Open design for large scale quantum computer
An international team of researchers have developed an open design for a trapped ion quantum computer.
The international collaboration. led by the University of Sussex in the UK, includes scientists from Google US, Aarhus University in Denmark, RIKEN research institute in Japan and Siegen University in Germany. They developed a simpler modular design that used electric fields to transfer ions without having to align laser beams, allowing for a scalable architecture.
通过将单个激光束对准单个离子，每种离子都形成量子位，可以使用捕获的离子进行小尺度的量子计算。一台具有数十亿个量子的大型机器需要数十亿个精确对齐的激光器，使其不切实际。
苏塞克斯量子技术中心主任温弗里德·亨辛格（Winfried Hensinger）教授温弗里德·亨辛格（Winfried Hensinger）教授说：“多年来，人们说完全不可能建造一台实际的量子计算机。”他一直在研究困住的离子量子技术已有十多年了。
‘通过我们的工作，我们不仅表明可以做到这一点，而且我们已经提供了一个螺母和螺栓施工计划来构建实际的大型机器。现在，我们正在基于此设计建立一个原型。”他说。
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该模块将所有操作作为独立单元控制，并使用当前5NM技术的硅微加工技术构建。这些模块使用长波长（微波）捕获的离子量子门，使用电场在单个模块之间传输量子位。研究人员说，这比使用光纤链路快100,000倍，并允许连接许多模块以构建大型设备。
A high error–threshold surface error correction code can be implemented in the proposed architecture to execute fault-tolerant operations, and the design is suitable for other trapped ion quantum computer architectures, such as schemes using photonic interconnects.
Quantum computer design
每个模块都使用微型离子陷阱X结数组，其中两个或更多离子被困。这些区域具有三个不同的区域，具有基于微波炉的门区域，状态读数区域和一个加载区。一旦离子被困在加载区域，高保真离子穿梭操作将离子转移到栅极区域。在那里，可以使用局部可调的磁场单独解决离子，并使用静态磁场梯度与全局微波炉和RF场结合使用。
When the state of the qubit needs to be detected, the ion is transferred to the readout zone, where global laser fields and on-chip photo detectors are used for state readout. A second ion species is used to sympathetically cool the qubit ion without affecting its internal states.
所有相干量子操作和执行controlled by on-chip electronics, relying only on global microwave and rf fields. In this architecture, laser light is only required for state preparation and detection, photoionization, and sympathetic cooling. This means the laser beams can have much less stringent requirements than those used for quantum gate realization. The laser beams do not need to have high intensity, and do not need to be phase-stable.
5nm process technology
The X-junction structures occupy an area of 2.5 × 2.5 mm2and can be fabricated in large numbers on a silicon wafer to form the scalable quantum computer module. A total of 1296 individual X-junctions can be monolithically fabricated onto a 90 × 90–mm2 silicon wafer piece, compatible with standard 150-mm wafer sizes.
However, the challenge is the power supply. If all of these X-junctions are electrically connected together, the capacitance and power dissipation will become too large to be driven with a standard helical resonator of high-quality factor. As a result, the vacuum system for the trapped ions and the cooling system restrict the size of the modules.
The team used the Advanced Design System software tool from Keysight Technologies to simulate 6 × 6 junctions connected together in an electrical submodule. This keeps the capacitance below 80 pF, and a quality factor of Q > 200 is achievable using a compact helical resonator of 15 mm in diameter.
An additional requirement to achieve a high-quality factor is to use a substrate with low rf loss, typically with a bulk resistivity of 50 kΩ·cm. Compact resonators are placed inside the system underneath the module and connected with shielded cables to the electrical submodules. All resonators are attached to the same frequency source, and the resonant circuits are tuned into resonance with the frequency source using variable capacitors.
电截面的紧密接近度将导致谐振器之间的电容耦合，从而导致谐振器和相邻的RF电极的相匹配。仔细设计离子芯片上的连接路径用于避免相关的RF电极之间的不可忽略的相位差异。
Each electrical submodule features 1224 static voltage electrodes and 108 individual local gradient current wires. The required static voltages and currents are supplied by DACs inside the vacuum system, fabricated on separate silicon substrates, which are attached to the ion trap substrate using TSV and wafer-stacking technology.
Stacked readout electronics
Each wafer layer features four DACs with 160 analog outputs in total (the AD5370 has sufficient outputs and was used as an example, but a modified version will be required that operates at higher update rates) and, combined with the required TSV and RC filters, occupies an area of no more than 15 × 15 mm2. Generating enough analog outputs requires a total of nine wafer layers that will be stacked together. An additional layer is used to house an electronic control unit, which controls the in-vacuum DACs and detection system.
嵌入的铜线生成磁场梯度的路线以使每个模块只需要四个高电流连接，但是这些连接经过10 a的大电流，其横截面（〜30×60μm2）。这使得最终的热量必须有效分布并从模块中运输。另外，还需要将离子陷阱结构耗散的功率和空中电子设备远离模块。通过将硅底物冷却至100 k以下，可以避免电线结构的熔化。
The team propose using a liquid nitrogen microchannel cooler integrated into the back wafer of the modules to efficiently remove the heat from the modules. Deep trenches are etched into the backside of the last wafer, forming channels through which liquid nitrogen is passed. The channels are covered using an additional silicon wafer. Fabricating the entire module including the liquid cooler out of silicon prevents additional stress and wafer bow arising from different thermal expansion coefficients.
这些模块中的每个模块中的每个模块都可以用作具有1296个X界面的独立小型量子处理器模块。
The research is part of the UK Government’s National Quantum Technology Programme.
“通用量子计算机的可用性可能会对整个社会产生根本影响。毫无疑问，建造一台大型机器仍然具有挑战性，但是现在是将学术卓越转化为实际应用的时候，这是在英国技术中的优势基础上。” Winfried说。‘我们很高兴与行业和政府合作实现这一目标。”
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