  Quantum computing is on the verge of a transition from fundamental research
to practical applications. Yet, to make the step to large-scale quantum
computation, an extensible qubit system has to be developed. In classical
semiconductor technology, this was made possible by the invention of the
integrated circuit, which allowed to interconnect large numbers of components
without having to solder to each and every one of them. Similarly, we expect
that the scaling of interconnections and control lines with the number of
qubits will be a central bottleneck in creating large-scale quantum technology.
Here, we define the quantum Rent's exponent $p$ to quantify the progress in
overcoming this challenge at different levels throughout the quantum computing
stack. We further discuss the concept of quantum extensibility as an indicator
of a platform's potential to reach the large quantum volume needed for
universal quantum computing and review extensibility limits faced by different
qubit implementations on the way towards truly large-scale qubit systems.
