From barren plateaus through fertile valleys: Conic extensions of parameterised quantum circuits

Understand

Optimisation via parameterised quantum circuits is the prevalent technique of near-term quantum algorithms.

• However, the omnipresent phenomenon of barren plateaus - parameter regions with vanishing gradients - sets a persistent hurdle that drastically diminishes its success in practice.
• In this work, we introduce an approach - based on non-unitary operations - that favours jumps out of a barren plateau into a fertile valley.
• These operations are constructed from conic extensions of parameterised unitary quantum circuits, relying on mid-circuit measurements and a small ancilla system.