点亮离子阱内置于芯片中的光纤可以提供控制离子进行量子计算和传感所需激光。
Walk into a quantum lab where scientists trap ions, and you'll find benchtops full of mirrors and lenses, all focusing lasers to hit an ion “trapped” in place above a chip. By using lasers to control ions, scientists have learned to harness ions as quantum bits, or qubits, the basic unit of data in a quantum computer. But this laser setup is holding research back — making it difficult to experiment with more than a few ions and to take these systems out of the lab for real use.
Now, MIT Lincoln Laboratory researchers have developed a compact way to deliver laser light to trapped ions. In a recent paper published in Nature, the researchers describe a fiber-optic block that plugs into the ion-trap chip, coupling light to optical waveguides fabricated in the chip itself. Through these waveguides, multiple wavelengths of light can be routed through the chip and released to hit the ions above it.
“It's clear to many people in the field that the conventional approach, using free-space optics such as mirrors and lenses, will only go so far,” says Jeremy Sage, an author on the paper and senior staff in Lincoln Laboratory's Quantum Information and Integrated Nanosystems Group. “If the light instead is brought onto the chip, it can be directed around to the many locations where it needs to be. The integrated delivery of many wavelengths may lead to a very scalable and portable platform. We're showing for the first time that it can be done.”
走进科学家诱捕离子的量子实验室，你会发现台面上摆满了镜子和透镜，它们都在聚焦激光，以击中 "被困 "在芯片上的离子。通过使用激光来控制离子，科学家们已经学会了将离子作为量子比特或量子比特来利用，这是量子计算机中数据的基本单位。但是，这种激光设置阻碍了研究的进行--使其难以对超过几个离子进行实验，也难以将这些系统带出实验室用于实际用途。
现在，麻省理工学院林肯实验室的研究人员已经开发出一种紧凑的方法，将激光传递给被困的离子。在最近发表在《自然》杂志上的一篇论文中，研究人员描述了一个光纤块，它可以插入离子捕获器芯片，将光耦合到芯片本身制造的光波导上。通过这些波导，多种波长的光可以穿过芯片并释放出来，打到芯片上方的离子上。
"林肯实验室量子信息和集成纳米系统组的高级职员、论文作者杰里米-塞奇说："该领域的许多人都清楚，使用镜子和透镜等自由空间光学器件的传统方法只能走到这里。"如果把光带到芯片上，它可以被引导到它需要的许多地方。许多波长的集成传输可能会导致一个非常可扩展和便携的平台。我们首次展示了这是可以做到的"。
Multiple colors
Computing with trapped ions requires precisely controlling each ion independently. Free-space optics have worked well when controlling a few ions in a short one-dimensional chain. But hitting a single ion among a larger or two-dimensional cluster, without hitting its neighbors, is extremely difficult. When imagining a practical quantum computer requiring thousands of ions, this task of laser control seems impractical.
That looming problem led researchers to find another way. In 2016, Lincoln Laboratory and MIT researchers demonstrated a new chip with built-in optics. They focused a red laser onto the chip, where waveguides on the chip routed the light to a grating coupler, a kind of rumble strip to stop the light and direct it up to the ion.
Red light is crucial for doing a fundamental operation called a quantum gate, which the team performed in that first demonstration. But up to six different-colored lasers are needed to do everything required for quantum computation: prepare the ion, cool it down, read out its energy state, and perform quantum gates. With this latest chip, the team has extended their proof of principle to the rest of these required wavelengths, from violet to the near-infrared.
“With these wavelengths, we were able to perform the fundamental set of operations that you need to be able to control trapped ions,” says John Chiaverini, also an author on the paper. The one operation they didn't perform, a two-qubit gate, was demonstrated by a team at ETH Zürich by using a chip similar to the 2016 work, and is described in a paper in the same Nature issue. “This work, paired together with ours, shows that you have all the things you need to start building larger trapped-ion arrays,” Chiaverini adds.
