There is this most beautiful idea in quantum mechanics called the adiabatic approximation. It says: if a system changes slowly enough, it stays in its instantaneous eigenstate. In other words, if the Hamiltonian evolves gently, the system adapts without jumping, without panic, without losing its identity.
So what does this mean in the “macro” aka classical world?
Last weekend, while hobbling on crutches through the rolling hills of Oxfordshire, I found myself musing aloud about the adiabatic approximation to my partner – whose PhD is in mathematics (of course). We talked about how adiabatic evolution underpins adiabatic quantum computing and quantum annealers like the D‑Wave, where computation is not a sequence of discrete logical steps but a continuous deformation of an energy landscape. You don’t force the answer; you let the system settle into it.
And then my mind drifted back to my earlier life in experimental nuclear physics – stabilising magnetic fields, coaxing ultra‑clean vacuum systems into behaving, nursing cryogenic environments, and tuning low‑noise electronics until the universe finally whispered something measurable. The fundamentals haven’t changed. To let the quantum world reveal its structure, you still need patience, stability, and engineering that borders on devotion.
As we walked – slowly, with me navigating the meadows like a cautious eigenstate – it struck me that this strange quantum idea can be summarised quite simply: Move slowly enough across the gentle hills of Oxfordshire, and even on crutches, you won’t fall. Your feet follow the contours. You stay in your instantaneous eigenstate.
With her calm, unflappable mathematician beside her, this physicist was at her happiest. And somewhere between the hedgerows and the horizon, we figured out what I would write for my assignment this week.
It feels extraordinary to be a prospective PhD – or DPhil, as we say in Oxford – candidate again. Sweeter the second time round, because now I understand the landscape. And perhaps that is the real adiabatic lesson: move slowly enough and the path reveals itself.
21st April 2026
