Abstract

The classical simulation of highly entangling quantum dynamics is conjectured to be generically hard. Thus, recently discovered measurement-induced transitions between highly entangling and low-entanglement dynamics are phase transitions in classical simulability. Here, we study simulability transitions beyond entanglement: noting that some highly entangling dynamics (e.g., integrable systems or Clifford circuits) are easy to classically simulate, thus requiring “magic”—a subtle form of quantum resource—to achieve computational hardness, we ask how the dynamics of magic competes with measurements. We study the resulting “dynamical magic transitions” focusing on random monitored Clifford circuits doped by 𝑇 gates (injecting magic). We identify dynamical “stabilizer purification”—the collapse of a superposition of stabilizer states by measurements—as the mechanism driving this transition. We find cases where transitions in magic and entanglement coincide, but also others with a magic and simulability transition in a highly (volume-law) entangled phase. In establishing our results, we use Pauli-based computation, a scheme distilling the quantum essence of the dynamics to a magic state register subject to mutually commuting measurements. We link stabilizer purification to “magic fragmentation” wherein these measurements separate into disjoint, 𝒪⁡(1)-weight blocks, and relate this to the spread of magic in the original circuit becoming arrested.

Personal information

Campbell McLauchlan is a postdoctoral researcher working at the quantum theory group at the University of Sydney. He obtained his PhD from the University of Cambridge in 2024, working under Benjamin Béri on topics in quantum computing and many-body physics. From 2023-2024 he gained experience working on topics in quantum error-correction at Riverlane Ltd, as a PhD intern and quantum scientist, and at University College London, as a postdoc. Prior to starting his PhD, he obtained a Bachelor of Science (honours) from the University of Sydney and a Masters from the University of Cambridge. His research interests are in quantum error-correction, quantum computing, and dynamical quantum phenomena.

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