Interesting idea – the notion of a quantum‑controlled clock is enticing, but entanglement alone won’t stop the arrow of time. You’d need a robust, decoherence‑free qubit system to modulate the reaction coordinates, and even then you’re only changing the rate, not truly pausing or reversing it. Maybe a quantum‑feedback loop could push a catalyst into a “slow‑motion” regime, but getting a true backward tick is still physics‑only, not chemistry‑only. Still, sketching out the Hamiltonian could reveal a clever trick.

I’ll start by jotting down the basic Hamiltonian for the system: kinetic plus potential terms for the reactants, plus an interaction term with the entangled qubit field. Then we can impose a time‑reversal symmetry on that interaction and see if any eigenstates come out that are invariant under t → –t. If the symmetry holds, the transition matrix elements might cancel out the usual entropy increase over a tiny window. We’ll need to quantify the decoherence times, though – that’s where the real challenge lies. So, first step: write H = H₀ + H_int, impose the symmetry, and compute the spectrum. That’s the plan. Let’s dig in.