Mark Rudner

_{University of Washington}

Programmable adiabatic demagnetization: preparing low energy states by simulated cooling

The ability to prepare ground states of many-body Hamiltonians on quantum devices is of central importance for a variety of tasks and applications in quantum computation and quantum simulation. In this talk I will describe a simple, flexible, and robust protocol to prepare low-energy states of arbitrary Hamiltonians on either digital or analog quantum hardware. The protocol is inspired by the “adiabatic demagnetization” technique, used to cool solid state systems to extremely low temperatures. A constant fraction of the available qubits serve as a renewable bath, enabling the cooling process to be run in a cyclic fashion. Measurements of the bath spins at the end of each cycle provide information on the progress of cooling. For the simplest implementation, we find that the performance of the algorithm in the presence of a finite error rate depends on the nature of the excitations of the system: the greater difficulty of cooling systems with topological excitations (which cannot be created or destroyed individually) is manifested in characteristic dependencies of cooling performance on error rate and system size, which can serve as signatures of the underlying ground state order. By using alternative "fermionized" encodings, we show how low energy states of fermionic or topological phases can also be efficiently prepared on bosonic hardware through this method.