No, quantum computing did not come of age with Google’s Sycamore, a 53-qubit computer solving in 200 seconds a problem that would take even a supercomputer 10,000 years. Instead, it is the first step, showing that a functional computation can be done with a quantum computer, and it does indeed solve a special class of problems much faster than conventional computers.
This is not evidence that quantum computers have now superseded classical computers. The hype is misleading as it is based on a very narrow definition quantum supremacy; quantum supremacy is one quantum computer beating all classical computers for one specially constructed task.
The bad news – for the science-fiction enthusiasts – is that it is not going to replace our current computers but will be useful only for a special class of problems. Its construction requires conditions such as super-low temperatures that can be created only in a special environment. We are not going to wear it on our sleeves, or use it on our mobile phones. At least not yet, and not with today’s physics. And our encryption algorithms on which all our Internet protocols and the world’s financial transactions are based are safe, at least for now.
Nevertheless, this is a huge step forward, in which billions are being spent by nations and companies in the belief that this will open up areas of computations that have been closed for now. This is the first example of where a specific problem has been solved, even if it is a very particular one and specially created for quantum computing. Such problems could be extended to the real world: They could be about creating novel materials, new drugs to attack autoimmune illnesses and different types of cancers. These findings could slingshot humanity’s understanding of the best building materials for space, or provide insight into cures for a range of illnesses or diseases that we are not yet able to treat.
Quantum supremacy has been watched by cryptographers with dread. Throughout the world, all Internet protocols, financial transactions, and blockchain-based systems (such as Bitcoin) work via cryptography. With quantum computers, all current cryptography including the popular RSA private-public key systems would be easily broken, a real nightmare for nation-states and financial players that rely on such encryption systems. Of course, new systems can be created that would withstand such quantum tools, but at a huge cost.
It seems that a researcher at the US National Aeronautics and Space Administration inadvertently loaded a draft version of a research paper from Google’s team into NASA’s server for technical papers. As it so happened, Google Scholar routinely scours such servers, and notified the existence of this paper to quantum researchers worldwide. It was Google that outed Google’s unpublished paper.
The paper – “Quantum Supremacy Using a Programmable Superconducting Processor,” by Google’s AI Quantum and collaborators as authors – has now been removed from the NASA server, but the cat is out of the bag.
How is a quantum computer different from a traditional one?
All our computers in everyday use – “classical computers” – store information in bits that exist as either a 0 or a 1 (False=0, True=1). The quantum computer has quantum bits – qubits – that exist in two different states simultaneously, using the quantum phenomenon of superposition. This allows a quantum computer to have many more states, and therefore much more information than what a classical computer does.
The second difference between the classical computer bits and the qubits is that quantum phenomena have another property, that of entanglement. Each of the qubits is entangled with other qubits, giving rise to many more possible states that they can be in together, than each of them could be in individually. In other words, the same number of qubits can handle a much larger number of possibilities than can classical bits, and therefore compute a certain class of problems much faster. And the beauty of such computations is that the size of the problem does not really matter: The qubits working together crunch the bigger problems almost as fast as smaller problems.
The laws of physics in the subatomic world – quantum physics – differ from the everyday physics with which we are familiar. The possibility of a quantum computer, first proposed by the late Nobel laureate Richard Feynman, was that a computer running on quantum principles would solve quantum world problems in physics and chemistry. That means that if we are trying to simulate quantum phenomena on computing machines, the only way to compute the quantum world in reasonable time would be to build the machines themselves on quantum phenomena, that is, quantum computers. It was in essence a thought experiment by Feynman to show why quantum simulations on computers based on classical physics would not work in quantum physics.
So why do the machines based on classical physics not work on quantum phenomena? Simply put, the calculations of such systems would grow exponentially with the size of the system – or how far in the future the state of the system being computed is. Any effort to understand the future states in the quantum world does not have definitive answers, but rather distributions of probability, which is exactly how a quantum computer provides its results.
While the quantum computers have an advantage in handling a large class of problems, because of the nature of the quantum phenomena and the extreme environment – near zero – in which the qubits exist, any small change or disturbance in the environment produces noise, meaning it introduces errors in the calculations. A quantum computer therefore needs an error-correction mechanism, in effect additional qubits for this task. Given that the qubit environment is a very difficult one, this may require a fairly large number of qubits for error correction. By the way, all digital communications including within classical computers also need error correction. The question here is the number of qubits we have to set aside for error correction.
Classical computers won’t be replaced by quantum computers; they are a complementary technology. They address a different class of problems than classical computers, particularly those that deal with probabilities rather than definite solutions. These are also the problems that cannot be solved by classical computers, at least in any reasonable time frame.
So what is the use of such a machine? Let us look at the flip side. Charles Babbage, the “father of computers,” had used machines with mechanical devices that could be programmed for solving mathematical problems. The technology, various mechanical devices, could do individual arithmetic operations according to a “program” and solve problems that were difficult to solve by hand. But they were too costly to make and cumbersome to run. It is the Babbage machines that have now morphed into actual computers after we mastered the technology of vacuum tubes, and later electronic circuits. It took about a hundred years. The current qubit technologies are analogous to Babbage machines. So the future will combine both: a combination of classical and quantum processors so that a problem can be partitioned into its classical and quantum parts.
Currently, all governments have a huge stake in hack-proofing of their digital infrastructure and using quantum communications for this purpose. No country can be left behind, at least if it has geopolitical aspirations – but only the US and China are serious players in the field at the moment.
The US is currently ahead of the game with Google, IBM building 50-qubit architecture, and others close behind. D-Wave Systems, a Canadian company, has a functioning 2,048-qubit system, and is building a much larger 5,000-qubit machine. But these can solve only a special class of problems, and they are not programmable general-purpose quantum computers that others are building.
The Chinese are behind the US and its companies in the quantum-computer race, but have achieved a functional 18-qubit entanglement, a different approach to quantum computing, and a 24-qubit machine. China does seem to have many more researchers in this area, and also a larger number of patents and research papers. All other countries exploring quantum computers, including in the European Union and India, are spending only a fraction of what the US and China are spending.
Google’s latest exercise has brought quantum computing – or a technology embedding uncertainty – knocking at our door. When and how it will enter is still to be seen. But it is no longer a question of if, but when.