While the refrigeration system can bring temperatures down to extremes, it can’t remove heat very quickly – so if you have a chip in there that’s creating a lot of heat, you’re going to have a problem.
“You’re probably familiar with the power dissipation of an FPGA,” Clarke said. “This would overwhelm the refrigeration cooling capacity. At the lowest level of a fridge, you typically have about a milliwatt of cooling power. At the four Kelvin stage [higher up in the fridge], you have a few watts.”
Future fridge designs are expected to improve things, but it’s unlikely to massively increase the temperature envelope. “That imposes limitations on the power dissipation of your control chips.”
Beyond the quantum chip itself, quantum systems need other hardware to make them fully-fledged computers. Some of that can be handled outside the fridge, with most quantum computers paired with a few racks of conventional HPC servers. But other parts, particularly the control chip, need to be inside the fridge itself – which means yet another thing that needs to work under extreme conditions, while not giving out too much energy itself.
“Controlling the quantum chip is actually pretty difficult, and that’s what we do with Horse Ridge,” Clarke said. Its second-generation Horse Ridge II is a CMOS chip, “with more than 100 million transistors, produced on our 22-nanometer node.”
The hardware, revealed this month, is verified at operating at 4 Kelvin, but the company hopes to push that in the years ahead. “Going forward, you would probably focus not only on additional capability, but on additional power optimization of Horse Ridge inside the fridge,” Clarke said.
“And you would probably also focus on improving the cooling capacity of the fridge, which our external suppliers and the quantum ecosystem is working on.”
But Intel has taken a slightly different route than some of its quantum competitors. Like Google and IBM, it has worked on superconducting qubit systems, including its 49-qubit Tangle Lake processor, but in recent years it has started to shift to researching spin qubits with partner QuTech.
A debate rages about which approach is best, but one huge advantage of spin qubits, which more closely resemble existing semiconductor components, is that they are expected to have a ‘much’ higher operating temperature. Instead of 20 millikelvins, they can run at around one degree. That might not sound like a huge difference, but “believe it or not, it makes things tremendously easier.”
Spin qubits are far smaller than superconducting ones, with a square millimeter theoretically having enough room for up to a billion spin qubits. But we’re a long way from that, and currently efforts to build more powerful quantum systems will require bigger fridges.
“I think the form factor of the fridge will be bigger than it is now,” Clarke said. “Right now, it’s about the size of a keg of beer. When I think about what it would take to get to a million qubits, you still aren’t at a point where you’re worried about the form factor of the fridge or the system – it’s not going to be a similar size as a typical supercomputer, with rows, rows, and rows of racks.
“It may, in the end, look like one of the gas cylinders you might see out of a hospital, that sort of size, but that’s still I think, quite manageable from a form factor in the data center.”