One of the limiter for scaling many quantum computers based up the superconducting, spin qubit, or topological technologies is the need to provide control lines for controlling the qubits as well as lines for readout of the qubit data. All of this data needs to have a pathway from a classical computer running at room temperature to the qubits which are at millikelvin temperatures and vice-versa. Current machines use individual coaxial cables to accomplish this, but it creates a rats nest of wires as well as potential heat issues that make scaling these machines to the thousands or millions of qubits extremely challenging.

Example of the Wiring in Google’s Sycamore Processor, Credit: Google

To solve this problem, researchers are developing a generation of cryo-CMOS control chips that will operate at a few kelvin or millikelvin temperatures that move most of the control logic down very close to the qubits. These cryo-CMOS will help solve the wiring problem as they will only need a handful of wires to be routed to the room temperature classical computer and not 2 or 3 wires for every qubit. We had previously reported on two generations of cryo-CMOS control chips developed by Intel and its partner QuTech called Horseridge and Horseridge 2. Now Microsoft has shown its approach in a blog article on their web site and a paper published in Nature.

Microsoft’s Vision of a Quantum Stack. Credit: Microsoft
One of the control chips is codenamed “Gooseberry” and sits in the Quantum plane and runs at a 100 millikelvin temperature. The other control chip is the cryo-computer core located at the bottom of the classical compute stack and runs at 2 kelvin.

Interestingly, Microsoft is taking a slightly different technical approach from Intel because they have developed a two chip solution versus Intel’s single chip solution. Intel’s Horseridge is designed to run at a 3-4 kelvin temperature and can currently support 128 qubits. This may be OK for spin qubits which may be able to run at those temperatures. But if the qubit technology requires millikelvin temperatures, it would still require some routing of control wires to the qubits at the lower temperatures. Microsoft has divided the control functionality into two chips. The first is a very low temperature chip codenamed Gooseberry that sits right next to the qubits and runs at a 100 millikelvin temperature which requires a dilution refrigerator for cooling. The challenge is that logic chips do generate power and heat and adds to the cooling load if they use too much power. So Microsoft has designed the chip in such a way that the maximum consumption of Gooseberry would be 2 milliwatts and within the limits of a dilution refrigerators capability. The current generation of Gooseberry can handle 32 qubits, but Microsoft projects this design can be scaled up to 1000 qubits and stay within the 2 milliwatt limit. Gooseberry has been designed so that it can potentially be used with topological, spin, or gatemon (superconducting) qubits.

The cryo-control core runs at 2 kelvin and does not face the same power constraints. Although the different between 100 millikelvin and 2 kelvin may not seem like a big difference, it is. Control electronics running at 2 kelvin can be cooled with liquid helium that can provide much great cooling power than a dilution refrigerator at 100 millikelvin. So it can provide more powerful general purpose computing functionality versus Gooseberry which is limited to more simple control signal generation. The two chips communicate with each other using a four wire serial peripheral interface (SPI).

For more details about Microsoft’s cryo-CMOS control chips you can view a blog article on the Microsoft web site here and a Nature article which describes the Gooseberry chip here.

January 30, 2021