Avoiding thermal noise to enter low temperature while not using dissipative element
Usually, in these situations what really matters is to avoid attenuation in the return path from the cryogenic device under test (shown at the top of the sketch), since you are trying to detect a signal from it with good SNR. Attenuation in the excitation signal coming from the high temperature is much less of a problem, since the signal source can have whatever power is required. If this is your situation, you can use a cryogenic circulator and attenuator.  (The circulator sends a signal entering port 1 to port 2, entering port 2 to port 3, and entering port 3 to port 1, ideally with very little attenuation or reflection.)  The excitation signal passes through a cryogenic attenuator before it goes to the circulator, to remove broadband high-temperature thermal noise. Broadband noise coming from the amplifier ends up in the attenuator, rather than in the device under test.