nanoquebec 2007-08-16 08:56
Ultrafast quantum computer uses optically controlled electrons
[size=5][b]Ultrafast quantum computer uses optically controlled electrons [/b][/size]
[img]http://img172.imageshack.us/img172/8052/qubus9d984con0.jpg[/img]
[i]The loop-qubus quantum computer involves a semiconductor chip with a loop of cavities containing quantum dots. By focusing optical pulses at individual quantum dots, the electron spins within the dots rotate, changing the state of the bit. Credit: Clark, et al.[/i]
[b]【纳米科技世界快讯】Scientists have designed a scheme to create one of the fastest quantum computers to date using light pulses to rotate electron spins, which serve as quantum bits. This technique improves the overall clock rate of the quantum computer, which could lead to the fastest potentially scalable quantum computing scheme of which the scientists are aware.
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Susan Clark and Kai-Mei Fu, both of Stanford University, and Thaddeus Ladd and Yoshihisa Yamamoto, both with Stanford University as well as the National Institute of Informatics in Tokyo, have published their results on the new scheme in a recent issue of Physical Review Letters.
“We still don't know what a final quantum computer will look like,” Ladd explained to PhysOrg.com. “Large scale quantum computation is a technology that is still very far away from being implemented, and will probably incorporate many new ideas that have not been imagined yet. The important development in this paper is finding a physical implementation of an existing theoretical idea [using phase gates to couple non-local spins] and estimating the speed.”
On a single semiconductor chip, the researchers combine fast single-bit rotations and fast two-qubit gates, both of which are optically controlled. In quantum computing, the orientation and phase of the electron spin serve as the bit state, and the gates are responsible for performing reversible operations on input data to produce output data.
The semiconductor chip is a square millimeter in size, and consists of a loop of cavities—together, this apparatus is called a “loop-qubus.” Each cavity holds a quantum dot, which is a small piece of semiconductor that contains, in this scheme, a single electron. By focusing optical pulses at individual quantum dots, the electron spins rotate, changing the state of the bit.
The architecture is built on the idea of using phase gates to couple non-local spins. The optical pulses can provide a means to couple distant electron spins, or qubits, so that the phase of one qubit can depend on the phase of another qubit. When coupled, the qubits’ spin states form a “qu-bus,” which is the basis of a two-qubit gate.
The operating speed of a quantum computer is measured by its clock signal, which could take many different forms. In the optical control scheme, the pulses, which could be supplied by a laser, provide a clock rate for the system. Ladd explained that there are several limitations on speed for quantum computers.
“In quantum computing, not only is the state of the bit (0 or 1) important, but also the phase of the bit,” he said. “How quickly we can control the phase of the qubit, in our scheme, depends on the magnetic field. Increasing the magnetic field increases how fast the phase for any single qubit changes in time and ultimately sets the limit of how fast we can control our qubits. In the article, we give the limit of about 100 GHz, which is assuming a very high magnetic field, which would require superconducting magnets to achieve.
“The second limitation on speed is the time it takes for the phase of one qubit to change the phase of another,” he continued. “This must be done with pulses that are slower than the rate light moves in and out of each optical cavity, so this brings the speed down to more like 10 GHz. Finally, as the computer gets bigger, the amount of time it takes for light to propagate around the system will also limit speed, perhaps bringing the speed of physical qubits down to GHz compared to classical computers.”
However, Ladd added that the proposed architecture, with its speedy physical operations, non-local couplings, potential for monolithic semiconductor implementation, and non-reliance on single photon sources or detectors, is still much faster than other schemes for quantum computing, such as ion traps.
“As opposed to classical computers, quantum computers critically depend on error correcting schemes,” Clark said, explaining the complexity of calculating a quantum computer’s speed. “Techniques for correcting errors can get complicated; however, in general, they require many physical qubits and qubit operations to represent one fault-tolerant logical operation (and therefore more time). The speed of the physical qubit manipulation makes the computer appear faster than it is. Proper error correction may reduce the speed of the quantum computer to 1-10 MHz.”
Besides speed, the proposed scheme also has benefits in terms of scalability and manufacturing potential. Because the system can create two-qubit gates between distant qubits, the scheme favors scalability compared with systems that rely on adjacent qubit interactions. Also, the system doesn’t require the two qubits to have the same frequency, which matches other proposals in its potential for large-scale fabrication.
“In terms of building this computer, we are working on that one step at a time,” Clark said. “We are starting by putting the quantum dot qubits in cavities, performing rotations on those qubits, and then coupling them via qubus. But a complete, scalable device remains many years away.”
Citation: Clark, Susan M., Fu, Kai-Mei C., Ladd, Thaddeus D., and Yamamoto, Yoshihisa. “Quantum Computers Based on Electron Spins Controlled by Ultrafast Off-Resonant Single Optical Pulses.” Physical Review Letters, 99, 04051 (2007).
