Improving the Adiabaticity of Adiabatic Quantum Computers with Unknown Energy Spectrum

Adiabatic quantum computing brings powerful insights into the realization of many-body ground states and the solution to optimization problems. For an adiabatic quantum algorithm to be successful, the adiabatic criterion needs to be satisfied. However, as the energy gap between the quantum ground state and the excited states decreases with the number of qubits, the adiabaticity of the quantum evolution can be seriously violated. Previous approaches to improve the adiabaticity require either a priori knowledge of the energy spectrum or the construction of impractical many-body interactions. Here we present an approach that can significantly improve the adiabaticity of an adiabatic quantum computer without the knowledge of the energy spectrum, where an ancilla provides nonlinear control over the adiabatic quantum computer. We show that in the vicinity of the bifurcation points in this nonlinear system, the dynamics of the adiabatic quantum computer can be slowed down and the adiabaticity can be greatly enhanced. We illustrate this approach with a quantum two-level-system, an exactly-1 3SAT instance on six qubits, and a transverse field Ising model.