After decades of research, quantum computers are approaching the scale at which they could outperform their "classical" counterparts on some problems. They will be truly practical, however, only when they implement quantum error correction, which combines many physical quantum bits, or qubits, into a logical qubit that preserves its quantum information even when its constituents are disrupted. Although this task once seemed impossible, theorists have developed multiple techniques for doing so, including "surface codes" that could be implemented in an integrated-circuit-like planar geometry. For ordinary binary data, errors can be corrected, for example, using the majority rule: A desired bit, whether 1 or 0, is first triplicated as 111 or 000. Later, even if one of the three bits has been corrupted, the other two "outvote" it and allow recovery of the original data.

After decades of heavy slog with no promise of success, quantum computing is suddenly buzzing with almost feverish excitement and activity. Nearly two years ago, IBM made a quantum computer available to the world: the 5-quantum-bit (qubit) resource they now call (a little awkwardly) the IBM Q experience. That seemed more like a toy for researchers than a way of getting any serious number crunching done. But 70,000 users worldwide have registered for it, and the qubit count in this resource has now quadrupled. In the past few months, IBM and Intel have announced that they have made quantum computers with 50 and 49 qubits, respectively, and Google is thought to have one waiting in the wings. "There is a lot of energy in the community, and the recent progress is immense," said physicist Jens Eisert of the Free University of Berlin.

Here we discussed the advantages and limitations of seven key qubit technologies for designing efficient quantum computing systems. The seven qubit types are: Superconducting qubits, Quantum dots qubits, Trapped Ion Qubits, Photonic qubits, Defect-based qubits, Topological Qubits, and Nuclear Magnetic Resonance (NMR) . They are the seven pathways for designing effective quantum computing systems. Each one of them have their own limitations and advantages. We have also discussed the hierarchies of qubit types.

The high technological and strategic stakes mean major technology companies as well as ambitious start-ups and government-funded research centers are all in the race to build the world's first universal quantum computer. In contrast to today's classical computers, where information is encoded in bits (0 or 1), quantum computers process information stored in quantum bits (qubits). These are hosted by quantum mechanical objects like electrons, the negatively charged particles of an atom. Quantum states can also be binary and can be put in one of two possibilities, or effectively both at the same time--known as quantum superposition--offering an exponentially larger computational space with an increasing number of qubits. This unique data crunching power is further boosted by entanglement, another magical property of quantum mechanics where the state of one qubit is able to dictate the state of another qubit without any physical connection, making them all 1's for example.

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 that quantum computers have now superseded classical computers. The hype of quantum supremacy 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.