### Undecidability of the Spectral Gap (short version)

**Toby Cubitt, David Perez-Garcia, Michael M. Wolf**

Nature 528, 207-211 , (2015)

**arXiv.org**: 1502.04135

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**Abstract:**The spectral gap -- the difference in energy between the ground state and the first excited state -- is one of the most important properties of a quantum many-body system. Quantum phase transitions occur when the spectral gap vanishes and the system becomes critical. Much of physics is concerned with understanding the phase diagrams of quantum systems, and some of the most challenging and long-standing open problems in theoretical physics concern the spectral gap, such as the Haldane conjecture that the Heisenberg chain is gapped for integer spin, proving existence of a gapped topological spin liquid phase, or the Yang-Mills gap conjecture (one of the Millennium Prize problems). These problems are all particular cases of the general spectral gap problem: Given a quantum many-body Hamiltonian, is the system it describes gapped or gapless? Here we show that this problem is undecidable, in the same sense as the Halting Problem was proven to be undecidable by Turing. A consequence of this is that the spectral gap of certain quantum many-body Hamiltonians is not determined by the axioms of mathematics, much as Goedels incompleteness theorem implies that certain theorems are mathematically unprovable. We extend these results to prove undecidability of other low temperature properties, such as correlation functions. The proof hinges on simple quantum many-body models that exhibit highly unusual physics in the thermodynamic limit.