End-to-End Efficient Quantum Thermal and Ground State Preparation Made Simple

We propose new quantum algorithms for thermal and ground state preparation based on system-bath interactions. These algorithms require only forward evolution under a system-bath Hamiltonian in which the bath is a single reusable ancilla qubit, making them especially well-suited for early fault-tolerant quantum devices. By carefully designing the bath and interaction Hamiltonians, we prove that the fixed point of the dynamics accurately approximates the desired quantum state. Furthermore, we establish theoretical guarantees on the mixing time for several physically relevant models, thereby providing a rigorous justification for the end-to-end efficiency of system-bath interaction models in thermal and ground state preparation.