Quantum digital cooling

We introduce a method for digital preparation of ground states of 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 nonperturbatively. Studying this cooling method on a 1-qubit system toy model, we optimize two cooling protocols based on weak-coupling and strong-coupling approaches. Extending the insight from the 1-qubit system model, 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. We test LogSweep on the 1D transverse-field Ising model, demonstrating approximate ground-state preparation with an error that can be made polynomially small in the computation time for all three phases of the system. The BangBang protocol extends the strong-coupling approach, and exploits a heuristics for local Hamiltonians to maximize the probability of deexciting system transitions in the shortest possible time. Although this protocol does not promise long-time convergence, it allows for a rapid cooling to an approximation of the ground state, making this protocol appealing for near-term demonstrations.

Automated summary

The method introduced is inspired by natural cooling, using digital quantum hardware to prepare ground states of simulated Hamiltonians. By simulating a cold bath with a single ancillary qubit and optimizing weak-coupling and strong-coupling protocols, the study extends to scalable protocols for larger systems. The LogSweep protocol sweeps energies to resonantly match targeted transitions, while the BangBang protocol maximizes the probability of deexciting system transitions in the shortest time possible, allowing for rapid ground state approximation.