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

量子比特技术再添新进展：量子点新方案与金刚石空位

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. 如果你关注量子计算新闻足够久，就会发现任何能够在两个截然不同的能态之间切换的量子系统，似乎都可以被用作量子比特。原子、离子、光子、电子以及人造器件，每种技术都有其支持者。吸引支持者的关键因素之一是该技术的可扩展性。我们需要大量高质量的量子比特才能开始进行复杂的计算，而任何技术能否实现这一目标，目前仍是争论的焦点。

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. 因此，尽管有些技术现在已经能支持数千个量子比特，但一些竞争对手仍致力于研发仅含少量量子比特的系统——背后的公司坚信它们具有更高效的扩展潜力。其中一种技术涉及容纳单个电子的量子点。它们的优势在于，我们可以利用制造传统处理器的相同技术来生产它们，这种方法已被证明具有可扩展性。本周发表的两篇新论文描述了使用量子点的不同方法，其中一种方案极具吸引力，以至于 IBM 收购了开发该技术的公司。此外，另一家公司发布了一款处理器，展示了在金刚石缺陷中容纳 100 个独立电子的技术，这在以前被认为难以扩展。

Flipping spins

翻转自旋

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. 第一则新闻来自 HRL Laboratories 在《自然》杂志上发表的一篇论文，该公司是霍华德·休斯（Howard Hughes）发起的科研项目的后继者。HRL 使用的是人造量子点，这与显示器中使用的量子点技术不同，尽管它们依赖于一些相同的物理原理。两种方法都使用一种结构，将电子捕获在小于电子波长的区域内。在显示器中，这可以控制材料发射的波长。对于量子比特而言，这些陷阱要小得多，其作用是将单个电子固定在特定位置。

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. 关键在于，它们是可以制造的；通过合适的布线，电磁相互作用可以将单个电子捕获在硅片的一小块区域内。一旦被捕获，电子的自旋（可以是向上、向下或两者的叠加态）就可以用作量子比特。虽然这项技术易于扩展——我们非常擅长在大规模硅片上进行布线——但电子自旋很难保持稳定，通常需要通过微波进行控制，这需要一套独立的控制系统。

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. 但“通常”并不意味着“总是”，HRL 描述了一种不同的技术。它需要三个独立的量子点，每个量子点容纳一个电子，周围的电子设备控制这三个电子自旋之间的相互作用程度。这一点至关重要，因为在特定条件下，没有两个电子可以具有相同的自旋。这解释了原子轨道为何以特定的方式填充：每个能级仅容纳一对电子，一个自旋向上，另一个自旋向下。使它们产生相互作用可以改变它们的自旋。

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. 要对这种量子比特进行操作，只需控制哪些电子在相互作用以及相互作用的程度即可。这是通过电子方式控制的，使我们能够完全摒弃微波。一切都通过布线处理，消除了将大量微波传输电缆引入保持硬件接近绝对零度的制冷系统中的需求。

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. HRL 在论文中花费了大量篇幅描述其控制系统。该系统位于制冷系统的中间层，由一个针对低温和低功耗操作优化的传统处理器组成，尽管采用 130 纳米工艺制造，但功耗不到 3.5 瓦。操作量子比特的指令在别处编译，然后加载到控制器中，之后控制器便可自主运行。与容纳量子比特的芯片之间的通信由超导带状电缆处理。

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. HRL 描述的系统拥有 18 个量子比特，该公司在其上运行了一个简单的纠错码，证明逻辑错误率低于 1%。这远低于其他技术所取得的水平，但这是一个重要的验证，表明 HRL 面临的问题很可能属于工程领域，而非物理学领域。

Hybrid machines

混合机器

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 obvious fit at first, so we asked IBM Director of Research Jay Gambetta about the decision. 尽管这篇论文由 HRL 发表，但其取得的进展现已归 IBM 所有。这一点值得注意，因为 IBM 在竞争技术上投入了巨资，即量子比特被容纳在一种称为“超导量子比特”（transmon）的人造器件中，并使用微波脉冲进行控制——这正是 HRL 试图避免的控制系统。起初这看起来并不匹配，因此我们询问了 IBM 研究总监 Jay Gambetta 关于这一决定的看法。

“At a high level, I’m a strong believer in silicon technology—superconducting qubits are built on silicon, spins are built on silicon,” Gambetta told Ars. “It gives us flexibility to keep both parts going in parallel, build on the same foundation.” Right now, that foundation allows rapid iteration and testing of designs. IBM has typically released a new processor architecture every year, refining it over two or three generations before the next one arrives. “从宏观层面来看，我非常坚信硅技术——超导量子比特建立在硅上，自旋量子比特也建立在硅上，”Gambetta 告诉 Ars。“这给了我们灵活性，可以并行推进两部分技术，并建立在相同的基础之上。”目前，这一基础允许对设计进行快速迭代和测试。IBM 通常每年发布一种新的处理器架构，在下一代产品到来之前，会对其进行两到三代的改进。

But Gambetta suggested that in the longer term, this raises the possibility that HRL tech will appear in an IBM system, operating alongside IBM’s existing tech. “As we look at the error correction codes going forward, some codes are better at being memories, some are better at being magic state creation,” Gambetta said. “I want to have the flexibility to build my hardware that maximizes both of them.” 但 Gambetta 表示，从长远来看，这增加了 HRL 技术出现在 IBM 系统中并与现有技术协同运行的可能性。“当我们展望未来的纠错码时，有些代码更适合作为存储器，有些则更适合用于创建魔态（magic state），”Gambetta 说。“我希望拥有灵活性来构建我的硬件，从而最大化这两者的优势。”

Other spin tech

其他自旋技术

The same issue of Nature includes a separate paper on quantum-dot qubits from a group at Delft University of Technology. We covered their technology in May, but in short, it involves qubits based on single electrons in quantum dots and focuses on moving electrons and their spins around. One problem with quantum dots and other manufactured qubits is that their connections are dictated by the wiring of the chip they’re on. 同一期的《自然》杂志还收录了代尔夫特理工大学（Delft University of Technology）团队关于量子点量子比特的另一篇论文。我们在五月份报道过他们的技术，简而言之，它涉及基于量子点中单个电子的量子比特，并专注于移动电子及其自旋。量子点和其他人造量子比特的一个问题是，它们的连接方式受限于芯片本身的布线。