Posted

Jon Nelson, Joel Rajakumar, Michael J. Gullans (Oct 09 2025).
Abstract: It has been known for almost 30 years that quantum circuits with interspersed depolarizing noise converge to the uniform distribution at ω(logn)\omega(\log n) depth, where nn is the number of qubits, making them classically simulable. We show that under the realistic constraint of geometric locality, this bound is loose: these circuits become classically simulable at even shallower depths. Unlike prior work in this regime, we consider sampling from worst-case circuits and noise of any constant strength. First, we prove that the output distribution of any noisy geometrically local quantum circuit can be approximately sampled from in quasipolynomial time, when its depth exceeds a fixed Θ(logn)\Theta(\log n) critical threshold which depends on the noise strength. This scaling in nn was previously only obtained for noisy random quantum circuits (Aharonov et. al, STOC 2023). We further conjecture that our bound is still loose and that a Θ(1)\Theta(1)-depth threshold suffices for simulability due to a percolation effect. To support this, we provide analytical evidence together with a candidate efficient algorithm. Our results rely on new information-theoretic properties of the output states of noisy shallow quantum circuits, which may be of broad interest. On a fundamental level, we demonstrate that unitary quantum processes in constant dimensions are more fragile to noise than previously understood.

Order by:

Want to join this discussion?

Join our community today and start discussing with our members by participating in exciting events, competitions, and challenges. Sign up now to engage with quantum experts!