Four Ways to Make Bigger Quantum Computers

(IEEESpectrum) Today’s quantum-computer processors must operate inside cryogenic enclosures at near absolute zero, but the electronics needed for readout and control don’t work at such temperatures. So those circuits must reside outside the refrigerator. For today’s sub-100-qubit systems, there’s still enough space for specialized cabling to make the connection. But for future million-qubit systems, there just won’t be enough room. Such systems will need ultralow-power control chips that can operate inside the refrigerator. Engineers unveiled recently some potential solutions.
CryoCMOS:
Perhaps the most straightforward way to make cryogenic controls for quantum computers is to modify CMOS technology.
Microrelays:
In logic circuits, transistors act as switches, but they aren’t the only devices that do so. Engineers in Tsu-Jae King Liu’s laboratory at the University of California, Berkeley, have developed micrometer-scale electromechanical relays as ultralow-power alternatives to transistors.
Single-flux quantum logic:
Hypres, in Elmsford, N.Y., has been commercializing cryogenic ICs for several years. Seeking to steer its rapid single-flux quantum (RSFQ) logic tech into the realm of quantum computing, the company recently spun out a startup called Seeqc. Seeqc is now designing an entire system using the technology: a digital-control, error-correction, and readout chip designed to work at 3 to 4 K and a separate chip designed to work at 20 millikelvins to interface with the quantum processor.
Weyl semimetals:
Quantum computing is already strange, but it might take some even stranger tech to make it work. Scientists at Lund University, in Sweden, and at IBM Research–Zurich have designed a new device called a Weyl semimetal amplifier that they say could bring readout electronics closer to the qubits.