[Cryptography] Some quantum computers might need more power than supercomputers

[Jan Dušátko]

> If I take the cooling of an absolutely black body with an area of 
> ​​1m^2 from 20˚C to mK and μK, then I have some heat flow 
> characteristics. But if I take the Carnot machine calculation, then 
> the worst possible scenario produce list of needs. For cooling an 
> absolutely black body with a surface area of 1m^2 that means:
> - To a temperature of 1K requires an input of 122KW
> - To a temperature of 0.1K requires an input of 1.22MW
> - To a temperature of 0.01K requires an input of 12.2MW
> - To a temperature in mK requires an input of 122MW
> - To a temperature in μK requires an input of 122GW
>
> To cool the delivered 1W (Carnot cooling, the best possible option and 
> optimistic attitude) for perfectly insulated body, we need the 
> following cooling device power:
> - To a temperature of 1K requires an input of 293W
> - To a temperature of 0.1K requires an input of 2.93kW
> - To a temperature of 0.01 K requires an input of 29.3 kW
> - To a temperature of 1mK requires an input of 293W
> - To a temperature of 1μK requires an input of 293MW
>
> For my surprise, the possible heat conductivity by construction is 
> very low, about 2,0-2,2W. I estimated rod with a circular 
> cross-section of diameter 18,3mm. I choose it because QC in ball of 
> surface are 1m^2 probably cannot be heavier than iron ball. But 
> radiation could make things worse than thermal conductivity. This is a 
> reason why I did not count that.
>
Update:

In reality, layering of protections significantly reduces radiation 
transmission, where each additional layer can reduce transmission to 3% 
- 5% of the original flux. Ideally, two layers with attenuation of 5% of 
the original flux should give a total attenuation of 0.25%. However, no 
structure is perfect, so in reality we can get values ​​corresponding to 
more than the formula q≈1/(N+1). In such a case, somewhere around 50 
layers, the main heat transfer should be through the structure. The 
theoretical limit of insulation should be able to limit the radiation 
flux from 460W to 4.6µW. The above limitations are given by the physical 
properties of the material and quantum phenomena.

Regards

Jan