In the previous module, we introduced the “quantum-to-classical” interface as a means of storing quantum instructions on a classical computer, as well as classically interpreting the results of a quantum algorithm. In this lecture, Prof. Koen Bertels will introduce classical feedback, which is an important step in computations where the operation we wish to do next depends on a measurement result obtained during the computation itself. This is a necessity in quantum error correction, which we will introduce in the following module.

Prof. Bertels will also introduce the qubit addressing table, which is used to keep track of the positions of the various qubit states as they are routed in hardware. This routing is necessary, since a pair of qubits can only undergo an entangling interaction if the qubits are next to each other on the chip.

Main takeaways

• Certain quantum algorithms require different operations to be applied, depending on measurement results which are acquired during computation. This includes all known quantum error correction routines, which makes the feedback loop from quantum measurement data to new quantum instructions a crucial component of the stack.
• In order to execute algorithms on computers where not all pairs of qubits can be entangled with each other natively (using a single instruction), the microarchitecture is also able to re-assign qubit states to different addresses in hardware, thanks to the inclusion of a qubit addressing table.