The qubit package at 14 mK; its pointer reads the qubit stateThree 20 dB attenuators on the drive line: 60 dB in all, so about 0.0029 thermal photons reach the chipReadout back up: two isolators, a circulator, a 40 dB HEMT at 2.85 K, an amplifier at room temperatureA superconducting quantum computer. Its transmon qubit sits at the bottom of a cryogen-free dilution refrigerator, drawn here with the cans off: a pulse tube cools the 50 K and 4 K plates; below them the still, cold plate and mixing chamber plates hang on struts, cooled by He-3 diluting into He-4 in the mixing chamber (measured stages 35 K, 2.85 K, 0.88 K, 82 mK, 14 mK). A 5.0 GHz drive pulse starts with about 1,250 thermal photons at 300 K and loses 20 dB at 4 K, 20 dB at the cold plate and 20 dB at the mixing chamber, so the transmon sees about 0.0029; a 20 ns pulse flips it, the 7.4 GHz readout climbs back through isolators and a 40 dB HEMT, and the excited state decays with T1 of 85 µs.