触发器量子位:有望设计发明全新的量子芯片

诸平

据澳大利亚新南威尔士大学（University of New South Wales, UNSW）2017年9月6日提供的消息，美国橡树岭国家实验室（Oak Ridge National Laboratory）、美国普渡大学（Purdue University）与该大学的研究人员合作研究，他们发明了一种基于新奇的触发器量子位（'flip-flop qubits'）构建量子计算机的新方法，有望能以更简单、更廉价的方式批量生产量子芯片，相关研究成果于2017年9月6日在《自然通讯》（Nature Communications）杂志网站上发表——Guilherme Tosi, Fahd A. Mohiyaddin, Vivien Schmitt, Stefanie Tenberg, Rajib Rahman, Gerhard Klimeck, Andrea Morello. Silicon quantum processor with robust long-distance qubit couplings. Nature Communications, 06 September 2017, 8: 450. DOI: 10.1038/s41467-017-00378-x.

量子计算机是一种遵循量子力学规律进行高速数学和逻辑运算、存储及处理量子信息的物理装置，它能利用亚原子粒子的神奇力量来解决一些对于当前算机而言，过于复杂或过于耗时的疑难问题。而量子计算机的核心技术就是量子芯片，澳大利亚与美国科学家合作开发的新芯片设计方案,在《自然通讯》杂志网站发表的论文已经有详细的介绍,允许一种硅量子处理器,可以扩大并不需要以其他方法精确布置原子。重要的是允许量子位(quantum bits or 'qubits')这一量子计算机当中信息的基本单位,被放置相距数百纳米,而且依然存在耦合。这种新型芯片设计能让硅量子处理器克服当前所存在的2个局限性，其一是必须精准放置原子，另外就是原子必须分开放置，但又要相互连接。

澳大利亚悉尼新南威尔士大学量子计算和通信技术卓越ARC中心(UNSW-based ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) in Sydney)的项目负责人、也是构思发明“触发器量子位”（flip-flop qubit）的课题负责人安德里亚·莫雷洛（Andrea Morello）称，这种新设计允许人们使用与生产当前计算机芯片同样的设备技术来生产量子处理器。《自然通讯》杂志论文的第一作者吉列尔梅·托斯（Guilherme Tosi）也是CQC2T的一名研究人员,与CQC2T的安德里亚·莫雷洛、合作者法赫德·莫希亚丁（Fahd Mohiyaddin）、维维安·施密特（Vivien Schmitt）以及斯蒂芬妮·藤伯格（Stefanie Tenberg）一起推提出了先驱性的概念，与美国普渡大学的合作者拉杰卜·拉赫曼（Rajib Rahman）以及格哈德·凯利默克（Gerhard Klimeck）也就有关问题一起进行过商讨。

安德里亚·莫雷洛说：“这是一个杰出的设计,就像许多这样概念上的跳跃一样,令人惊异的是因为之前没有人想到它。”更多信息请注意浏览原文或者相关报道。

Flip-flop qubits: Radical new quantum computing design invented

                   September 6, 2017                                                                

       Artist's impression of the 'flip-flop' qubits exhibiting quantum entanglement. Credit: Tony Melov/UNSW    

Engineers at Australia's University of New South Wales have invented a radical new architecture for quantum computing, based on novel 'flip-flop qubits', that promises to make the large-scale manufacture of quantum chips dramatically cheaper - and easier - than thought possible

The new chip design, detailed in the journal Nature Communications, allows for a silicon quantum processor that can be scaled up without the precise placement of atoms required in other approaches. Importantly, it allows quantum bits (or 'qubits') - the basic unit of information in a quantum computer - to be placed hundreds of nanometres apart and still remain coupled.

The design was conceived by a team led by Andrea Morello, Program Manager in UNSW-based ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) in Sydney, who said fabrication of the new design should be easily within reach of today's technology.

Lead author Guilherme Tosi, a Research Fellow at CQC2T, developed the pioneering concept along with Morello and co-authors Fahd Mohiyaddin, Vivien Schmitt and Stefanie Tenberg of CQC2T, with collaborators Rajib Rahman and Gerhard Klimeck of Purdue University in the USA.

"It's a brilliant design, and like many such conceptual leaps, it's amazing no-one had thought of it before," said Morello.

"What Guilherme and the team have invented is a new way to define a 'spin qubit' that uses both the electron and the nucleus of the atom. Crucially, this new qubit can be controlled using electric signals, instead of magnetic ones. Electric signals are significantly easier to distribute and localise within an electronic chip."

