Qubit Recycling Revisited

Reducing the width of quantum circuits is crucial due to limited number of qubits in quantum devices. This paper revisit an optimization strategy known as qubit recycling (alternatively wire-recycling or measurement- and-reset ), which leverages gate commutativity to reuse discarded qubits, thereby reducing circuit width. We introduce qubit dependency graphs (QDGs) as a key abstraction for this optimization. With QDG, we isolate the computationally demanding components, and observe that qubit recycling is essentially a matrix triangularization problem. Based on QDG and this observation, we study qubit recycling with a focus on complexity, algorithmic, and verification aspects. Firstly, we establish qubit recycling’s NP-hardness through reduction from Wilf’s question, another matrix triangularization problem. Secondly, we propose a QDG-guided solver featuring multiple heuristic options for effective qubit recycling. Benchmark tests conducted on RevLib illustrate our solver’s superior or comparable performance to existing alternatives. Notably, it achieves optimal solutions for the majority of circuits. Finally, we develop a certified qubit recycler that integrates verification and validation techniques, with its correctness proof mechanized in Coq.