Tasks involving black boxes appear frequently in the theory of quantum information, with quantum channel discrimination as a central example that has been deeply studied. In this work, we experimentally study the discrimination between two unitary quantum channels in the multiple-shot scenario. We challenge the theoretical results concerning the probability of correct discrimination with the results collected from experiments performed on the IBM Brisbane. Our analysis shows that neither too deep quantum circuits nor circuits that create too much entanglement are suitable for the discrimination task. We conclude that circuit architectures which minimize entanglement overhead while preserving discrimination power are significantly more resilient to hardware noise if their depth does not exceed a threshold value. Consequently, our findings necessitate a paradigm shift: for execution on noisy hardware, the theoretically suboptimal circuit is, counterintuitively, often the superior choice.

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Nature

Researchers experimentally tested quantum channel discrimination on IBM Brisbane quantum hardware, comparing theoretical predictions with real-world performance. The study reveals that theoretically optimal circuits with deep architectures or high entanglement perform poorly on noisy hardware. Instead, simpler circuits that minimize entanglement while maintaining discrimination capability prove more resilient to hardware noise, provided circuit depth stays below a threshold. This finding challenges conventional wisdom: on current noisy quantum computers, theoretically suboptimal approaches often deliver superior practical results.

Experimental study of multiple-shot unitary channels discrimination using the IBM Q computers