Quantum computing roundup: Still more technologies making waves - Ars Technica

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If you follow quantum computing news for long enough, it can start to seem like any quantum system that can alternate between two well-separated energy states can be used as a qubit. Atoms, ions, photons, electrons, and manufactured devices all have their backers. One of the key things that attracts backers is the tech’s ability to scale. We’ll need a lot of high-quality qubits to start doing any complex computations, and the ability of any technology to get us there is the subject of debate.

So even as some technologies can now support thousands of qubits, some competitors are still working on a small handful of qubits—the companies behind them are convinced that they have the potential to scale more effectively.

One of those technologies involves quantum dots that hold a single electron. Their advantage is that we can manufacture them using the same tech we use to build traditional processors, an approach that has proven to be scalable. This week saw two new papers describing different ways of using quantum dots, one of which was appealing enough that IBM bought the company that developed it. Separately, another company has released a processor showing 100 individual electrons being held in diamond defects, technology that wasn’t obvious could scale.

Flipping spins

The first news comes from a publication in Nature from a company called HRL Laboratories, the descendant of the research program launched by Howard Hughes. HRL is using manufactured quantum dots, a distinct technology from the dots used in displays, although it relies on some of the same physics principles. Both approaches use a structure that traps electrons in a region smaller than the electron’s wavelength. In displays, this allows control of the wavelength that the material emits.

For qubits, the traps are far smaller and serve to hold a single electron in place. Critically, they can be manufactured; with the right wiring, electromagnetic interactions can trap a single electron in a small patch of silicon. Once trapped, the electron’s spin, which can be up, down, or a superposition of the two, can be used as a qubit. While this technology can scale easily—we’re very good at putting wiring into silicon at scale—electron spins are hard to keep stable and are typically controlled via microwaves, requiring a separate control system.

But “typically” doesn’t mean “always,” and HRL is describing a different tech. It requires three separate quantum dots, each holding an electron, with the surrounding electronics controlling how much the spins of these three electrons can interact. That’s critical because, under certain conditions, no two electrons can have the same spin. This explains why atomic orbitals fill up the way they do, with each energy level holding just a pair, one spin-up, the other spin-down. Enabling them to interact can alter their spins.

To do operations on this kind of qubit, you simply need to control which electrons are interacting and to what extent. That is controlled electronically, allowing us to eliminate microwaves entirely. Everything is handled via wiring, eliminating the need for lots of microwave-carrying cabling into the refrigeration system that keeps the hardware near absolute zero.

HRL spends much of the paper describing its control system, which sits at an intermediate level of refrigeration and consists of a traditional processor optimized for low-temperature and low-power operations, consuming less than 3.5 watts despite being manufactured on a 130 nm process. Instructions for operating the qubits are compiled elsewhere, then loaded into the controller, after which it operates autonomously. Communications with the chip that holds the qubits are handled by a superconducting ribbon cable.

The system HRL describes had 18 qubits, and the company ran a simple error-correction code on it, demonstrating a logical error rate of less than 1 percent. That’s well below what has been achieved with other technology, but it’s an important validation that the problems HRL is facing are likely to be in the realm of engineering rather than physics.

Hybrid machines

Although the paper was published by HRL, the progress it has made has since become the property of IBM. That’s notable because IBM has bet big on competing technology in which qubits are held in a manufactured device called a transmon and controlled with microwave pulses—precisely the control system that HRL was trying to avoid. It’s not an...