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.”

走進科學家誘捕離子的量子實驗室，你會發(fā)現(xiàn)臺面上擺滿了鏡子和透鏡，它們都在聚焦激光，以擊中 "被困 "在芯片上的離子。通過使用激光來控制離子，科學家們已經(jīng)學會了將離子作為量子比特或量子比特來利用，這是量子計算機中數(shù)據(jù)的基本單位。但是，這種激光設(shè)置阻礙了研究的進行--使其難以對超過幾個離子進行實驗，也難以將這些系統(tǒng)帶出實驗室用于實際用途。

現(xiàn)在，麻省理工學院林肯實驗室的研究人員已經(jīng)開發(fā)出一種緊湊的方法，將激光傳遞給被困的離子。在最近發(fā)表在《自然》雜志上的一篇論文中，研究人員描述了一個光纖塊，它可以插入離子捕獲器芯片，將光耦合到芯片本身制造的光波導上。通過這些波導，多種波長的光可以穿過芯片并釋放出來，打到芯片上方的離子上。

"林肯實驗室量子信息和集成納米系統(tǒng)組的高級職員、論文作者杰里米-塞奇說："該領(lǐng)域的許多人都清楚，使用鏡子和透鏡等自由空間光學器件的傳統(tǒng)方法只能走到這里。"如果把光帶到芯片上，它可以被引導到它需要的許多地方。許多波長的集成傳輸可能會導致一個非?？蓴U展和便攜的平臺。我們首次展示了這是可以做到的"。

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.?

多種顏色

用困住的離子進行計算需要精確地獨立控制每個離子。自由空間光學技術(shù)在控制短的一維鏈中的幾個離子時效果很好。但是，在一個較大的或二維的集群中擊中一個單一的離子，而不擊中其鄰居，是非常困難的。當想象一個需要成千上萬個離子的實用量子計算機時，這種激光控制的任務似乎不切實際。

這個迫在眉睫的問題導致研究人員找到了另一種方法。2016年，林肯實驗室和麻省理工學院的研究人員展示了一種內(nèi)置光學器件的新芯片。他們將紅色激光聚焦到芯片上，芯片上的波導將光線引向光柵耦合器，這是一種隆起帶，用于阻止光線并將其引向離子。

脈沖彩色激光的動畫

這個測量激光光束輪廓的動畫顯示了四個波長的激光被離子捕獲器芯片中的 "光柵耦合器 "發(fā)射出來。黃色的表面是芯片頂部的金屬電極層，用于捕獲上面的離子。

紅光對于進行一種稱為量子門的基本操作至關(guān)重要，該團隊在第一次演示中進行了這種操作。但是需要多達六種不同顏色的激光來完成量子計算所需的一切：準備離子，冷卻它，讀出它的能量狀態(tài)，并執(zhí)行量子門。有了這個最新的芯片，該團隊已經(jīng)將他們的原理證明擴展到這些所需波長的其余部分，從紫羅蘭到近紅外。

"這篇論文的作者John Chiaverini說："通過這些波長，我們能夠進行你需要控制被困離子的一組基本操作。他們沒有進行的一個操作，即雙比特門，由蘇黎世聯(lián)邦理工學院的一個團隊通過使用類似于2016年工作的芯片進行了演示，并在同一期《自然》雜志的一篇論文中進行了描述。"Chiaverini補充說："這項工作，與我們的工作搭配在一起，表明你擁有開始建造更大的陷落離子陣列所需的所有東西。

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.

纖維光學技術(shù)

為了實現(xiàn)從一個波長到多個波長的飛躍，該團隊設(shè)計了一種方法，將一個光纖塊直接粘合在芯片的側(cè)面。該塊由四根光纖組成，每根光纖都是針對某一波長范圍的。這些光纖與直接印在芯片上的相應波導排成一列。

"將光纖塊陣列對準芯片上的波導并涂上環(huán)氧樹脂，感覺就像做手術(shù)一樣。這是一個非常微妙的過程。我們有大約半微米的公差，而且它需要在冷卻到4開爾文的情況下存活下來，"領(lǐng)導實驗的羅伯特-尼芬格說，他是論文的第一作者。

在波導的頂部有一層玻璃。玻璃上面是金屬電極，它產(chǎn)生的電場將離子固定在原地；在光柵耦合器上的金屬上開了孔，光在那里被釋放出來。整個裝置是在林肯實驗室的微電子實驗室制造的。

設(shè)計能夠以低損耗將光傳遞給離子的波導，避免吸收或散射，是一個挑戰(zhàn)，因為損耗往往會隨著藍色波長的增加而增加。"這是一個開發(fā)材料、制作波導圖案、測試它們、測量性能和再次嘗試的過程。我們還必須確保波導的材料不僅能與必要的光波長一起工作，而且不會與捕獲離子的金屬電極發(fā)生干擾，"Sage說。

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.