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Title: Critical exponents of the two-dimensional quantum contact process
Plain: In a lattice of Rydberg atoms where an excited atom helps its neighbours become excited and excitations decay, activity either spreads or dies out. Whether quantum coherence changes the universal behaviour of this transition in two dimensions is disputed.
Precise: Quantum contact process: Lindblad dynamics with coherent facilitated flips $\Omega \sum_i (\sum_{j\,\mathrm{nn}\,i} n_j) \sigma^{x}_{i}$, decay at rate $\gamma$, and optional classical branching, on a square lattice. Determine the nature of the absorbing-state transition at $\gamma/\Omega$ of order one: directed percolation, tricritical, first order, or a new class. Answer: exponents $\beta, \nu_{\mathrm{perp}}, z$, or a demonstration of first-order behaviour.
Settled by: Large-scale 2D tensor-network or quantum-jump numerics with finite-size scaling, or Rydberg-array data.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: contested
Status note: In 1D, tensor-network work (Carollo et al. 2019) supports directed percolation; mean-field and some 2D studies indicate a first-order transition.
Posed since: (empty)