  In 2017, John Preskill defined Noisy Intermediate Scale Quantum (NISQ)
computers as an intermediate step on the road to large scale error corrected
fault-tolerant quantum computers (FTQC). The NISQ regime corresponds to noisy
qubit quantum computers with the potential to solve actual problems of some
commercial value faster than conventional supercomputers, or consuming less
energy. Over five years on, it is a good time to review the situation. While
rapid progress is being made with quantum hardware and algorithms, and many
recent experimental demonstrations, no one has yet successfully implemented a
use case matching the original definition of the NISQ regime. This paper
investigates the space, fidelity and time resources of various NISQ algorithms
and highlights several contradictions between NISQ requirements and actual as
well as future quantum hardware capabilities. It then covers various techniques
which could help like qubit fidelities improvements, various breeds of quantum
error mitigation methods, analog/digital hybridization, using specific qubit
types like multimode photons as well as quantum annealers and analog quantum
computers (aka quantum simulators or programmable Hamiltonian simulators) which
seem closer to delivering useful applications although they have their own mid
to longer-term scalability challenges. Given all the constraints of these
various solutions, it seems possible to expect some practical use cases for
NISQ systems, but with a very narrow window before various scaling issues show
up. Turning to the future, a scenario can be envisioned where NISQ will not
necessarily be an intermediate step on the road to FTQC. Instead, the two may
develop along different paths, due to their different requirements. This leaves
open a key question on the trade-offs that may be necessary to make between
qubit scale and qubit fidelities in future quantum computers designs.
