Correcting Connectivity in Arc-Based QUBO Models for Fixed-Fleet Vehicle Routing

Abstract

We revisit a degree-only arc Hamiltonian for fixed-fleet, homogeneous, uncapacitated vehicle routing. Because its local penalties define only a cycle cover, ground states may contain customer cycles disconnected from the depot. We construct a polynomial-size quadratic unconstrained binary optimization (QUBO) repair using capped single-commodity flow and prove that every ground-state routing is connected and cost-optimal under explicit penalty assumptions. For $N-1$ customers and $K$ nonempty routes, the unreduced encoding uses exactly $|E|(1+\lceil\log_2(N-K+1)\rceil)$ logical problem qubits. A reversible compute--phase--uncompute realization evaluates the flow penalties in $O(N^2\log N+N\log^2N)$ logical gates on a complete graph with $O(\log N)$ reusable workspace and no product register. On complete loopless graphs, a depot-delimited single-sequence position encoding uses fewer problem qubits and fewer written terms when the flow-word length grows. Conversely, the flow model achieves a smaller structured logical-gate upper bound under a common reversible accounting model. Exact audits of the Hamiltonian and circuit implementation, combined with a $1{,}200$-matrix classical benchmark, verify the formulation and quantify the connectivity gap. Finally, a 32,000-shot Amazon Braket task on IQM Emerald characterizes depth-one termwise Ising circuits on a diagnostic $N = 4,\, K = 1$ counterexample instance. In the degree-only circuit, $78.05\%$ of selected $p=1$ shots realize the invalid disconnected ground state; the reduced 14-qubit flow-augmented circuit yields no fully feasible sample. These device results characterize mapped Hamiltonians and compilation rather than an asymptotic routing solution advantage.