Source: PhysOrg.com.
nano 2007-08-19 01:56
[size=5]科学家制定出利用光脉冲控制电子旋转制造超速量子计算机计划[/size]
[img]http://www.physorg.com/newman/gfx/news/qubus.png[/img]
图注：量子总线线圈由多个圈在一起的腔洞组成。通过把激光脉冲聚焦到单量子点上，电子自旋便开始旋转，从而改变量子比特的状态。
据2007年8月15日physorg网站报道，科学家已制定出一项雄心勃勃的计划：利用光脉冲使充当量子比特的电子产生自旋，从而创造出一台迄今为止速度最快的量子计算机。该技术将全面提升量子计算机的时钟速率，从而使科学家能够按计划设计出一种既能够达到最快速度而又具有升级潜力的量子计算方案。
斯坦福大学的四名研究员，苏珊·克拉克、傅凯梅、撒迪厄斯·拉德和Yoshihisa Yamamoto将有关这项新计划的研究成果发表在了最新一期的《物理评论快报》上。撒迪厄斯·拉德和Yoshihisa Yamamoto同时也为东京的日本信息研究院工作。“我们仍然不知道量子计算机最终将会是个什么样子，”拉德解释称：“大规模量子计算技术距离实际应用还有很长的路要走，也许还将采用许多没有想象出来的新主意。这篇论文的目的是依据现有的理论探索一种物理上的实践方法，并且预测量子计算机的速度。”
在单一半导体芯片上，科研人员集成了快速单比特旋转和快速双量子比特阐门，这两个部分都是光控的。在量子计算中，电子自旋的方向和相位相当于比特状态，而门则负责对输入数据进行可逆转操作，从而产生输出数据。这种半导体芯片只有一平方毫米大小，由多个圈在一起的腔洞组成，这种结构被称为量子回路总线（loop-qubus）。每一个腔洞内都有一个量子点，也就是含有一个电子的一小片半导体。通过把激光脉冲聚焦到某一单个量子点上，电子自旋便开始旋转，从而改变量子比特的状态。
这种体系结构基于这样一种原理，即利用相位门使非本地自旋发生耦合。光脉冲可以使远距离的电子自旋或者量子比特发生耦合，这样一来，某个量子比特的相位可以由另一个量子比特的相位来决定。当两者发生耦合时，量子比特的自旋状态将形成一个“量子总线”，而“量子总线”是双量子比特门的基础。
量子计算机的时钟信号可以测量出自身的运行速度，而时钟信号可以有很多不同的表现形式。在光控方案当中，可以通过激光产生脉冲，从而为系统提供时钟速度。拉德解释称，对于量子计算机而言，存在几种速度上的限制。“在量子计算当中，不仅比特（0或1）的状态很重要，比特的相位也很重要，”拉德说：“在我们的方案中，我们控制量子比特相位的速度取决于磁场。增加磁场强度不仅可以提高任何一个单量子比特相位的变化速度，还能够最终决定我们究竟能够以多快的速度控制量子比特。在这篇论文当中，我们给出的极限速度是大约10万兆赫（100GHz），这需要非常高的磁场才行，也许只有超导磁体才能达到这一速度。第二种速度限制是一个量子比特相位转变为另一个量子比特相位所需的时间，这肯定需要利用脉冲才能完成，而这种脉冲的速度应慢于光在每个光学腔洞中进出的速度，这样才能使得速度降低到1万兆赫左右。最终，随着计算机越来越大，光在整个系统周围传播的时间也越来越长，这也将使速度受到限制，也许会把物理量子比特的速度降低到与普通电脑相似的兆赫级水平。”拉德补充称，尽管如此，由于上述体系结构具备快速的物理处理能力、非本地耦合能力和单片集成电路半导体功能，而且无需依赖单光子源或光子探测器，因此与离子阱等其它量子计算方案相比，这种体系结构的速度要快得多。
“与普通电脑相比，量子计算机非常依赖于严格的纠错方案，”克拉克进一步解释了计算出量子计算机运算速度的复杂性：“纠错技术可能是相当复杂的，但是，总得来说，纠错技术需要许多量子比特和量子比特处理才能实现逻辑容错（因此需要更多的时间）。物理量子比特处理的速度将提高比算机自身的速度。合理的纠错技术将把量子计算机的速度降低到1兆赫－10兆赫。”
除了速度之外，上述方案还将具有升级和制造潜力。由于这种系统可以在距离较远的量子比特之间创造双量子比特门，因此该方案有利于实现升级，而其它系统则需要依赖于邻近量子比特的相互作用，因此无法实现升级。此外，该系统不要求两个量子比特具有相同的频率，因此，与其它方案相比，该方案具有实现大规模制造的潜力。
“为了制造这种计算机，我们将一切从头开始，”克拉克说：“我们正在设法把量子点比特放入腔洞中，让这些量子比特产生电子自旋，然后使它们与量子总线发生耦合。但是我们需要许多年的时间才能制造出一台完善的、可升级的量子计算机。”
出处：中国科技信息网Chinainfo。