Please help transcribe this video using our simple transcription tool. You need to be logged in to do so.


The classical description of quantum states is in general exponential in the number of qubits. Can we get polynomial descriptions for more restricted sets of states such as ground states of interesting subclasses of local Hamiltonians? This is the basic problem in the study of the complexity of ground states, and requires an understanding of multi-particle entanglement and quantum correlations in such states. We propose a combinatorial approach to this question, based on a reformulation of the detectability lemma introduced by us in the context of quantum gap amplification \cite{ref:Aha09b}. We give an alternative proof of the detectability lemma which is not only simple and intuitive, but also removes a key restriction in the original statement, making it more suitable for this new context. We then provide a one page proof of Hastings' proof that the correlations in the ground states of Gapped Hamiltonians decay exponentially with the distance, demonstrating the simplicity of the combinatorial approach for those problems. As our main application, we provide a combinatorial proof of Hastings' seminal 1D area law \cite{ref:Has07} for the special case of frustration free systems. Area laws provide a fundamental ingredient in the study of the complexity of ground states, since they offer a way to bound in a quantitative way the entanglement in such states. An intricate combinatorial analysis allows us to significantly improve the bounds achieved in Hastings proofs, and derive an exponentially better scaling in terms of the inverse spectral gap and the dimensionality of the particles. This holds out hope that the new approach might be a promising route towards resolving the 2D case and higher dimensions, which is one of the most important open questions in Hamiltonian complexity.

Questions and Answers

You need to be logged in to be able to post here.