Quantum information

Microwave ion-trap quantum computing

A new type of ion-trap quantum technology has been developed that uses microwave radiation to perform computations. It will considerably simplify the practical implementation of large-scale quantum computers. See Letters p.181 & p.185

The strange phenomena of quantum physics, such as the possibility of a single atom being in two different places simultaneously, have mystified experienced physicists and students alike. These phenomena are not just theoretical curiosities: a new, practical, field of quantum physics has become particularly vibrant during the past ten years. It concerns harnessing the power of quantum effects to produce an innovative type of technology — quantum technology. This has the potential to revolutionize computing. Although ways to implement this technology are being pursued in various physical systems, quantum computing using trapped ions1,2,3 has undoubtedly been the most successful so far. In this issue, Ospelkaus et al.4 (page 181) and Timoney et al.5 (page 185) describe a trapped-ion approach that further bolsters the enormous potential of this system for the implementation of large-scale quantum computers.

Ospelkaus et al.4 report the realization of quantum gates using microwave radiation instead of laser beams, which have until now been used to implement such gates. Quantum gates are the analogues of classical logic gates, and are used to perform computations in a quantum processor. Timoney et al.5 discuss a new approach to quantum computing — involving microwaves — that is highly resilient to noise (often referred to as decoherence) acting on the physical system used to execute the computations. Decoherence has the potential to destroy quantum effects and interfere with the operation of a quantum processor. Timoney and colleagues' scheme effectively shields the quantum processor from decoherence.

Quantum gates executed in a quantum processor are based on the creation of entanglement. Entanglement is one of the non-intuitive phenomena of quantum physics, whereby the properties of multiple systems, such as groups of ions, are correlated. Ordinarily, the outcome of measuring a particular property on an ion yields a completely random result. But measuring the same property on each ion of a set of entangled ions produces correlated results. Non-entangled ions do not produce such correlations. An easy way to understand the situation is to imagine two people each tossing a coin. The outcome of each coin toss is random. However, if the coins were entangled in a particular way, the outcome of the coin toss would always be the same for both coins — with both people getting heads or tails.

The past few years have witnessed ground-breaking advances in quantum-information processing using trapped ions, including the entanglement of up to 14 ions6, and the development of other entanglement-based protocols, such as the realization of a number of quantum algorithms3 and of teleportation7,8. These advances have been made in experiments in which laser beams are used to perform entanglemewnt operations. In 2001, Mintert and Wunderlich had the visionary idea9 of implementing quantum gates using long-wavelength radiation, such as microwaves or radio waves. Whereas laser beams must be carefully aligned to interact with the trapped ions that are to be entangled, microwaves can be applied via waveguides (structures that can guide radiation) that are part of the chip on which the ion trap is integrated10, and so do not require alignment.

What's more, it is much easier and less costly to generate microwave radiation than it is to use the complicated laser systems currently employed, and highly stable microwave sources are readily available. Large-scale quantum computers may require many millions of individually trapped ions, each constituting a single quantum bit (the basic unit of information storage in a quantum computer). As a result, creating the required number of laser beams to entangle the ions may entail significant engineering and come at a considerable cost. By contrast, the use of microwave radiation for the same purpose would be much easier and would make the construction of a large-scale ion-trap quantum-information processor much simpler.

Based on their proposal11 to make use of the oscillating magnetic fields that are inherent to microwave radiation (the original proposal by Mintert and Wunderlich9 requires static magnetic fields in addition to microwaves), Ospelkaus et al.4 have realized the first microwave quantum gate. They achieved this by using a waveguide integrated into a microchip (Fig. 1) that holds the ion-trap structure. The microchip contains electrodes that produce electric fields capable of trapping two ions just above the chip's surface. Multiple pulses of microwave radiation are then applied to the trapped ions through the waveguide, effectively entangling the two ions and successfully executing a quantum gate.

Figure 1: Microwave ion-trap chip.
figure1

Ospelkaus and colleagues' approach4 to entangling pairs of ions and generating a quantum gate involves launching microwaves into a waveguide incorporated on a chip.

Meanwhile, Timoney et al.5 trapped individual ions and applied a number of microwave pulses to them. This approach sets the ions to a state in which they are decoupled from outside noise. An easy way to visualize this is by considering the suspension of a common car. Springs in the car's suspension system decouple the car frame from the wheels, largely isolating the driver from vibrations caused by uneven road surfaces. In a similar way, Timoney and colleagues' scheme allows trapped ions to be isolated from external disturbances that would otherwise have the potential to disturb the operation of a microwave trapped-ion quantum processor.

The achievements of Ospelkaus et al.4 and Timoney et al.5 constitute step-changing innovations for quantum computing with trapped ions because they will probably aid the production of large-scale ion-trap quantum computers on foreseeable timescales. Quantum computing is likely to revolutionize many areas of science, and we have only just started to appreciate its true potential.

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Correspondence to Winfried K. Hensinger.

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Hensinger, W. Microwave ion-trap quantum computing. Nature 476, 155–156 (2011). https://doi.org/10.1038/476155a

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