Quantum computing, which was proposed in the 1980s, is just beginning to become available, albeit with small numbers of qubits, high decay rates, and significant amounts of noise. As we’ll discuss, IBM and Microsoft are beginning to offer quantum computer and quantum simulator access in their clouds.
Elsewhere, Google has demonstrated quantum computing capabilities (and claimed quantum supremacy) in its labs, but hasn’t announced public access. Intel is also working on quantum chips and systems, but hasn’t announced commercial availability.
Quantum computing definitions
Qubits are quantum bits. Classical bits have two possible values, 0 and 1. Quantum bits can take on an infinite number of values, all of which are combinations or superpositions of the two classical states. Any two-dimensional column vector of real or complex numbers with norm 1 represents a possible quantum state held by a qubit.
You can picture that in 3D as vectors lying anywhere on the surface of a unit sphere, called a Bloch sphere (see image below). When you measure a qubit in a superposed state, the quantum state will randomly resolve to one of the two classical states. You can also explicitly set a qubit to a classical value (0 or 1). Measuring a qubit that has a classical value does not affect its state.