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March 13, 2019 10:30 pm

Google Builds Circuit to Solve One of Quantum Computing's Biggest Problems

Researchers at Google, the University of Massachusetts at Amherst, and the University of California Santa Barbara has solved one of the biggest limitations with quantum computing: all the control and readout circuits of quantum computer systems must be at room temperature, while their superconducting qubits live in a cryogenic enclosure at less than 1 kelvin. "For today's sub-100-qubit systems, there's enough space for specialized RF cabling to come in and out of the enclosure," reports IEEE Spectrum. "But to scale up to the million-qubit systems needed to do really cool stuff, there just won't be enough room." At the IEEE International Solid-State Circuits Conference in San Francisco last month, the researchers reported making a key control circuit in CMOS that will work at cryogenic temperatures. They described it as "a high-performance, low-power pulse modulator needed to program the qubits." From the report: "The current approach is OK for now," says Joseph Bardin, a University of Massachusetts at Amherst associate professor of electrical and computer engineering who designed the IC while on sabbatical at Google. "But it's not scalable to a million qubits." For Google's 72-qubit quantum processor there are already 168 coaxial cables going into the refrigerator and connecting to the 10-millikelvin quantum processor. The pulse modulator IC Bardin worked on is used to encode quantum states on a qubit in order to execute a program. Quantum computers get their parallelizing power because qubits don't have to be just 0 or 1, like the bits in an ordinary computer. Instead, they can be a mix of those states. The pulse modulator uses a specific set of RF frequencies to produce that mix. "The biggest challenge is heat dissipation," explains Bardin. The qubits are at 10 millikelvins, but the control circuits, which necessarily throw off heat, can't be held that low. The researchers aimed for 4 K for the control IC. "However, at 4 K, thermodynamics limits the efficiency of cooling. The best you're going to get is about 1 percent efficiency. In practice it's worse." So the power dissipated by the electronics per qubit had to be only in the milliwatt range. That power constraint had to be balanced with the need for control accuracy, Bardin says. This was complicated by how differently CMOS transistors behave at 4 k, which is a more than 200 degrees below what silicon foundries' simulation models can deal with. Bardin and the Google team managed to design the IC in a way that compensates for these problems and achieves the balance between power consumption and performance. The resulting IC consumed less than 2 mW, yet it was able to put a qubit through its paces in testing.

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