Noise limits quantum circuits
Better noise control is needed, or carefully engineered designs to exploit specific noise properties.
A theoretical analysis of noise build-up in quantum circuits concludes that noise sets a practical limit on how many operational steps can be applied one after another.
Researchers from EPFL, Switzerland; the Free University of Berlin, Germany; and the University of Copenhagen, Denmark, collaborated on the project, which involved analysing large families of quantum circuits made of simple two-qubit operations, with realistic noise affecting each individual qubit following each step.
They tracked how each layer's influence propagates through the circuit when noise is present. They demonstrated that only the final steps affect operations in most noisy quantum circuits.
In other words, a quantum computer that is tasked with estimating a physical quantity, such as the energy or the state of a qubit, will only produce results that reflect what happens towards the circuit’s end point. Earlier operations ‘fade from memory’ as noise builds up.
Critically, this explains why noisy circuits can still be adjusted or ‘trained’ for simple tasks. The researchers explain that changing a circuit’s settings will alter the result, but only because the final layers remain active.
The scientists conclude that it is only possible to continue to train noisy circuits because noise has already weakened most of the power. Treating real hardware noise as a simple blur can lead to false expectations, they warn.
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