  Quantum computers have been proposed to solve a number of important problems
such as discovering new drugs, new catalysts for fertilizer production,
breaking encryption protocols, optimizing financial portfolios, or implementing
new artificial intelligence applications. Yet, to date, a simple task such as
multiplying 3 by 5 is beyond existing quantum hardware. This article examines
the difficulties that would need to be solved for quantum computers to live up
to their promises. I discuss the whole stack of technologies that has been
envisioned to build a quantum computer from the top layers (the actual
algorithms and associated applications) down to the very bottom ones (the
quantum hardware, its control electronics, cryogeny, etc.) while not forgetting
the crucial intermediate layer of quantum error correction.
