Physicists May Have Found a Hard Limit on The Performance of Large Quantum Computers

For circuit-based quantum computations, the achievable circuit complexity is limited by the quality of timekeeping. That's according to a new analysis published in the journal Physical Review Letters exploring 'the effect of imperfect timekeeping on controlled quantum dynamics.'

An announcement from the Vienna University of Technology explains its significance. 'The research team was able to show that since no clock has an infinite amount of energy available (or generates an infinite amount of entropy), it can never have perfect resolution and perfect precision at the same time. This sets fundamental limits to the possibilities of quantum computers.'

ScienceAlert writes:
While the issue isn't exactly pressing, our ability to grow systems based on quantum operations from backroom prototypes into practical number-crunching behemoths will depend on how well we can reliably dissect the days into ever finer portions. This is a feat the researchers say will become increasingly more challenging...

'Time measurement always has to do with entropy,' says senior author Marcus Huber, a systems engineer who leads a research group in the intersection of Quantum Information and Quantum Thermodynamics at the Vienna University of Technology. In their recently published theorem, Huber and his team lay out the logic that connects entropy as a thermodynamic phenomenon with resolution, demonstrating that unless you've got infinite energy at your fingertips, your fast-ticking clock will eventually run into precision problems. Or as the study's first author, theoretical physicist Florian Meier puts it, 'That means: Either the clock works quickly or it works precisely — both are not possible at the same time....'

[F]or technologies like quantum computing, which rely on the temperamental nature of particles hovering on the edge of existence, timing is everything. This isn't a big problem when the number of particles is small. As they increase in number, the risk any one of them could be knocked out of their quantum critical state rises, leaving less and less time to carry out the necessary computations... This appears to be the first time researchers have looked at the physics of timekeeping itself as a potential obstacle. 'Currently, the accuracy of quantum computers is still limited by other factors, for example the precision of the components used or electromagnetic fields,' says Huber. 'But our calculations also show that today we are not far from the regime in which the fundamental limits of time measurement play the decisive role.'

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