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What is quantum supremacy?

Quantum supremacy is a milestone that has been long sought in computing, and now Google has announced it has reached it. A look at the science behind the concept, and what was really achieved and how much remains.

This week, Google announced that it has achieved a breakthrough called quantum supremacy in computing. What does that mean, and why is it important?

So, what is quantum supremacy?
It is a term proposed in 2012 by John Preskill, professor of theoretical physics at the California Institute of Technology. It describes the point where quantum computers can do things that classical computers cannot. In Google’s case, researchers at the University of California, Santa Barbara have claimed to have developed a processor that took 200 seconds to do a calculation that would have taken a classical computer 10,000 years.

But what is a quantum computer?
Our traditional computers work on the basis of the laws of classical physics, specifically by utilising the flow of electricity. A quantum computer, on the other hand, seeks to exploit the laws that govern the behaviour of atoms and subatomic particles. At that tiny scale, many laws of classical physics cease to apply, and the unique laws of quantum physics come into play.

Developing such a computer has been a goal of scientists for nearly four decades. In 1981, the physicist Richard Feynman wrote: “Trying to find a computer simulation of physics seems to me to be an excellent program to follow out… Nature isn’t classical… and if you want to make a simulation of Nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem, because it doesn’t look so easy.”

What difference would such a simulation make?
It is about processing speed. Let us look at how a classical computer processes information. Bits of information are stored as either 0 or 1. Every string of such digits (bitstrings) represents a unique character or instruction; for example, 01100001 represents the lowercase “a”.

In a quantum computer, information is stored in quantum bits, or qubits. And a qubit can be both 0 and 1 at the same time. Quantum physics involves concepts that even physicists describe as weird. Unlike classical physics, in which an object can exist in one place at one time, quantum physics looks at the probabilities of an object being at different points. Existence in multiple states is called superposition, and the relationships among these states is called entanglement.

The higher the number of qubits, the higher the amount of information stored in them. Compared to the information stored in the same number of bits, the information in qubits rises exponentially. That is what makes a quantum computer so powerful. And yet, as Caltech’s Preskill wrote in 2012, building reliable quantum hardware is challenging because of the difficulty of controlling quantum systems accurately.

Is that what Google has achieved?
The researchers demonstrated what a quantum computer is capable of. They built an architecture of 54 qubits with Sycamore, Google’s quantum computer. While one of these did not perform, the other 53 qubits were entangled into a superposition state.

The team composed a random sequence of about 1,000 operations. Each time they then ran this random algorithm, the quantum computer would produce a bitstring.

Now, some bitstrings are more likely to occur than others, and it is possible to identify which ones are more likely. However, the more complex the random quantum circuit, the tougher for a classical computer to identify the likelier bitstrings — and the difficulty grew exponentially. Supremacy was achieved when they demonstrated that the quantum processor just took 200 seconds to compute a super complex random algorithm, while the fastest supercomputer would have taken 10,000 years, Google said in an email.

So, what good does that do?
None, as far as practical applications are concerned. The task performed isn’t super important for this milestone; it’s much more about the fact that the milestone happened in the first place, the email from Google said. It cited the Wright brothers as an analogy: “For them to demonstrate that aviation is possible, it didn’t matter so much where the plane was headed, where it took off and landed, but that it was able to fly at all.”

Is everyone convinced?
IBM has disputed Google’s assertion that its quantum calculation could not be performed by a traditional computer. In a blog post, IBM has claimed that the computation described by the Google researchers could be achieved by an existing computer in less than two-and-a-half days, not 10,000 years.

Incidentally, IBM itself claimed a quantum computation breakthrough on Thursday. Its researchers made a breakthrough in controlling the quantum behaviour of individual atoms, demonstrating a versatile new building block for quantum computation, IBM said on its website. The paper is published in the journal Science. Google’s research appears in Nature.

What next?
The scientists are looking to improve on their work, including detecting and fixing errors. The University of California, Santa Barbara noted that the research has already achieved a very real tool for generating random numbers. Random numbers can be useful in a variety of fields — including protecting encrypted keys for decryption, which could be a potentially thorny issue for governments.

Quantum computers could one day result in huge advances in science research and technology. Among areas that stand to gain are artificial intelligence, and new drug therapies. All that, however, is a long way away.

Source: The Indian Express

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