Quantum Digital Cooling

We introduce a new method for digital preparation of ground states of a simulated Hamiltonians, inspired by cooling in nature and adapted to leverage the capabilities of digital quantum hardware. The cold bath is simulated by a single ancillary qubit, which is reset periodically and coupled to the system non-perturbatively. Studying this cooling method on a 1-qubit system toy model allows us to optimize two cooling protocols based on weak-coupling and strong-coupling approaches. Extending these, we develop two scalable protocols for larger systems.
The LogSweep protocol extends the weak-coupling approach by sweeping energies to resonantly match any targeted transition, demonstrating preparation of an approximate ground state for ferromagnetic and critical Ising chain, with error that can be made polinomially small in time.
The BangBang protocol extends the strong-coupling approach, and exploits a heuristics for local Hamiltonians to approximately match transition energies, allowing rapid cooling to an approximation of the ground state, which makes this protocol appealing for near-term simulation applications.