Undecidability of the Spectral Gap (short version)

The spectral gap - the energy difference between the ground state and first excited state - is central to quantum many-body physics. Many challenging open problems, such as the Haldane conjecture, existence of gapped topological spin liquid phases, or the Yang-Mills gap conjecture, concern spectral gaps. These and other problems are particular cases of the general spectral gap problem: given a quantum many-body Hamiltonian, is it gapped or gapless? Here we prove that this is an undecidable problem. We construct families of quantum spin systems on a 2D lattice with translationally-invariant, nearest-neighbour interactions for which the spectral gap problem is undecidable. This result extends to undecidability of other low energy properties, such as existence of algebraically decaying ground-state correlations. The proof combines Hamiltonian complexity techniques with aperiodic tilings, to construct a Hamiltonian whose ground state encodes the evolution of a quantum phase-estimation algorithm followed by a universal Turing Machine. The spectral gap depends on the outcome of the corresponding Halting Problem. Our result implies that there exists no algorithm to determine whether an arbitrary model is gapped or gapless. It also implies that there exist models for which the presence or absence of a spectral gap is independent of the axioms of mathematics.

Note:

v2: published version. 7 pages, 3 figures. See long companion paper arXiv:1502.04573 (same title and authors) for full technical details. v3: Added supplementary material as ancilliary file (previously part of arXiv:1502.04573v2)

gap: spectral
spin: topological
temperature: low
spin: liquid
many-body problem
critical phenomena
Hamiltonian
correlation function
thermodynamical
excited state

References(2)

Figures(3)

[1]

The spectral gap problem is algorithmically undecidable: there cannot exist any algorithm which, given a description of the local interactions, determines whether the resulting model is gapped or gapless. This is the same sense in which the Halting Problem is undecidable.12

[2]

Undecidability of the Spectral Gap (short version)

- Nature 528 (2015) 207-211
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e-Print:
- 1502.04135
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DOI:
- 10.1038/nature16059