Figure 3: Thermalization of two CBTs at a refrigerator temperature ≤2.8 mK.
(a) Cooling of one CBT in vacuum (circles) and one immersed in the 3He/4He refrigerant of the dilution refrigerator (triangles). In both cases, the CBTs are cooling after being warmed above 10 mK by temporarily increasing the refrigerant temperature. The CBT in vacuum is extremely slow to thermalize. By comparison, the CBT immersed in 3He/4He thermalizes significantly faster. (b) Cooling of the immersed CBT after it has been heated by a large DC drive current (50, 40 and 30 nA for run 1, 2 and 3, respectively). Fitting to an exponential decay (solid line) yields a time constant of 570 s and a saturation temperature of 3.8 mK. (c) Schematic of the immersion cell used to cool a CBT in the 3He/4He mixture of a dilution refrigerator.
