Ask HN: Why does a quantum computer look like that? | Hacker News Reader

Ask HN: Why does a quantum computer look like that?

18 pointsrussol7 comments

There are a lot of tiny pipes connected with each other through disks made of gold. And why gold by the way? And what are the roles for the "pipes"? Why are some with rings? Is there any clear description for newbies, why a quantum computer look like that?

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For reference, op is probably referring to something like this:

https://www.ibm.com/blogs/research/wp-content/uploads/2017/1...

As a side note, this was probably also the computer that inspired Alex Garland's tv series Devs since the show's quantum computer looked like this:

https://media.wired.com/photos/5e5ede941e06690008353d15/Back...

There are two things relevant to this design:

1. what you are looking at is basically a freezer; For certain effects you need a very very low temperature.

2. Gold in general is a very good material as it is very stable and doesn't react with a lot of stuff (it is not very corrosive). It also conducts electricity very well.

I would also argue, that what you see is normally the open version of it or it was opend to show the inside: https://www.information-age.com/ibm-commercial-quantum-compu... they are in tubes; Might even be vacuum.

Also heat goes up, cold stays down; might also have influecned the design but actually you are probably better of asking a physics person :D

The "gold" is brass, which is just used because it's well-suited for for making this kind custom scientific apparatus.

Most of what you see is support machinery: cooling, vacuum, shielding, signaling, the actual "quantum" bit is a tiny thing deep inside it.

The apparatus you’re thinking of is called a dilution refrigerator.

The “tiny pipes” are actually coaxial signal cables carrying RF and DC control signals. These terminate at a “package” which contains the chip on which the superconducting qubits are located.

The disks (we call them plates) are different temperature stages for the refrigerator, with lower stages being colder than upper stages. The last stage has operating temperature ~10-20mK. Then 100 mK, 1K, 3K, 50K and lastly room temperature. Most of the other things on the refrigerator are involved either in signal filtering, thermal anchoring, or thermal isolation. When operational, all of the stages are enclosed in multiple cylindrical thermal shields. The outermost layer is the “vacuum can” which is airtight and allows for the whole thing to operate under internal vacuum.

Edit: This is just one type of quantum computing device. Others, such as trapped ion or neutral atom rigs would look radically different. Larger vacuum enclosures, different ways of performing control/read operations (lasers!), etc.

It sounds like you may be talking about the IBM Q (there are in fact many different types of Quantum CPU) but that one is a superconducting type so the pipes will be for the cooling medium.

Hey there. I happen to be lucky enough to work close to some of these beautiful machines; so I relayed your question along, and one of my coworkers produced this useful response for you:

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As a couple of people have already pointed out, the machine you're looking at isn't really the quantum computer, it's the apparatus that's used to keep the processor in a nice cold, quiet environment (a very necessary and complex endeavour when working with delicate quantum systems). Superconducting processors require dilution refrigeration, a process that uses the two common isotopes of helium to extract heat from the fridge and its payload (although other QC implementations may also require some cryogenics, the specific apparatuses may look quite different); Oxford Instruments (one of many commercial fridge manufacturers) has some relatively accessible documentation here: http://home.agh.edu.pl/~kozlow/fizyka/otrzymywanie%20niskich...

Gold is a commonly used material in cryogenics for a number of reasons: 1) it's soft and conducts heat really well, so by coating pieces made of other materials like copper with it and pressing them together, we can get good thermal contact; 2) it's shiny and doesn't oxidize so it reflects light really well since radiation (light) is another form of heat transfer, this again reduces heat absorption. Overall, material choice/design in cryogenics is complicated because many materials' properties change drastically at low temperatures (fun fact: dental floss is often used for this reason).

Depending on which images you're looking at, the "tubes" may indeed be coaxial cabling to carry microwave signals to the processor, but there are also tubes that act to circulate helium and as structural support of the various stages.

D-Wave's quantum annealing processor requires very few microwave signal lines, one of the reasons we've been able to scale it to over 5000 qubits.

For further exploration of quantum computing systems, I recommend this tour of D-Wave's lab: https://www.youtube.com/watch?v=VfxNdBTH8wY and this WebGL interactive 3-D tour showcasing the D-Wave Advantage processor: https://www.dwavesys.com/d-wave-advantage-showcase

I believe a new lab tour video is being produced right now and should be available soon, so check back on the youtube channel.

Thank you for the great explanation to you and your coworker! The documentation by Oxford Instruments and the tour are really cool. As far as I understand, almost the entire construction is a kind of a cooler. The actual quantum processor is located at the bottom of the upside-down pyramid. To make it work properly, the construction should be loaded through a ceiling to a special room with no air. It takes around 24 hours to meet working conditions.

I'm still trying to guess what the rings on pipes are standing for? [1]

[1] https://www.economist.com/sites/default/files/images/print-e...

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