Yet more qubit tech: New quantum dot options, diamond vacancies

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. One problem with quantum dots and other manufactured qubits is that their connections are dictated by the wiring of the chip they’re on. Since error correction codes are based on the geometry of the connections among qubits, this means committing to one or a small number of error correction codes when the chip is manufactured. If better code is developed or if some operations are easier to perform with a different code, you’re out of luck until the next generation of chips. Qubits based on trapped ions or neutral atoms don’t have this limitation. The qubits can be moved around so that any one can hypothetically be connected to any other. This provides a great deal of algorithmic flexibility and efficiency; their backers expect it’s enough to overcome the relatively slow operations compared to silicon-based qubits. In May, however, the Delft team showed that an individual spin could be moved from one quantum dot to a neighboring, unoccupied one and that this approach could scale—the spin could be moved through several intervening dots and end up on the opposite side of the chip from where it started. In theory, this means any two spins could be moved around arbitrarily, brought into proximity, and entangled. The approach combines the advantage of manufacturing with the flexibility of atom-based systems. A separate technology went pretty quiet after we looked at it a decade ago: nitrogen vacancies in diamonds. A diamond is a regular array of carbon atoms, all sharing bonds with their neighbors. But impurities can be scattered throughout the array, including the incorporation of nitrogen. Nitrogen can form one less bond than carbon, so it leaves a neighboring carbon with a single unbonded electron. The spin of that electron can be manipulated just like the spin of one held in a quantum dot. The problem has been that it’s very difficult to control where nitrogen vacancies end up within the diamond. While it’s relatively easy to run wires to them after they’re identified, every chip made to work with nitrogen vacancies is bespoke; if you use a standardized chip layout, there’s no guarantee that there will be one—and only one—nitrogen vacancy where the wiring expects it to be. Saxon Q, a spinout of the Universität Leipzig, has apparently developed a technique that can put nitrogen vacancies within a 10-nanometer radius. The company’s hardware is striking in several ways, most notably because it runs at room temperature. The machines it is building fit in a standard rack mount and plug right into a normal power supply. The other notable feature is its modular design; the company builds a “core” with eight qubits and can integrate multiple cores in a single rack. It announced last week that it is selling units with over 100 qubits total, which it expects to begin shipping within a few months. Larger systems are expected next year. Both will be too small to do interesting error-corrected computations. But Saxon Q says its individual hardware qubits have a gate fidelity of over 99.9 percent, making the technology competitive with others on the market. So once again, we return to where we started: The key question remains whether the technology can scale.