Monitored quantum circuits exhibit entanglement transitions at certain measurement rates. Such a transition separates phases characterized by how much information an observer can learn from the measurement outcomes. We study SU(2)symmetric monitored quantum circuits, using exact numerics and a mapping onto an effective statisticalmechanics model. Due to the symmetry's nonAbelian nature, measuring qubit pairs allows for nontrivial entanglement scaling even in the measurementonly limit. We find a transition between a volumelaw entangled phase and a critical phase whose diffusive purification dynamics emerge from the nonAbelian symmetry. Additionally, we identify a “spinsharpening transition.” Across the transition, the rate at which measurements reveal information about the total spin quantum number changes parametrically with system size.
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Observation of measurementinduced quantum phases in a trappedion quantum computer
Manybody open quantum systems balance internal dynamics against decoherence from interactions with an environment. Here, we explore this balance via random quantum circuits implemented on a trapped ion quantum computer, where the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a faulttolerent threshold. We probe the "pure" phase, where the system is rapidly projected to a deterministic state conditioned on the measurement outcomes, and the "mixed" or "coding" phase, where the initial state becomes partially encoded into a quantum error correcting codespace. We find convincing evidence of the two phases and show numerically that, with modest system scaling, critical properties of the transition clearly emerge.
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 Award ID(s):
 1818914
 NSFPAR ID:
 10339334
 Date Published:
 Journal Name:
 ArXivorg
 ISSN:
 23318422
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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