Two Strange Ideas for a Megaqubit Quantum Computer Story-level

The perpetual problem with scaling up most quantum computers is a seemingly mundane one: too many wires. Experts say that quantum computers could need at least a million qubits held close to absolute zero to do anything computationally remarkable. But connecting them all by coaxial cable to the control and reading electronics, which work at room temperature, would be impossible.

Computer giants like IBM, Google and Intel hope to solve that problem with cryogenic silicon chips that can operate close to qubits. But researchers have recently come up with some more exotic solutions that could pick up the pace.

In it IEEE International Electronic Devices Meeting (IEDM) in December, two groups of researchers suggested that silicon might not be the best answer. Instead, their solutions are based on semiconductors and transistors more commonly targeted at near terahertz radio frequency. And in February in the IEEE International Solid State Circuits Conference (ISSCC) a separate research group proposed a technology that could use terahertz radio to eliminate communication cables altogether.

shared quantum wells

A type of device made of compound semiconductors like indium gallium arsenide instead of silicon and called a high electron mobility transistor (HEMT) is a natural for amplifying the kind of RF signals needed to interact with qubits. But researchers at the Korea Institute for Advanced Technology (KAIST) and IBM Zurich and École Polytechnique Fédérale de Lausanne (EPFL) reckon it could also do the job of reducing routing, multiplexing and demultiplexing cables. Crucially, it could do it with little power loss, which is important because in the coldest parts of the cryogenic chambers used for quantum computers, the cooling system can remove only a couple of watts of heat.

HEMTs have a layered semiconductor structure that creates a super-narrow region of free electrons, called a two-dimensional electron gas. Charge moves quickly and with little resistance through this “quantum well,” hence the HEMT’s ability to amplify high-frequency signals. The KAIST and Swiss teams reasoned that at cryogenic temperatures, 2D electron gas could carry signals with less resistance than metal.

To prove it, they built demultiplexer circuits made up of several transistors and tested them at 5 Kelvin. Instead of connecting each transistor to its neighbor with a metal interconnect, they made them share the quantum well. The only metal involved was where the signal entered the multiplexing network and where it exited. “It doesn’t matter how many transistors there are between the input and output, there are only two sources of resistance,” he says. sang heyon kimassociate professor of electrical engineering at KAIST.

The Switzerland-based team built similar structures, measuring a 32 percent reduction in resistance between two transistors connected by a metal interconnect and two connected by a quantum well. A 1 to 8 multiplexer may need 14 transistors for the resistance enhancement to add up quickly.

“We are doing a lot of things with this technology, some still in the planning phase,” he says. Cezar B. Zota, a member of the IBM Zurich research staff. His team plans to expand their test device from two transistors to a full matrix switcher. While Kim’s lab focuses on integrating multiplexers with low-noise amplifiers and other electronic devices through 3D stacking.

Qubit control signals could be multiplexed to reduce the number of wires going to the quantum computing chip. Transmitting those signals into the quantum well. [blue] High electron mobility transistors generate less heat.IBM Research Zurich

backscatter trays

Multiplexing can reduce the number of signal wires going to the qubit chip, but what if they could be eliminated entirely? MIT researchers, led by an associate professor of electrical and computer engineering Han Ruonan, tried a scheme that would use terahertz waves instead. They settled on radiation close to terahertz, specifically 0.26 THz, because, among other reasons, it was too high a frequency to interfere with qubit operations, and it worked with small enough antennas.

A full power terahertz transceiver would throw off too much heat to place near the qubit chip. Instead, the MIT team designed a terahertz “backscatter” system. The system would consist of two transceiver chips, one at the top of the cooler, where it is warmest and power consumption is less of an issue, and one at the bottom as part of a 4 Kelvin Cryo Control Chip linked to the chip. of quantum computer.

Terahertz radiation is injected into the cooler where it is channeled to the hot top transceiver chip. In “downlink” mode, that transceiver encodes data into terahertz radiation. The signals travel through the cooler to the bottom, where they are picked up by an array of patch antennas on the cold transceiver.

Instead of using wires to connect external electronic devices to quantum computers, the MIT researchers propose using terahertz radiation.WITH

To get data from the quantum computing chip, the system switches to uplink mode. The warm transceiver sends a steady beam of terahertz radiation to the cold transceiver. Switches on that chip alter the antenna’s circuitry, causing it to reflect radiation instead of absorbing it, thus sending data to the warm transceiver.

In system tests, the uplink could send 4 gigabits per second and add only 176 femtojoules per bit of heat. The downlink was even more power efficient, at just 34 femtojoules per bit.