Quantum computing is an endeavor that's been promised to upend everything from codebreaking, to drug development, to machine learning. With so much hype, it's easy to get lost marveling at the possibilities, without grasping what quantum computing actually is.

Quantum computers work differently from the classical computers. Exploiting the principles of quantum mechanics, they can easily tackle computational problems that may be tough for the classical computer as the size of the numbers and number of inputs involved grows bigger. Quantum computers do not look like desktops or laptops that we associate the word ‘computer’ with. Instead (and there are only a handful of them) they resemble the air-conditioned server rooms of many offices or the stacks of central processing units from desktops of yore that are connected by ungainly tangled wires and heaped in freezing rooms. Conventional computers process information in ‘bits’ or 1s and 0s, following classical physics under which our computers can process a ‘1’ or a ‘0’ at a time. The world’s most powerful super computer today can juggle 148,000 trillion operations in a second and requires about 9000 IBM CPUs connected in a particular combination to achieve this feat. Quantum computers compute in ‘qubits’ (or quantum bits). They exploit the properties of quantum mechanics, the science that governs how matter behaves on the atomic scale. In this scheme of things, processors can be a 1 and a 0 simultaneously, a state called quantum superposition. While this accelerates the speed of computation, a machine with less than a 100 qubits can solve problems with a lot of data that are even theoretically beyond the capabilities of the most powerful supercomputers. Because of quantum superposition, a quantum computer — if it works to plan — can mimic several classical computers working in parallel. 

Quantum supremacy:

Google claims to have built the first quantum computer that can carry out calculations beyond the ability of today’s most powerful supercomputers, a landmark moment that has been hotly anticipated by researchers. A paper by Google’s researchers claimed that their processor was able to perform a calculation in three minutes and 20 seconds that would take today’s most advanced classical computer, known as Summit, approximately 10,000 years. The researchers said this meant the “quantum supremacy”, when quantum computers carry out calculations that had previously been impossible, had been achieved.

The system can only perform a single, highly technical calculation, according to the researchers, and the use of quantum machines to solve practical problems is still years away. But the Google researchers called it “a milestone towards full-scale quantum computing”. They also predicted that the power of quantum machines would expand at a “double exponential rate”, compared to the exponential rate of Moore’s Law, which has driven advances in silicon chips in the first era of computing. While prototypes of so-called quantum computers do exist, developed by companies ranging from IBM to start-ups such as Rigetti Computing, they can only perform the same tasks classical computers can, albeit quicker. Quantum computers, if they can be built at scale, will harness properties that extend beyond the limits of classical physics to offer exponential gains in computing power.

Quantum winter:

Although grand claims have been made about a looming revolution in computing, and private investment has been flowing into quantum technology, it is still early days, and no one is sure whether it is even possible to build a useful quantum computer. Some researchers have raised the possibility that, if quantum computers fail to deliver anything of use soon, a quantum winter will descend: enthusiasm will wane and funding will dry up before researchers get anywhere close to building full-scale machines. 

The hope is that quantum computers could eventually help revolutionize our understanding of chemistry and material science by performing simulations that are too complicated for classical computers. Quantum computers could also be used to crack some forms of encryption that keep the internet secure.

Quantum computing in India:
There are no quantum computers in India yet. In 2018, the Department of Science & Technology unveiled a programme called Quantum-Enabled Science & Technology (QuEST) and committed to investing ?80 crore over the next three years to accelerate research. The ostensible plan is to have a quantum computer built in India within the next decade. Phase-1 of the problem involves hiring research experts and establishing teams with the know-how to physically build such systems.