I was excited yesterday to tag an extremely stable point for accuracy and precision across the full "layer stack" of Qrack, but excitement with Qrack's stability lead to more exhaustive testing, which indicated one more bug intrinsic to QUnit. The point of v7.1.1 is only to create a tag as a reference point for this even more stable version, after a couple of critical commits were added to main branch. QUnit uses a system of constrained 2 qubit gate buffers to try to attain better Schmidt decomposition. These buffers only allow phase and "inversion" (Pauli X-like or Y-like) singly-controlled gate target "payloads." We are able in cases to commute Hadamard gates through the controlled gate buffers, when symmetry permits an exact logical equivalence after simple transformation of the gate buffers, to move the Hadamard gate to the first-to-be-applied side of the circuit. Specifically, besides cases of Hadamard gates applied on buffer control qubits, Hadamard gates can be moved to the opposite side of the gate buffer target qubits when applied to a target qubit with exactly equal or exactly opposite phases on phase or inversion gate components pairs. Starting from singly-controlled phase and inversion gates, same phase produces a new singly-controlled phase gate, and opposite phase produces a new singly-controlled inversion gate. However, while all phase gates generally commute, the buffering of controlled inversion gates requires an explicit order of application of buffers. Since the buffer system is designed to assume general commutation of buffers, with limited exception to handle one-off inversion gate "fusion" and control qubit commutation, we can't generally commute Hadamard gates around opposite-phase buffers without accounting for the order in which inversion buffers will be applied. The bottom line is that the previous implementation of this buffer system optimistically assumed that arbitrary commutation of opposite-phase buffers would preserve exact observable state, but this seems proven wrong. The fix in this version, v7.1.1, opts to omit opposite-phase target qubit commutation with Hadamard gates entirely, since this passes hundreds of thousands of random mirror circuit assertions in contiguous sequence with ~0 bit-flip errors, (or less than about 1 per hundred thousand assertions, on my development machine). More aggressive optimization can be done, (and probably will be soon,) but I'm trying to tag the most accurate and precise version of Qrack I can attain, particularly for QUnit layer. (We're aiming for a "100% bug-free" tag of QUnit in v7.1.x, if this isn't a Sisyphean goal in software development in general.)

Daniel Strano considers Benn Bollay to be a coauthor, among the Qrack contributors.