Tosi said the design sidesteps a challenge that all spin-based silicon qubits were expected to face as teams begin building larger and larger arrays of qubits: the need to space them at a distance of only 10-20 nanometres, or just 50 atoms apart.

"If they're too close, or too far apart, the 'entanglement' between quantum bits - which is what makes quantum computers so special - doesn't occur," Tosi said.

Researchers at UNSW already lead the world in making spin qubits at this scale, said Morello. "But if we want to make an array of thousands or millions of qubits so close together, it means that all the control lines, the control electronics and the readout devices must also be fabricated at that nanometric scale, and with that pitch and that density of electrodes. This new concept suggests another pathway."

At the other end of the spectrum are superconducting circuits - pursued for instance by IBM and Google - and ion traps. These systems are large and easier to fabricate, and are currently leading the way in the number of qubits that can be operated. However, due to their larger dimensions, in the long run they may face challenges when trying to assemble and operate millions of qubits, as required by the most useful quantum algorithms.

"Our new silicon-based approach sits right at the sweet spot," said Morello, a professor of quantum engineering at UNSW. "It's easier to fabricate than atomic-scale devices, but still allows us to place a million qubits on a square millimetre."

In the single-atom qubit used by Morello's team, and which Tosi's new design applies, a silicon chip is covered with a layer of insulating silicon oxide, on top of which rests a pattern of metallic electrodes that operate at temperatures near absolute zero and in the presence of a very strong magnetic field.

       Dr. Guilherme Tosi and Professor Andrea Morello at the UNSW labs with a dilution refrigerator, which cools silicon chips down to 0.01 K above absolute zero. Credit: Quentin Jones/UNSW    

At the core is a phosphorus atom, from which Morello's team has previously built two functional qubits using an electron and the nucleus of the atom. These qubits, taken individually, have demonstrated world-record coherence times.

Tosi's conceptual breakthrough is the creation of an entirely new type of qubit, using both the nucleus and the electron. In this approach, a qubit '0' state is defined when the spin of the electron is down and the nucleus spin is up, while the '1' state is when the electron spin is up, and the nuclear spin is down.

"We call it the 'flip-flop' qubit," said Tosi. "To operate this qubit, you need to pull the electron a little bit away from the nucleus, using the electrodes at the top. By doing so, you also create an electric dipole."

"This is the crucial point," adds Morello. "These electric dipoles interact with each other over fairly large distances, a good fraction of a micron, or 1,000 nanometres.

"This means we can now place the single-atom qubits much further apart than previously thought possible," he continued. "So there is plenty of space to intersperse the key classical components such as interconnects, control electrodes and readout devices, while retaining the precise atom-like nature of the quantum bit."

Morello called Tosi's concept as significant as Bruce Kane seminal 1998 paper in Nature. Kane, then a senior research associate at UNSW, hit upon a new architecture that could make a silicon-based quantum computer a reality - triggering Australia's race to build a quantum computer.

       Flop qubit processor illustration. Credit: Guilherme Tosui    

"Like Kane's paper, this is a theory, a proposal - the qubit has yet to be built," said Morello. "We have some preliminary experimental data that suggests it's entirely feasible, so we're working to fully demonstrate this. But I think this is as visionary as Kane's original paper."

Building a quantum computer has been called the 'space race of the 21st century' - a difficult and ambitious challenge with the potential to deliver revolutionary tools for tackling otherwise impossible calculations, with a plethora of useful applications in healthcare, defence, finance, chemistry and materials development, software debugging, aerospace and transport. Its speed and power lie in the fact that quantum systems can host multiple 'superpositions' of different initial states, and in the spooky 'entanglement' that only occurs at the quantum level the fundamental particles.

"It will take great engineering to bring quantum computing to commercial reality, and the work we see from this extraordinary team puts Australia in the driver's seat," said Mark Hoffman, UNSW's Dean of Engineering. "It's a great example of how UNSW, like many of the world's leading research universities, is today at the heart of a sophisticated global knowledge system that is shaping our future."

The UNSW team has struck a A$83 million deal between UNSW, telco giant Telstra, Australia's Commonwealth Bank and the Australian and New South Wales governments to develop, by 2022, a 10-qubit prototype silicon quantum integrated circuit - the first step in building the world's first quantum computer in silicon.

In August, the partners launched Silicon Quantum Computing Pty Ltd, Australia's first quantum computing company, to advance the development and commercialisation of the team's unique technologies. The NSW Government pledged A$8.7 million, UNSW A$25 million, the Commonwealth Bank A$14 million, Telstra A$10 million and the Federal Government A$25 million.                                                                

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