Clifford quantum cellular automata: Trivial group in 2D and Witt group in 3D
Abstract
We study locality preserving automorphisms of operator algebras on Ddimensional uniform lattices of prime pdimensional qudits quantum cellular automata (QCAs), specializing in those that are translation invariant (TI), and map every prime pdimensional Pauli matrix to a tensor product of Pauli matrices (Clifford). We associate antihermitian forms of the unit determinant over Laurent polynomial rings to TI Clifford QCA with lattice boundaries and prove that the form determines the QCA up to Clifford circuits and shifts (trivial). It follows that every 2D TI Clifford QCA is trivial since the antihermitian form in this case is always trivial. Furthermore, we prove that for any D, the fourth power of any TI Clifford QCA is trivial. We present explicit examples of nontrivial TI Clifford QCA for D = 3 and any odd prime p and show that the Witt group of the finite field Fp is a subgroup of the group C(D=3,p) of all TI Clifford QCA modulo trivial ones. That is, C(D=3,p≡1mod4)⊇Z2×Z2 and C(D=3,p≡3mod4)⊇Z4. The examples are found by disentangling the ground state of a commuting Pauli Hamiltonian, which is constructed by coupling layers of prime dimensional toric codes such that an exposed surface has an anomalous topological order that is not realizable by commuting Pauli Hamiltonians strictly in two dimensions. In an appendix independent of the main body of this paper, we revisit a recent theorem of Freedman and Hastings that any twodimensional QCA, which is not necessarily Clifford or translation invariant, is a constant depth quantum circuit followed by a shift. We give a more direct proof of the theorem without using any ancillas.
 Publication:

Journal of Mathematical Physics
 Pub Date:
 September 2021
 DOI:
 10.1063/5.0022185
 arXiv:
 arXiv:1907.02075
 Bibcode:
 2021JMP....62i2202H
 Keywords:

 Quantum Physics;
 Condensed Matter  Strongly Correlated Electrons;
 Mathematical Physics
 EPrint:
 43 pages and a calculation note in Mathematica, (v2, v3) a new section on boundary antihermitian forms (v4) more detail (v5) fixing Lemma III.11