Manufacturing qubits that can move
Manufacturing qubits that can move
可移动量子比特的制造
To get quantum computing to work, we will ultimately need lots of high-quality qubits, which we can tie together into groups of error-corrected logical qubits. Companies are taking distinct approaches to get there, but you can think of them as falling into two broad categories. Some companies are focused on hosting the qubits in electronics that we can manufacture, guaranteeing that we can get lots of devices. Others are using atoms or photons as qubits, which give more consistent behavior but require lots of complicated hardware to manage. 为了实现量子计算，我们最终需要大量高质量的量子比特，并将它们组合成纠错逻辑量子比特组。各家公司正采取不同的路径来实现这一目标，但大体上可以归为两类：一些公司专注于将量子比特托管在可制造的电子设备中，以确保能够获得大量器件；另一些公司则使用原子或光子作为量子比特，虽然它们表现更稳定，但需要复杂的硬件来管理。
One advantage of systems that use atoms or ions is that we can move them around. This allows us to entangle any qubit with any other, which provides a great deal of flexibility for error correction. Systems based on electronic devices, in contrast, are locked into whatever configuration they’re wired into during manufacturing. But this week, a new paper examined research that seems to provide the best of both worlds. It works with quantum dots, which can be manufactured in bulk and host a qubit as a single electron’s spin. The work showed that it’s possible to move these spin qubits from one quantum dot to another without losing quantum information. The ability to move them around could potentially enable the sort of any-to-any connectivity we see with atoms and ions. 使用原子或离子的系统有一个优势，即我们可以移动它们。这使得我们能够将任意两个量子比特纠缠在一起，从而为纠错提供了极大的灵活性。相比之下，基于电子设备的系统在制造时其布线配置就已经固定。然而，本周发表的一篇新论文探讨了一项研究，似乎兼顾了这两者的优点。该研究利用量子点，这种材料可以批量生产，并以单个电子的自旋作为量子比特。研究表明，在不丢失量子信息的情况下，将这些自旋量子比特从一个量子点移动到另一个量子点是可行的。这种移动能力有望实现我们在原子和离子系统中看到的“任意对任意”连接。
Quantum trade-offs
量子权衡
A quantum dot can be thought of as a way of controlling an electron’s behavior. Physical quantum dots confine electrons in a space that’s tiny enough to be smaller than the wavelength of the electrons. Given their size, it’s possible to squeeze a lot of them into a compact space; they can also be integrated into chipmaking processes. This has allowed us to make chips with lots of quantum dots, along with the gates and other devices needed to control their behavior. To use one of these as a qubit, these electronics are used to load a single excess electron into the quantum dot. Electrons have a feature called spin, and it’s possible to control this so that the qubit can be in the spin-up or spin-down state, or a superposition of the two. 量子点可以被视为一种控制电子行为的方法。物理量子点将电子限制在一个极小的空间内，其尺寸甚至小于电子的波长。由于体积小，我们可以在紧凑的空间内挤入大量量子点，并且可以将它们集成到芯片制造工艺中。这使我们能够制造出包含大量量子点以及控制其行为所需的门电路和其他器件的芯片。要将其中一个用作量子比特，需要利用这些电子设备向量子点中加载一个多余的电子。电子具有自旋特性，我们可以对其进行控制，使量子比特处于自旋向上、自旋向下或两者的叠加态。
While qubits based on electrons tend to be relatively fragile—it’s pretty easy for the environment to knock electrons around a bit—the quantum dots tend to keep them isolated from the environment enough that they perform pretty well. Like any other manufactured chip, the wiring that connects the quantum dots is locked into place during the chip’s manufacture. Since different error correction schemes require different connections among the qubits, this forces us to commit to specific error-correction schemes during manufacturing. If a better scheme is developed after a chip is made, it’s probably not possible to switch to it. Less complex algorithms may benefit from simpler error-correction schemes that require less overhead, but we wouldn’t be able to switch schemes with these chips. 虽然基于电子的量子比特往往相对脆弱——环境很容易干扰电子——但量子点能将它们与环境充分隔离，从而使其表现良好。像其他制造出的芯片一样，连接量子点的布线在芯片制造过程中就已经固定。由于不同的纠错方案需要量子比特之间不同的连接方式，这迫使我们在制造时就必须确定特定的纠错方案。如果芯片制造后开发出了更好的方案，通常无法切换。较简单的算法可能受益于开销更小的简单纠错方案，但我们无法在这些芯片上切换方案。
