The buzz around the much-anticipated quantum computers became louder following a recent announcement by Google that a quantum computer, built-in their laboratory, had achieved ‘quantum supremacy’. The ‘supremacy’ implies that a quantum computer has solved a problem—which even with the most powerful classical computer available today would have taken about 10,000 years to solve—in just three minutes.
If this claim survives closer scrutiny, it represents as momentous achievement as the first outer space sojourn of artificial satellites. The potential of quantum computers to impact our future could be as much, if not more, as what space exploration had on humanity since the 1950s. Quantum computers are expected to help discover exotic materials for a variety of requirements, provide foolproof cryptographic protection against online frauds, enable drug discovery to fight diseases, design efficient batteries, smarter devices and gadgets through better machine-learning algorithms.
Computing devices of one kind or the other have existed since the 17th century. Though programmable computers were built since the 1930s, ENIAC, commissioned at the University of Pennsylvania in 1946, is regarded as the first general-purpose computer. It weighed about 30 tonnes and occupied an area equal to two badminton courts. Since then, technology has rapidly evolved on all fronts. The smartphones we use today are a million times more powerful than ENIAC. Major improvements came from the invention of transistors, integrated circuits and faster memory devices.
Yet, what had not changed for over a century is how computing itself is done. In all these machines, from primitive computers of the 1930s to modern phones, information is stored in a sequence of bits of 1 and 0. Whether for a computer game on a laptop, or for listening to music on our cellphones, or for computing material properties on a supercomputer, computation is the process of manipulating bit sequences using sophisticated electronic circuits. This is the classical computer model.
However, a quantum computer is fundamentally a different beast and is based on the strange laws of quantum physics. Instead of manipulating bit sequences of 1 and 0, it manipulates quantum bits called qubits. These qubits could be the internal states of atoms manipulated by lasers. Unlike one bit that can store one unit of information, qubits can store a lot more. To store the data generated by Twitter per minute, classical computers will use 10^9 bits of storage space, but remarkably a quantum computer, in principle, needs just about 31 qubits. Unprecedented power of the quantum computer comes from its inherent parallelism, the ability to manipulate a large collection of qubits in one shot in ways that a classical computer will not be able to match up.
Imagine having to locate a lost car in a large city. The ‘classical’ solution would be to start walking from your present position, search all the roads one by one until the car is found. In the quantum approach, you will start walking from the same position, but every time you hit a road junction, say with four roads, you replicate yourself into four persons and continue searching until the car is located. Weird as the quantum process might appear, it is likely to locate the car much faster. It indeed does so. Quantum computers developed in the last decade are proof that this weird idea actually works in practice. Google’s achievement sets the bar high. Google’s quantum processor Sycamore took a few minutes to solve a carefully chosen problem, whereas a powerful classical computer would have taken 10,000 years.
The problem solved by Google, output of a random quantum circuit with 53 qubits, is not of immediate practical interest. However, like all proof of concepts, this is the first attempt at demonstrating quantum supremacy, a somewhat controversial term coined by John Preskill of Caltech in 2012, to indicate that quantum computers can do what classical computers practically cannot. Though Google’s work is published in the prestigious journal Nature, an IBM research team has cast doubts on the claim that a classical computer would take as much as 10,000 years. This debate and experimentation will continue, but will not detract from the fact that quantum computers are for real, not the stuff of science fiction.
Even if this turns out to be a false alarm, no one doubts the disproportionate advantages of a quantum computer. It will only be a matter of time before one is realised. After all, quantum computers are based on sound theoretical foundations, powered by the same laws that have given us lasers, MRI scanners and GPS. Even if Google’s claim is true, we are nowhere near putting quantum computers on every desk. Major scientific and technical problems related to controlling qubits need to be overcome before it becomes a product in the market. Mostly, this might not even happen. Yet, behind
every secure digital transaction in the future, there might be a quantum computer. Decades from now, quantum computers and associated technologies will silently work in the background to power what we might use in our daily lives. We just witnessed the first shot being fired in this pursuit.
M S Santhanam
Physicist and a professor at the Indian Institute of Science Education and Research, Pune