In 1981, nobel Laureate Richard Feynman challenged the computing community to build a quantum computer. We have come a long way. In 2015, McKinsey estimated there were 7,000 researchers working on quantum computing, with a combined budget of $1.5 billion.20 In 2018, dozens of universities, approximately 30 major companies, and more than a dozen startups had notable R&D efforts.a Now seems like a good time to review the business.
How do quantum computers work? Quantum computers are built around circuits called quantum bits or qubits. One qubit can represent not just 0 or 1 as in traditional digital computers, but 0 or 1 or both simultaneously—a phenomenon called "superposition." A pair of qubits can represent four states, three qubits eight states, and so on. N qubits can represent 2" bits of information, and even 300 qubits can represent information equal to the estimated number of particles in the known universe.21 To perform calculations, qubits exploit superposition and "entanglement." This refers to when two quantum systems (such as an electron or a nucleus), once they interact, become connected and retain a specific correlation in their spin or energy states (which represent combinations of 0 and 1), even if physically separate. Entanglement makes it possible for quantum bits to work together and represent multiple combinations of values simultaneously, rather than represent one combination at a time. Once a calculation is finished, you observe the qubits directly as 0 or 1 values to determine the solution, as with a classical computer.