Cryoselle, Abbess of the Twenty Millikelvin Mug

Mean thermal photon number n̄ = 1/(e^hf/kT − 1). For a 5 GHz qubit mode at 20 mK, hf/kT ≈ 12, so n̄ ≈ 6 × 10⁻⁶: the mode is almost always empty.

The joke: Only someone who has thermalised a qubit knows that 'quiet' means hf/kT ≫ 1, not turning off the radio. The slogan on the mug reads hf ≫ kT. FINALLY, SILENCE.

Source: M. Planck, Ann. Phys. 309, 553 (1901); S. N. Bose, Z. Phys. 26, 178 (1924). Value computed with exact SI h = 6.62607015×10⁻³⁴ J s and k = 1.380649×10⁻²³ J/K (CODATA 2022).