that will be crucial in quantum key distribution and quantum optical coupling.

Spintronics - The particle spin significantly effect the transport properties of charge careers and the spin dependent transport (spintronics) have attracted attention of scientists and technocrats as it has potential applications in information process and storage.

 Molecular coherent quantum electronics - There are attempts to use molecules information in storage and processing elements for computation and gradually the molecular electronics is becoming an essential component of computer architecture. Mainly substance are in employed in biological systems.

Quantum optical interferometry - It has been shown then the quantum photon entanglement has immense potential in optical interferometry and increase the sensitivity by many orders of magnitude. Typical applications include fibre optical gyroscopes, ground and orbiting optical interferometers for gravity wave detection.

Quantum lithography and microscopy - The quantum entanglement has revolutionarised the quantum interferometric lithography and has brought a revolution in lithographic resolution by suppressing the diffraction process.

Quantum teleportation is one of the most promising application in the transport of quantum information and quantum states. The only limit is that in order to copy a quantum state to a distant location, the original state needs to be destroyed. The same state can't exist at both the places.

Challenges ahead

As we know that quantum technology is revolutionary and promise a better future in all aspects, but on the other hand they have put some challenges before the scientists, engineers and entrepreneurs. A few of such challenges to be faced by the technocrats in the times to come are –

Lack of determinism : The quantum devices are inherently probabilistic, rather than deterministic. This probabilistic nature will surely limit the efficiency of the system as it will generate probabilities for a number of possible solutions.

Logic circuits : The quantum logic circuits are a set of instruction sequences rather than electronics circuits. They constitute a series of instructions do be executed in a sequence. The wave function the main basis of quantum technology.

Collapse : The quantum wave function represents the complete state of a quantum device in terms of probability. For each of the quantum states, we cannot examine the wave function of a quantum system without causing it to collapse. Although, the collapse of a wave function is not a complete loss, but the monitoring mechanism doesn't seem to be exact.

Error correction : The quantum technology is very sensitive to noise (disturbance) and interact actively with environment there by producing errors in working and downgrading the performance. Therefore, reliable error management scheme is required for such devices.

Scalability : The quantum technologies have demonstrated very highly efficient performance, but they are still relatively smaller as compared to the classical devices. Quantum devices require scaling to higher levels and simultaneously maintaining the levels of coherence is a major challenge.

Hardware development : There are many quantum technologies, each having their own strengths and weaknesses. There is a need to develop a scalable fault correction technology. Software development : The quantum algorithms and software development tools are still in their primitive stages of development and there is a requirement of new programming languages, compilers and other efficiency management tools to harness the quantum technology to the maximum possible extent.

Classical interface : The quantum technology is not going to replace the classical one completely and there is a dire need of an interface mechanism that could tackle the efficient transferring of data and instructions between the classical and the quantum devices, which is essential for practical applications.

Standards and protocols : A well defined set of standards and protocols for all the tools using quantum technology need to be developed essentially to ensure better performance.

Human resource : Trained quantum manpower is available only in small numbers at present and finding the right person for a job is difficult. Proper training modules need to be developed for ensuring availability of skilled manpower.

Overall expense : The quantum technology is in a stage of constant research and development, which requires huge inflow of capital. For ensuring this, a strong bridge between the institutional scientists and the entrepreneurs is the need of the hour.

Qubit quality : The large scale working qubits capable of generating instructions and controlling operations are required and at present the cubits available on cloud based quantum computers are not enough to operate the large scale systems. Errors have been noticed in operations running between two qubits which resulted in wrong results.

Too many wires : Today we need multiple wires, or multiple light sources like lasers to create a qubit. Imagine what would happen when we try to construct a million qubit chip, as this will involve many millions of wires connected to the device. Mechanism needs to be develop to reduce the amount of wires.

The Indian scenario

Quantum Technology has wide range of applications in the fields of computing, cyber security and communication. New frontiers with numerous economic value are expected to come up from the ongoing theoretical studies. The First attempt to accelerate the development of quantum capabilities in India was initiated in 2017 by the Interdisciplinary Cyber Physical Systems (ICPS) division of the Department of Science and Technology (DST) by launching the Quantum Information Science and Technology program (QuST). The program was aimed at identifying quantum technology as a forerunner, creating path for subsequent programs. The program envisioned to revolutionarize the future computation and communication systems, their influence on nation and our society. The program aimed at encouraging scientists and academicians. In 2018, the DST renamed the project QuST as Quantum Enabled Science and Technology (QuEST). A capital outlay of rupees eight thousand crores was provided for a period of five years in 2020, formally announcing the quantum mission which is popularly known as the National Mission on Quantum technology and Applications (NMQTA). Although the mission is on its way, it has produced significant results in the development of indigenous quantum communication technologies contributing to the India's quantum ecosystem.

As we have seen that the concept of quantum physics is a theoretical concept whose application to the modern technology has come up in the recent times. The research and development in any field of Physics has two branches that is theoretical and experimental. In India, scientists are working on both the aspects so in the Indian Quantum Mission, both the aspects have been kept in mind while designing the focus and core areas.

Theoretical research includes development of quantum algorithms that will contribute to the techniques of pattern recognition and photon sampling, development of simulation codes to transform theories into applications by making use of the number theory, sampling and analytical protocol development of quantum information system, development of quantum control and feedback mechanism along with hardware development to minimise noise, development of quantum enabled communication that will revolutionarise the information transfer with high speed and accuracy in a safe environment. Experiments will include development of colloidal semiconductors, promotion of research in quantum optics, developing technology to achieve ion trapping that will be useful in controlling the qubit and will also find applications in spectroscopy,  promotion of research concerning qubit design, quantum control, quantum topology,  development of low temperature devices as these will be necessary for working of quantum devices.

The responsibility of coordinating and managing the resources concerning the research and development of quantum technology have been interested to the Centre for Development of Quantum Technology (C-DOQ) and the research units will be spread throughout the country. C-DOQ will be the coordinating agency between various ministries and departments as a closed coordination is required due to the fact that field of quantum technology is an interdisciplinary field and efficient output can only be extracted by properly coordinated effort of scientists from all the branches. C-DOQ will be setting up Technology Business Incubators (TBIs) and with also develop centres of excellence. The centre will also publish education material for all levels of education which will help in producing skilled manpower to work with devices based on Quantum Technology. The mission governing board will be supported by various committees is like Scientific Advisory Committee (SAC) and the Inter Departmental Stakeholders Committee (IDSC). The mission will be sourced from public funds and no prospects of private funding have been indicated.

In April 2023, the Union Minister for Information & Broadcasting informed that an amount of over rupees six thousand core has been provided in the budget for the National Quantum Mission. The mission envisages to promote transformative technology including quantum communication, quantum computers and computing, quantum key distribution, quantum sensing, encryption, crypt analysis, quantum clock, etc. Under the mission, focus will be on fundamental science, developing technology, generating human and infrastructure resources and innovations leading to start-ups. The government’s organisational and financial support will ensure that both the private and the public sectors are benefited and this will establish standards for research which will help in simulating a pipeline to support research and applications in the future also. The aforesaid  development will be of immense importance and will boost the sectors of aerospace engineering, simulations, weather prediction, safer financial transactions, high speed and clear communication, cyber security, manufacturing, education, health,  agriculture with special focus on his skilled jobs, start-ups, entrepreneurship leading to economic growth of the country there by establishing a quantum economy and influencing the quality of life of the masses.