# Cooling below 1 K: How long can it stay cool?

1. May 17, 2013

### rollingstein

While reading up the recent hoopla about DWave's new "Quantum Computer" I came across a practical question about cryogenics:

How long could you keep a 10 kg mass at below 80mK without any external cooling if you could design the best practical insulation system? DWave claims it is "several months to years" but I was a bit skeptical. Assume you can use a 10'x10'x10 cube at most to contain it and heat loss is to ambient air.

Could one really make insulation that good? What do ballpark heat flux numbers show? Since DWave sells the darn thingy for $15 Million I'll assume we are allowed access to the best possible material / vacuums etc. If you really care, say you get an awesome budget of$1 Million to build your insulating system! :)

More details:

http://www.dwavesys.com/en/dev-tutorial-hardware.html [Broken]

Reduction of the temperature of the computing environment below approximately 80mK is required for the processor to function, and generally performance increases as temperature is lowered - the lower the temperature, the better. 20mK is targeted as the lowest temperature that can be easily reached as an operating point. The processor and parts of the input/output (I/O) system, comprising roughly 10kg of material, must be cooled to these temperatures. Most of the physical volume of the current system is due to the large size of the refrigeration system. The refrigeration system used to cool the processors is known as a dilution refrigerator.

The inset in Figure 7 shows the chip packaging attached to the cooling apparatus. Note that the area around the chip has now been closed up to protect it from being damaged. When the computer is being operated, this part is sealed inside a vacuum chamber Because quantum processors require low temperatures for the quantum effects to be sustained, the entire piece shown in the inset of figure 7 is cooled to around 20mK, which is approximately 100 times colder than interstellar space.

To reach the near-absolute zero temperatures at which the system operates, the refrigerators use liquid Helium as a coolant. The type of refrigerator inside the D-Wave OneTM system is known as a "dry" dilution refrigerator. This means that all the liquid helium resides inside a closed cycle system, where it is recycled and recondensed using a pulse-tube technology. This makes them are suited to remote deployment, as there is no requirement for liquid helium replenishment on-site.

The specialized equipment to allow cooling to these temperatures is available commercially and runs reliably. The refrigeration technology is also mature enough that the system has a turnkey operation. The computer can be cooled down to operating temperature within several hours, and once this temperature is reached remain cold for months or years.

Last edited by a moderator: May 6, 2017
2. May 17, 2013

### 256bits

I am not sure that he means that it will keep cold for months or years with the refrigeration system shut off, but he could have said explicitly. As it is the description seems amateurish, but that could be the whole point in not devulging too much infomation for competators.

3. May 17, 2013

### rollingstein

Good insulation is one way to interpret it. Say, you shut your cooling when it is at 80mK and then can you design insulation so good that the heat flux loss is so low that in a year your 10kg sample still hasn't heated up to 1 K? (say)

4. May 17, 2013

### rollingstein

Another interpretation could be evaporative cooling? Are they simply boiling off enough He to keep it cool?

Note that it is a closed system. So they'd have to store the He vapor till they restart their refrigeration cycle after one year.

5. May 17, 2013

### cjl

I don't interpret that statement as saying that it will stay cool for months or years with the refrigeration shut off. They are simply saying that the system can stay cold (and presumably operational) for months or years uninterrupted, and I would assume (from the way it is phrased) that the refrigeration system would be on the entire time.

6. May 17, 2013

### rollingstein

If so, that sounds a lot more believable, thanks!

7. May 17, 2013

### 256bits

That's exactly what i put across. Too bad you didn't pick up on how a refrigeration system works earlier.