多种颜色
用困住的离子进行计算需要精确地独立控制每个离子。自由空间光学技术在控制短的一维链中的几个离子时效果很好。但是，在一个较大的或二维的集群中击中一个单一的离子，而不击中其邻居，是非常困难的。当想象一个需要成千上万个离子的实用量子计算机时，这种激光控制的任务似乎不切实际。
这个迫在眉睫的问题导致研究人员找到了另一种方法。2016年，林肯实验室和麻省理工学院的研究人员展示了一种内置光学器件的新芯片。他们将红色激光聚焦到芯片上，芯片上的波导将光线引向光栅耦合器，这是一种隆起带，用于阻止光线并将其引向离子。
脉冲彩色激光的动画
这个测量激光光束轮廓的动画显示了四个波长的激光被离子捕获器芯片中的 "光栅耦合器 "发射出来。黄色的表面是芯片顶部的金属电极层，用于捕获上面的离子。
红光对于进行一种称为量子门的基本操作至关重要，该团队在第一次演示中进行了这种操作。但是需要多达六种不同颜色的激光来完成量子计算所需的一切：准备离子，冷却它，读出它的能量状态，并执行量子门。有了这个最新的芯片，该团队已经将他们的原理证明扩展到这些所需波长的其余部分，从紫罗兰到近红外。
"这篇论文的作者John Chiaverini说："通过这些波长，我们能够进行你需要控制被困离子的一组基本操作。他们没有进行的一个操作，即双比特门，由苏黎世联邦理工学院的一个团队通过使用类似于2016年工作的芯片进行了演示，并在同一期《自然》杂志的一篇论文中进行了描述。"Chiaverini补充说："这项工作，与我们的工作搭配在一起，表明你拥有开始建造更大的陷落离子阵列所需的所有东西。
Fiber optics
To make the leap from one to multiple wavelengths, the team engineered a method to bond a fiber-optic block directly to the side of the chip. The block consists of four optical fibers, each one specific to a certain range of wavelengths. These fibers line up with a corresponding waveguide patterned directly onto the chip.
“Getting the fiber block array aligned to the waveguides on the chip and applying the epoxy felt like performing surgery. It was a very delicate process. We had about half a micron of tolerance and it needed to survive cooldown to 4 kelvins,” says Robert Niffenegger, who led the experiments and is first author on the paper.
On top of the waveguides sits a layer of glass. On top of the glass are metal electrodes, which produce electric fields that hold the ion in place; holes are cut out of the metal over the grating couplers where the light is released. The entire device was fabricated in the Microelectronics Laboratory at Lincoln Laboratory.
Designing waveguides that could deliver the light to the ions with low loss, avoiding absorption or scattering, was a challenge, as loss tends to increase with bluer wavelengths. “It was a process of developing materials, patterning the waveguides, testing them, measuring performance, and trying again. We also had to make sure the materials of the waveguides worked not only with the necessary wavelengths of light, but also that they didn't interfere with the metal electrodes that trap the ion,” Sage says.
纤维光学技术
为了实现从一个波长到多个波长的飞跃，该团队设计了一种方法，将一个光纤块直接粘合在芯片的侧面。该块由四根光纤组成，每根光纤都是针对某一波长范围的。这些光纤与直接印在芯片上的相应波导排成一列。
"将光纤块阵列对准芯片上的波导并涂上环氧树脂，感觉就像做手术一样。这是一个非常微妙的过程。我们有大约半微米的公差，而且它需要在冷却到4开尔文的情况下存活下来，"领导实验的罗伯特-尼芬格说，他是论文的第一作者。
在波导的顶部有一层玻璃。玻璃上面是金属电极，它产生的电场将离子固定在原地；在光栅耦合器上的金属上开了孔，光在那里被释放出来。整个装置是在林肯实验室的微电子实验室制造的。
设计能够以低损耗将光传递给离子的波导，避免吸收或散射，是一个挑战，因为损耗往往会随着蓝色波长的增加而增加。"这是一个开发材料、制作波导图案、测试它们、测量性能和再次尝试的过程。我们还必须确保波导的材料不仅能与必要的光波长一起工作，而且不会与捕获离子的金属电极发生干扰，"Sage说。
Scalable and portable
The team is now looking forward to what they can do with this fully light-integrated chip. For one, “make more,” Niffenegger says. “Tiling these chips into an array could bring together many more ions, each able to be controlled precisely, opening the door to more powerful quantum computers.”
Daniel Slichter, a physicist at the National Institute of Standards and Technology who was not involved in this research, says, “This readily scalable technology will enable complex systems with many laser beams for parallel operations, all automatically aligned and robust to vibrations and environmental conditions, and will in my view be crucial for realizing trapped ion quantum processors with thousands of qubits.”
An advantage of this laser-integrated chip is that it's inherently resistant to vibrations. With external lasers, any vibration to the laser would cause it to miss the ion, as would any vibrations to the chip. Now that the laser beams and chip are coupled together, the effects of vibrations are effectively nullified.
This stability is important for the ions to sustain “coherence,” or to operate as qubits long enough to compute with them. It's also important if trapped-ion sensors are to become portable. Atomic clocks, for example, that are based on trapped ions could keep time much more precisely than today's standard, and could be used to improve the accuracy of GPS, which relies on the synchronization of atomic clocks carried on satellites.
“We view this work as an example of bridging science and engineering, that delivers a true advantage to both academia and industry,” Sage says. Bridging this gap is the goal of the MIT Center for Quantum Engineering, where Sage is a principal investigator. “We need quantum technology to be robust, deliverable, and user-friendly, for people to use who aren't PhDs in quantum physics,” Sage says.
Simultaneously, the team hopes that this device can help push academic research. “We want other research institutes to use this platform so that they can focus on other challenges — like programming and running algorithms with trapped ions on this platform, for example. We see it opening the door to further exploration of quantum physics,” Chiaverini says.
