Imagine you're trying to build a very long, complicated chain of dominoes. The aim is that each domino hits the next one perfectly, all the way down the line, producing an amazing result at the end. A quantum circuit is like a domino chain: a long chain of tiny steps ("operations") that work together to process information together in a powerful way.
Now imagine that every domino is a little wobbly. In the quantum circuit, that wobble is called "noise." It might not look like much—after all, all regular systems are subjected to some kind of "noise"—but noise in quantum circuits can accumulate and build up to a crescendo of problems.
The question is, if every domino piece is wobbly—if noise is inevitable and eventually destructive—does it still make sense to build complex chains? Quantum circuits lie at the heart of the next generation of technology, like quantum computers, which promise to solve certain problems far beyond the reach of today's machines.
A team of scientists has now carried out a broad theoretical analysis of how noise affects quantum circuits. Their findings show that noise sets a surprisingly tight practical limit on how deep such circuits can be—in other words, how many steps can be applied one after another in a quantum circuit—while also making parts of them easier to simulate on classical computers.
The work was led by researchers Armando Angrisani and Yihui Quek at EPFL, Antonio Anna Mele at the Free University of Berlin and Daniel Stilck França at the University of Copenhagen. It is published in Nature Physics.
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