SEEQC Developing a Quantum Chip with a New Approach for the Control Electronics

SEEQC System. Credit: SEEQC

SEEQC SFQ Control Chip. Credit: SEEQC

SEEQC has developed a prototype quantum processor called the SEEQC System Red that is based upon superconducting qubits along with a cryogenic control chip based on SFQ (Single Flux Quantum) logic that is placed on the same multi-chip module as the superconducting qubit chip. To understand the significance of this, it is important to understand two significant mechanical engineering issues that make it challenging to scale up superconducting quantum processors to a large number of qubits.

The first challenge deals with the enormous number of control wires needed for control and measure the qubits. Typically, you need at least two or three wires per qubit to connect the analog signals coming from an external waveform generators to the resonators at the qubit. In today's quantum processors, the waveform generators are placed outside the dilution refrigerator and the engineers use special coaxial cables that go all the way through the dilution refrigerator to connect the room temperature external electronics to the qubits running at the millikelvin temperatures. Although this can be done if your processor has a few dozen or perhaps a few hundred qubits, try doing this when you have a processor with several tens or hundreds of thousands of qubits. You would need to route tens or thousands of coaxial cables and they just won't fit.

To solve this problem, several companies including Intel, Microsoft, IBM, Google, and Raytheon BBN have been working on cryoCMOS chips that move the control electronics down to inside the fridge so that very short connections can be made to the qubits. Intel has been most public with this and we expect them to be using their Horse Ridge II cryoCMOS control chip with a processor they will make public later this year. These chips use standard CMOS logic and will typically run at temperatures of 2-3 degrees kelvin and would have short connections to the qubit chips which are placed at the next lower level and run at millikelvin temperatures. You can view some previous articles we have published in the QCR about cryoCMOS chips here, here, and here.

But the CMOS chips can run into the second mechanical engineering issue which would be the heat budget of the dilution refrigerators. CMOS logic dissipates heat and the fridge only has a certain capacity to remove the heat. If there is too much heat being generated, it won't be able to maintain the cold temperature. And, of course, this problem gets worse as you start adding more qubits which need more control.

This is where SFQ logic comes in. SFQ is a digital logic family that is based upon superconducting Josephson junctions. It is not to be confused with a quantum qubits as SFQ does not rely upon quantum mechanical principles like superposition or entanglement. However, circuits made with SFQ are very fast and use orders of magnitude less power than CMOS. Also, they can run at the same millikelvin temperatures as the qubit chips. The technology that SEEQC is using stems from significant Josephson junction research performed by IBM in the 1960's and 1970's when they were considering building ultrafast mainframe computers using this technology. SEEQC will be placing the SFQ chips on a multi-chip module right next to the qubit chips and this approach may solve some of the mechanical engineering problems inherent in other designs. The only other company we have heard that is considering using SFQ is D-Wave on their gate-level processor development program.

Using this approach, SEEQC has been testing an 8 qubit module and is working on a version that can control up to 64 qubits which is currently in fabrication. Preliminary performance metrics on some early devices include a leading two-qubit average gate speed of 39 nanoseconds with average gate fidelities of 98.4%.

You can view SEEQC's announcement about this development in a press release here.

March 16, 2023