  Quantum information processing is expressed using quantum bits (qubits) and
quantum gates which are arranged in the terms of quantum circuits. Here, each
qubit is associated to a quantum circuit wire which is used to conduct the
desired operations. Most of the existing quantum circuits allocate a single
quantum circuit wire for each qubit and, hence, introduce a significant
overhead. In fact, qubits are usually not needed during the entire computation
but only between their initialization and measurement. Before and after that,
corresponding wires may be used by other qubits. In this work, we propose a
solution which exploits this fact in order to optimize the design of quantum
circuits with respect to the required wires. To this end, we introduce a
representation of the lifetimes of all qubits which is used to analyze the
respective need for wires. Based on this analysis, a method is proposed which
"recycles" the available wires and, by this, reduces the size of the resulting
circuit. Experimental evaluations based on established reversible and
fault-tolerant quantum circuits confirm that the proposed solution reduces the
amount of wires by more than 90% compared to unoptimized quantum circuits.
