How many qubits are needed to out-perform conventional computers, how to protect a quantum computer from the effects of decoherence and how to design more than 1000 qubits fault-tolerant large scale quantum computers, these are the three basic questions we want to deal in this article. Qubit technologies, qubit quality, qubit count, qubit connectivity and qubit architectures are the five key areas of quantum computing are discussed. Earlier we have discussed 7 Core Qubit Technologies for Quantum Computing, 7 Key Requirements for Quantum Computing. Spin-orbit Coupling Qubits for Quantum Computing and AI, Quantum Computing Algorithms for Artificial Intelligence, Quantum Computing and Artificial Intelligence, Quantum Computing with Many World Interpretation Scopes and Challenges and Quantum Computer with Superconductivity at Room Temperature. Here, we will focus on practical issues related to designing large-scale quantum computers.

A computer that uses the quirks of quantum physics to work on data should be capable of things far beyond any machine in use today. Governments and large tech companies have spent huge sums trying to prove out that idea. Yet quantum computers have sometimes seemed like one of those technologies that are always 20 years away. Recently some leading research groups have come to think they can see a path to shortening that time considerably. Yesterday Google and researchers from the University of the Basque Country in Bilbao, Spain, published results that could lead to a shortcut to the long-awaited first conclusive demonstration of the power of quantum computing.

Quantum computing is now ready to go – or is it? Google appears to have reached an impressive milestone known as quantum supremacy, where a quantum computer is able to perform a calculation that is practically impossible for a classical one. But there are plenty of hurdles left to jump over before the technology hits the big time. For a start, the processors need to be more powerful. Unlike classical computers, which store data as either a 0 or a 1, quantum computers use qubits that store data as a mixture of these two states.

Chips (above) hold quantum bits that are at the heart of the search for a universal quantum computer. For 30 years, researchers have pursued the universal quantum computer, a device that could solve any computational problem, with varying degrees of success. Now, a team in California and Spain has made an experimental prototype of such a device that can solve a wide range of problems in fields such as chemistry and physics, and has the potential to be scaled up to larger systems. Both IBM and a Canadian company called D-Wave have created functioning quantum computers using different approaches. But their devices are not easily scalable to the many quantum bits (qubits) needed for solving problems that classical computers cannot.

Google's cryostats reach temperatures of 10 millikelvin to run its quantum processors. From aspects of quantum entanglement to chemical reactions with large molecules, many features of the world cannot be described efficiently with conventional computers based on binary logic. The solution, as physicist Richard Feynman realized three decades ago1, is to use quantum processors that adopt a blend of classical states simultaneously, as matter does. Many technical hurdles must be overcome for such quantum machines to be practical, however. These include noise control and improving the fidelity of operations acting on the quantum states that encode the information.