By encoding information in the quantum states of individual atoms, quantum computers have the potential to make the latest Intel Pentium chip look as outmoded as the abacus. But before such a quantum device can even begin to compete with its classical counterpart, we first need to find a way to scale up individual quantum bits, or qubits. Physicists in the US have now made an important step towards this goal by creating an ion trap on a semiconductor chip. Such micrometre-scale traps could allow many qubits to be integrated in a workable quantum computer ( Nature Physics at press).
By encoding information in the quantum states of individual atoms, quantum computers have the potential to make the latest Intel Pentium chip look as outmoded as the abacus. But before such a quantum device can even begin to compete with its classical counterpart, we first need to find a way to scale up individual quantum bits, or qubits. Physicists in the US have now made an important step towards this goal by creating an ion trap on a semiconductor chip. Such micrometre-scale traps could allow many qubits to be integrated in a workable quantum computer ( Nature Physics at press).
Trapped atomic ions are standards for quantum information processing, serving as quantum memories, hosts of quantum gates in quantum computers and simulators, and nodes of quantum communication networks. Quantum bits based on trapped ions enjoy a rare combination of attributes: They have exquisite coherence properties, they can be prepared and measured with nearly 100% efficiency, and they are readily entangled with each other through the Coulomb interaction or remote photonic interconnects. The outstanding challenge is the scaling of trapped ions to hundreds or thousands of qubits and beyond, at which scale quantum processors can outperform their classical counterparts in certain applications. We review the latest progress and prospects in that effort, with the promise of advanced architectures and new technologies, such as microfabricated ion traps and integrated photonics.
This paper reports on a promising technology for constructing an ultrapowerful quantum computer, called the ion trap. Researchers have found a way to build ion traps from semiconductors in much the same way as computer chips. By linking several of these ion trap chips, researchers are able to manipulate much larger numbers of ions and demonstrate rudimentary components of a quantum computer over the next few years. A major benefit of chip-based ion traps is that they could be combined into large arrays. The goal is to create a big switchyard in which hundreds or thousands of ions are constantly being shuttled around in small groups, controlled by conventional electronics.
Trapped ions, trapped atoms, and NMR spin systems are all fine ways of building small “toy” quantum computers, each with its own advantages and disadvantages. There are also many other techniques which have been suggested, although these three so far remain in the lead for general-purpose quantum computing. However, the most powerful general-purpose quantum computers constructed to date have only about a dozen qubits, and this is not nearly large enough to make quantum computers useful rather than merely interesting. (Larger systems have been used to demonstrate particular quantum information processing techniques, but these cannot as yet be used to implement arbitrary quantum algorithms.) Although it is not completely clear how complex a general-purpose quantum computer needs to be, it is clear that such a device will involve thousands or even millions of qubits, rather than the dozens involved today. It is, therefore, important to consider whether there is any hope of scaling up these technologies to useful sizes, and we will consider each of the three approaches in turn, before turning briefly to alternative technologies which have not been discussed so far. Trapped ions Trapped ions initially look very promising as a candidate for scaling up, as it is possible to trap thousands of ions while keeping a reasonable distance between them. Early experiments relied on particular tricks which only work with systems of two ions, but this is not true of more recent work, and there is no reason in principle why these large strings of ions could not be controlled.
The electromagnetic confinement of atomic ions has provided a useful testbed for many different applications in atomic physics, including laser cooling {1}, mass spectrometry {2}, and precision control of nearly-pure quantum states {3}. Recently, ion traps have been effectively applied to the growing field of quantum computation {4}, where the ability to isolate a single ion from its environment has made it an attractive architecture for a large-scale quantum information processor {5, 6, 7}. While many of the fundamental quantum computing building blocks have been demonstrated with trapped ions {8}, the technology for scaling to large numbers of ion quantum bits (qubits) is just beginning to develop. In this paper, we describe an important milestone on this path with the successful operation of an ion trap fabricated on an integrated gallium arsenide (GaAs) heterostructure, which could in principal scale to host a large array of ions.
A full-scale quantum computer would work like the standard desktop computer, in that it would have a place to store data, a place where a program manipulates the data, and interconnections to move the data from one to the other. In the computer you are using now, bits of data - stored as quantity of charge or its absence - are transferred from memory to a processor in the form of levels of voltage. At the processor the computer's program determines which logic operations the bits will be subjected to. Once the logic operations are completed, the bits are converted to amounts of charge and stored in memory again. Similarly, in an ion-trap computer, stored qubits would be called from a storage trap to a logic trap, the kind we've been building so far. The two traps would be connected by a long trap that acts like an interconnect or a data bus. It sounds simple, but such a structure would have to be repeated and connected many dozens of times on the same chip to handle the number of ions we'd need. Therefore, a quantum computer equivalent of very-large-scale integration would be required to handle the control circuitry just to move the ions around. The small-scale quantum algorithms that scientists are running today and plan to run in the near future will almost certainly lead to insights that could make full-scale quantum computing, if not easy, at least more tractable.
Abstract The first generation of quantum computers are on the horizon, fabricated from quantum hardware platforms that may soon be able to tackle certain tasks that cannot be performed or modelled with conventional computers. These quantum devices will not likely be universal or fully programmable, but special-purpose processors whose hardware will be tightly co-designed with particular target applications. Trapped atomic ions are a leading platform for first-generation quantum computers, but they are also fundamentally scalable to more powerful general purpose devices in future generations. This is because trapped ion qubits are atomic clock standards that can be made identical to a part in 1015, and their quantum circuit connectivity can be reconfigured through the use of external fields, without modifying the arrangement or architecture of the qubits themselves. In this forward-looking overview, we show how a modular quantum computer with thousands or more qubits can be engineered from ion crystals, and how the linkage between ion trap qubits might be tailored to a variety of applications and quantum-computing protocols.