Reaches 2 mK[ref]. youtu.be/upw9nkjawdy?t=487 from Video "Building a quantum computer with superconducting qubits by Daniel Sank (2019)" mentions that 15 mK are widely available.
Used for example in some times of quantum computers, notably superconducting quantum computers. As mentioned at: youtu.be/uPw9nkJAwDY?t=487, in that case we need to go so low to reduce thermal noise.
Articles by others on the same topic
A dilution refrigerator removes heat at very low temperatures by transferring helium-3 from a concentrated phase into a dilute phase rich in helium-4. The enthalpy per circulating helium-3 atom is higher in the dilute phase, so crossing the phase boundary absorbs energy from the mixing chamber of a dilution refrigerator. This is the useful cooling process.
Continuous operation requires a circulation loop. A dilution refrigerator still preferentially evaporates helium-3 from the returning mixture; pumps return it through a condensing and precooling path. Heat exchangers let the outgoing cold stream cool the incoming stream before it reaches the mixing region.[ref]
How Does a Dilution Refrigerator Work? by Bluefors. Phase Separation of Helium Isotopes and Operation of the Dilution Unit: enthalpy difference, mixing chamber, still and return heat exchangers
A heater attached to the cold stage can impose a known test load, allowing a measurement of cooling power versus temperature. Unwanted heat conduction through supports and wiring, and absorbed thermal radiation, enter that same load budget. The refrigerator therefore needs both the microscopic mixing mechanism and careful thermal coupling to the experiment.