Non-destructive measurements

[eddieschoute]

Is your feature request related to a problem? Please describe. It is common to use non-destructive measurements in fault-tolerant quantum computing. Depicting a sequence of these measurements using yquant is currently not obvious because a measurement only has one output: classical or quantum.

Describe the solution you'd like A non-destructive measurement should not act only on one register but have two outputs: One classical and one quantum. The classical output could just be a (bit-)label that is used later in the circuit as classical control, which could have it's own notation so as to not draw explicit classical wires.

Describe alternatives you've considered

• I considered using a dmeter with qubit output but it's not easy to describe the classical output.
• I considered a box with $I + (-1)^b P$ for measuring $P$, but it's wide and not as natural.

Additional context One depiction of such circuits can be found in my paper "Edge-disjoint paths for Surface Code Compilation" (sorry for the self-plug). Even simple-circuits will have many classical bits floating around and need a non-obtrusive way to represent them. See Figure 18, for example, as well as Figure 8 for a more complex one.