A qubit stores information in the difference between two energy levels. At room temperature, the surrounding heat contains enough energy to push the qubit between those levels randomly. Cooling to 15 millikelvin — about 200 times colder than outer space — makes thermal energy so small compared to the qubit's energy gap that the qubit stays where you put it long enough to run a circuit. Without extreme cold, quantum information dissolves into noise before you can use it.

Imagine trying to balance a marble on a ridge between two valleys. At room temperature, the ground shakes so violently that the marble bounces between valleys thousands of times per second. Cooling the system is like calming the ground until it barely trembles — the marble stays put long enough for you to do something useful with its position.

The thermal occupation of a qubit's excited state follows the Boltzmann distribution: $n_{th}=1/(\exp (hf/k_{B}T)-1)$. For a 5 GHz transmon qubit at 300 K, $n_{th}$ is approximately 1250 — the qubit is overwhelmed by thermal photons. At 15 mK, $n_{th}$ drops below 0.0001, meaning the qubit starts in its ground state with over 99.99% probability. The two main decoherence channels — energy relaxation (T1) and dephasing (T2) — both improve dramatically at lower temperatures because fewer thermal excitations interact with the qubit.