What Does "Macroscopic" Mean in Macroscopic Quantum Tunnelling? A Terminology Note

The 2025 Nobel Prize in Physics honours the discovery of macroscopic quantum tunnelling
and energy quantisation in an electric circuit. This note asks what the word “macroscopic” refers
to. We distinguish three readings: the physical size of the device, the thickness of the barrier,
and the nature of the tunnelling variable. In current-biased Josephson-junction experiments the
barrier that is crossed is not the nanometre-thick oxide layer, whose pair tunnelling underlies
the Josephson effect recognised in 1973. It is a barrier in the effective potential of a collective
phase variable that describes billions of Cooper pairs. We argue that “macroscopic” is best read
as describing this variable, and that this reading is consistent with the temperature-independent
escape rate observed at low temperature. We also note that “macroscopic” has no single sharp
definition. The note is a clarification and reports no new results.