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New Delhi: The National Quantum Mission approved by the Union cabinet last week is a venture into the unknown, and not just because quantum computing is still a fledgling field of study. The clich is also true of the very science that the quest for quantum computers is based on.

Quantum mechanics is a mysterious world where a particle can exist in two states at once, or when a cat, famously named after Erwin Schrdinger, is both alive and dead (or neither) provided you dont look at it, because when you do, it will definitely be dead.

I think I can safely say that nobody understands quantum mechanics, the legendary American physicist Richard Feynman said at Cornell Universitys Messenger Lectures in 1964. The following year, he would win the Nobel Prize for Physics for his work on quantum mechanics.

From quantum mechanics emerged the quest for quantum computers a couple of decades later, seeking to harness the strange properties of nature at atomic levels. Such computers, in theory, would be several times faster than traditional computers.

In fact, it was none other than Feynman who, in 1981, proposed the idea of finding a computer simulation of physics. The real use of it would be with quantum mechanics Nature isnt classical and if you want to make a simulation of nature, youd better make it quantum mechanical, and by golly its a wonderful problem, because it doesnt look so easy, he said at a conference organised by the Massachusetts Institute of Technology and IBM.

Quantum computing is one of the four domains for which thematic hubs will be set up in top academic and national research and development institutes under the National Quantum Mission approved last week with a budget of 6,000 crore. The other three domains are quantum communication, which seeks to transmit information that would be difficult to eavesdrop on; quantum sensing and metrology, or the use of quantum phenomena to make precise measurements; and quantum materials and devices, such materials being solids with exotic properties.

Quantum technology is a field where research still has miles to go, especially as far as building a "usable" quantum computer is concerned. However, some key milestones have been reached, particularly over the last couple of decades.

Classical physics cannot explain much of the behaviour of matter and energy at subatomic levels, but can still explain much of the physical world. Quantum mechanics studies matter at the atomic and subatomic levels, where the laws of classical physics cease to apply.

In Feynmans words, Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.

The physicist said this in a lecture delivered at the California Institute of Technology. You can view it on the universitys website.

Quantum mechanics is all about weird concepts: wave-particle duality, a property that allows matter and energy, such as light, to behave both as a wave and as a stream of particles; superposition, when an object exists in multiple possible states at the same time; and entanglement, when two or more particles or photons can exist in a shared state, both behaving the same way, even if they are far apart. The 2022 Nobel Prize for Physics honoured Alain Aspect, John Clauser and Anton Zeilinger for their work on superposition.

While the benefits of quantum mechanics have engaged scientists for generations, these depend on the problem that one aims to address, said Apoorva Patel, convener of the Quantum Technology Initiative at the Indian Institute of Science (IISc).

Quantum physics was invented because certain physical phenomena could not be explained at all by classical theories. The practical advantage can be proven when such phenomena are at the core of the problems to be tackled, he said, citing the examples of superposition and entanglement among others.

Of course, classical theories explain many physical phenomena, and when that is the case, quantum technology will hardly offer any advantage in addressing them, Patel said.

The challenge is that quantum dynamics is highly fragile. Environmental disturbances rapidly destroy quantum signals. So, the quantum effects can be observed only in highly protected and cooperative settings. They do not survive in hostile situations. The need to construct a carefully protected setting is what makes quantum technology expensive, Patel said.

As such, there will be many situations in which classical technology will be more robust, cheaper and efficient. Quantum technology, therefore, will be useful only in special-purpose-devices, he said.

It requires in-depth knowledge of quantum physics to figure out what such devices would be. The rest is all hype, Patel said.

Special-purpose-devices can do many useful things. One obvious answer is that the first rewards will come in the development of high-precision sensors and measuring instruments, which will definitely bring many benefits to society. The real challenges are all in the design of such systems and not in their usage. That is where the investment must be made. Whether the government and industry will pay attention to this or not is a different story, he said.

High-precision sensors and measuring instruments would come under the domain of quantum sensing and metrology. Such sensors are vital to devices such as atomic clocks, platforms used in the making of quantum computers, and various areas of science that require high precision.

A quantum computer would be superior to classical ones in several aspects, key among them being processing speed and stronger encryption of information. A classical computer stores information in terms of bits, which are in the form of combinations of 0s and 1s. A quantum computer, on the other hand, would store information in quantum bits, or qubits. A qubit can be both 0 and 1 at the same time, and because such information can probabilistically exist in multiple forms simultaneously, the information stored rises exponentially with the number of qubits.

In quantum communication, the nature of cryptography would make eavesdropping impossible without being detected. A widely studied method is to transmit a quantum key via a series of photons. If anyone were to eavesdrop on the communication, some of the properties of the key would be altered just like Schrdingers cat would be dead once observed and so the sender of the information would know there has been a breach.

The challenges remain the fragility of quantum states and the design of such systems. At the heart of a quantum computer are its qubits, created as an array of atoms of a suitable element or isotope. These are levitated in free space in a vacuum environment. Storing and manipulating information in this exotic form requires sophisticated control of the underlying materials, Princeton University scientists said in a paper in Science in 2021, and called on materials scientists to take up the challenge of developing hardware for quantum computing.

While a usable quantum computer is still far away, the quest has progressed since Feynmans observations in 1981.

In 1985, Oxford researcher David Deutsch published a theoretical paper describing a universal quantum computer. What provoked greater interest, however, was an algorithm proposed in 1994 by Massachusetts Institute of Technology professor Peter Shor, then working for American telecommunication giant, AT&T.

Shor proposed a method using entanglement of qubits and superposition to find the prime factors of an integer. This was potentially important because finding factors of large numbers is so difficult that many encryption systems exploit this difficulty. Shors idea led to a storm of research, but what he proposed in theory proved hard to achieve.

No other algorithms to rival the potential of Shors were found. Despite disappointment, momentum was not lost and the field branched into different directions, Nature observed in an editorial, 40 years of quantum computing, in January 2022.

During the current century, universities and companies have made further strides. According to Washington University, the record for the highest number of qubits is currently 72, on a chip developed by Google. In 2017, Microsoft released Q#, a language for quantum algorithms. And in January of 2019, IBM announced one of the first commercial quantum computers.

Also in 2019, Google announced that its collaborators at the University of California, Santa Barbara, had achieved quantum supremacy, the stage at which a quantum computer performs tasks that a classical computer cannot. The universitys researchers claimed to have developed a processor that took 200 seconds to do a calculation that would have taken a classical computer 10,000 years. The claim was, however, disputed by IBM.

Most existing quantum computers use metal-insulator-metal sandwiches that are turned into superconducting qubits, by being lowered to extremely low temperatures, a write-up on the US department of energy website notes. But, scientists routinely using quantum computers to answer scientific questions is a long way off, it added.

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Quantum computing: Where we are now, and how we got there - Hindustan Times