What is a quantum computer? The question feels deceptively simple. It is a computer that uses quantum effects to run calculations – or at least, that is what I have been saying to friends and acquaintances for years. Because I report about the latest advances in quantum computing nearly constantly, I have felt confident in this answer. Recently, however, I have had to re-examine that confidence. A more detailed question makes clear why: though I can name many different components and functions of a quantum computer, could I identify exactly when they all become a quantum computer? One recent philosophical study suggests that this is at the root of the difficulty of forecasting the future of quantum computing. For Christophe Jurczak at investment firm Quantonation, it all started at a conference that had nothing to do with quantum computing. Researchers were discussing a different emerging technology, and Jurczak realised that he had first heard about it 30 years ago. Back then, everyone was saying that it was only “five years away”. Déjà vu. Then, a worry set in – could quantum computers also be such a “perpetual five-year technology” (PFYT)? This drove him to do a year-long deep dive into philosophical literature. The paper I read was its outcome. In it, he argues that, right now, quantum computers are firmly in the PFYT category. All attempts to forecast when quantum computers will truly arrive are misguided so long as they presuppose that we already know what quantum computers are, he writes. “If the identity of a quantum computer is still being settled as the machine develops, that presupposition fails.” Advertisement As someone who thinks about quantum computing an awful lot, I wanted to know what exactly brought Jurczak to this somewhat alarming conclusion. In the nearly five years that I