Confusion with attenuator noise
If the attenuator is at a low temperature (50K) the added thermal noise is lower. It is &quot;kb Tatt B&quot;  where &quot;Tatt&quot;  is the attenuator temperature = 50K.
A lot of people are not aware that it is not only because a device has a given attenuation that its output noise is fully determinated.
Of course in most situation the device is passive and at 25°C, and in this case its noise contribution is directly deductible from the attenuation.
But, effectively, if a passive attenuator is at a lower temperature, you need to know the temperature to deduce its contribution to noise.
Another case is an active attenuator at ambient temperature. Then it is perfectly possible to have an output noise lower than what it would be for the same passive attenuation (even with ouput impedance matched, 50 Ohms typiccaly).
From all this, the main thing to retain is that all valid explanations that explain the contribution to noise of a attenuator (in particular the famous &quot;noise factor&quot; = &quot;attenuation&quot;) should use the hypothesis : passive, at ambient temperature (and maybe impedance matched, but this is another story)