Quantum walks on graphs have been shown in certain cases to mix quadratically faster than their classical counterparts. Lifted Markov chains, consisting of a Markov chain on an extended state space which is projected back down to the original state space, also show considerable speedups in mixing time. Here, we construct a lifted Markov chain on a graph with n2D(G) vertices that mixes exactly to the average mixing distribution of a quantum walk on the graph G with n vertices, where D(G) is the diameter of G. Moreover, the mixing time of this chain is D(G) timesteps, and we prove that computing the transition probabilities for the lifted chain takes time polynomial in n. As an immediate consequence, for every quantum walk there is a lifted Markov chain with a faster mixing time that is polynomial-time computable, as the quantum mixing time is trivially lower bounded by the graph diameter. The result is based on a lifting presented by Apers, Ticozzi and Sarlette (arXiv:1705.08253).
We analyze the sum-of-squares rank of unweighted instances of the Minimum Knapsack (MK) problem, i.e., minimization of ∑i=1nxi for 0/1 variables under the constraint ∑i=1nxi≥q, with q∈R. Such instances have long served as a testbed for understanding the limitations of lift-and-project methods in Boolean optimization. For example, both the Lovász-Schrijver and Sherali-Adams hierarchies require (maximal) rank n to solve them, already when q=1/2 is constant. The SOS hierarchy requires only \emph{sublinear} rank O(n) to solve unweighted MK when q=1/2. On the other hand, when q is allowed to vary with~n, the SOS rank of the problem may become linear. Interestingly, this is known to happen both when q is large, and when q is very small ($0