From time crystals to wormholes: When is a quantum simulation real?

WHEN scientists reported they had created a space-time wormhole in November last year, the world’s media were all over the story, even though they struggled to make sense of it. A journalist for the website UNILAD put it neatly when they wrote: “So, you might have to bear with us here a bit, because it’s all very complicated and new.”

As far as many observers could see, physicist Maria Spiropulu at the California Institute of Technology and her colleagues had in fact merely used a quantum computer to simulate a wormhole. Good luck flying a spaceship through that. What confused matters was that the team insisted the work amounted to more than just a simulation. The quantum computation, the researchers said, was fully equivalent to the creation of a wormhole.

If you find that hard to swallow, you aren’t alone. Ask other physicists about Spiropulu’s claims and you tend to get a lot of long pauses, chin-stroking and disagreement. It seems there is genuine confusion about if and when a quantum computation can create real entities or just simulate them.

The putative wormhole isn’t the only thing said to have been conjured up by quantum computers recently – there is also the alluringly named time crystal, as well as strange particles called nonabelions, touted as the ideal ingredient for next-generation quantum computers. But whether these amount to instances of true creation or not is a question that takes us into deep waters. It is a new twist on the riddle that has haunted physics since quantum mechanics was devised in the early 20th century: what is truly real?

Regular computers use …