So, quantum dots appear to typify the trade-offs that we’re facing with quantum computing: it’s easier for us to make lots of quantum dots and all the hardware needed to manipulate them, but it’s seemingly not possible for them to benefit from the flexibility that other types of qubits have. The whole point of this new paper is to show that this isn’t necessarily true. 因此，量子点似乎代表了我们在量子计算中所面临的权衡：制造大量量子点及其所需的操控硬件相对容易，但它们似乎无法像其他类型的量子比特那样具备灵活性。这篇新论文的核心意义在于证明这种观点并非绝对。
Moveable dots
可移动的量子点
The new work was done in collaboration between researchers at Delft University of Technology and the startup QuTech. The team built a chip that had a linear array of quantum dots, and they started out with single electron spins at each end. Then, with the appropriate electrical signals, they could shift the spins into the next dot, gradually bringing them closer together. (And, by gradually, we mean a fraction of a second here, but relatively slowly compared to basic switching in electronics.) Once the electrons were close enough, the spin wavefunctions overlapped, allowing the researchers to perform two-qubit gates on them. These manipulations can be used to entangle the two spins and are thus needed to build error-corrected logical qubits; these gates are also needed for performing calculations. 这项新工作由代尔夫特理工大学的研究人员与初创公司 QuTech 合作完成。团队制造了一块包含线性量子点阵列的芯片，并在两端各放置了一个单电子自旋。随后，通过适当的电信号，他们能够将自旋移动到下一个量子点，逐渐使它们靠近。（这里的“逐渐”是指在几分之一秒内完成，但与电子设备中的基本开关速度相比相对缓慢。）一旦电子足够接近，自旋波函数就会重叠，使研究人员能够对它们执行双量子比特门操作。这些操作可用于纠缠两个自旋，是构建纠错逻辑量子比特所必需的；这些门操作也是进行计算所必需的。
The researchers then confirmed that they could move the electrons back to their starting positions, after which measurements confirmed that their spins were entangled. And since quantum teleportation also requires a two-qubit gate, they showed that the process could be used for teleportation. Teleportation can enhance the sort of mobility provided by moving the qubits around, since it can be used to move states around after the qubits have been widely separated. (Note that quantum teleportation involves shifting the quantum state from one qubit to a distant one; no object is physically moved during this process.) 研究人员随后确认，他们可以将电子移回起始位置，测量结果证实它们的自旋仍然保持纠缠状态。由于量子隐形传态也需要双量子比特门，他们证明了该过程可用于隐形传态。隐形传态可以增强移动量子比特所带来的灵活性，因为它可以在量子比特被广泛分离后移动量子态。（注意，量子隐形传态涉及将量子态从一个量子比特转移到另一个遥远的量子比特；在此过程中没有物体被物理移动。）
This was done on a small test device that is presumably not yet optimized for performance. But the operations were done with pretty reasonable fidelity. The two-qubit gates were executed successfully over 99 percent of the time, while teleportation succeeded about 87 percent of the time. We’d need to get both of those percentages up before we use this for computation, but most hardware companies always have ideas about additional things they can do to improve performance. 这项工作是在一个小型测试设备上完成的，该设备可能尚未针对性能进行优化。但操作的保真度相当不错。双量子比特门的执行成功率超过 99%，而隐形传态的成功率约为 87%。在将其用于计算之前，我们需要提高这两个百分比，但大多数硬件公司总是有办法通过额外手段来提升性能。
On the dot
总结
The researchers briefly lay out the kinds of things they envision this enabling. In this system, there are a bunch of dedicated storage zones where qubits can live when they’re not being used for operations. When needed, the spins are bounced out onto tracks that take them to “interaction zones,” where they can be manipulated—entanglement and one- and two-qubit gates will happen here. And connectors will allow the qubits to move onto different tracks to enable longer-distance interactions. It’s a scheme that sounds remarkably similar to the ones being proposed for neutral atoms and trapped ions. But it also offers the benefits of bulk manufacturing and very co… 研究人员简要阐述了他们设想的实现前景。在该系统中，有一系列专用的存储区，量子比特在不进行操作时可以驻留于此。需要时，自旋会被移动到轨道上，送往“交互区”进行操控——纠缠以及单、双量子比特门操作将在此处发生。连接器将允许量子比特移动到不同的轨道上，以实现更长距离的交互。这种方案听起来与针对中性原子和囚禁离子提出的方案非常相似，但它同时也具备了批量制造和高度兼容的优势。