Important Step Towards Quantum Computers

A team of scientists at the National Institute of Standards and Technology (NIST) in Boulder, USA, has managed to entangle ions using microwave fields for the first time. According to their publication in the journal Nature (“Microwave quantum logic gates for trapped ions”), the team has implemented a method that could be important for the realization of an integrated quantum computer with trapped ions.

Christian Ospelkaus, professor within the Cluster of Excellence QUEST (Centre for Quantum Engineering and Space-Time Research) at Leibniz Universität Hannover and Physikalisch-Technischen Bundesanstalt, Braunschweig since December 2010, has realized the experiment together with colleagues at NIST.

Entanglement is a concept at the heart of the bizarre quantum world. We take it for granted that, if two coins are flipped simultaneously, each of them alone would show a random pattern of heads or tails, independent of the other coin. In a quantum world, the two coins could be manipulated such that, if one of them shows heads, the other one will too, and vice versa. This is called an ‘entangled state’ of the two ions. If we identify ‘heads’ and ‘tails’ with the two values of ‘zero’ and ‘one’, this operation represents a so-called entangling quantum logic gate. Such gate operations are the crucial step in building a ‘quantum computer’. These devices might one day exploit the laws of quantum mechanics to solve certain problems in physics, mathematics and cryptography much faster than classical supercomputers can do.

Ions (charged atoms) are one of the experimentally most advanced technologies on the route towards a practical quantum computer. In a number of previous ground-breaking experiments by the NIST group and other groups in the field, operations on the ion quantum bits or ‘qubits’ had been carried out using laser beams. In their article, the NIST team shows that these operations can be carried out using miniaturized microwave electronics routinely used e. g. in cellphones, rather than with a complex laser system that fills a whole room. In order to generate the entanglement, the microwave source is integrated into the electrodes of a so-called ‘chip trap’, a microscopic structure for trapping and manipulation of ions located inside a vacuum enclosure. “Because microwave fields can typically be generated in a less cumbersome and more easily controllable way than laser beams, this technique might help us build more powerful and error-resilient experiments”, says Christian Ospelkaus.

Image of ion trap (square gold structure in the center). The setup is located in a vacuum enclosure, and the ions hover 30 micrometers above the chip surface. The microwave signals are brought in using the three broad lines on the right-hand side. (Image: Yves Colombe/NIST)

The team characterized how well they can create the entangled state and find that it works 76% of the time. The laser-based approach that has been developed for several years now is currently still better (99.3%), but the new experiment has one important advantage: It requires only about one tenth of the footprint of a laser-based experiment, and the footprint might further be shrunk based on this pioneering experiment. In their publication, the team points out a number of steps that can be taken to increase the quality of operations. ‘The ability to integrate the control of the qubits into the trapping structure, rather than having to build a huge laser system, is an important step. In the future, this method might help us process more and more qubits’, says Christian Ospelkaus.

With his group within the cluster of excellence ‘QUEST’, Christian Ospelkaus is working on developing a microscopic experimental model system to help understand the behaviour of quantum many-body systems. The microwave quantum logic techniques he helped develop at NIST are an important part of this scheme. The group also pursues extensions to precision spectroscopy.

The research at NIST was supported by United States Intelligence Advanced Research Projects Activity (IARPA), Office of Naval Research (ONR), Defense Advanced Research Projects Agency (DARPA), National Security Agency (NSA) and Sandia National Laboratories.

Since November 2007, the Cluster of Excellence QUEST – Centre for Quantum Engineering and Space-Time Research – at Leibniz Universität Hannover has been supported by the Excellence Initiative of the German federal and state governments. Its main research areas are quantum engineering and space-time research. Apart from six institutes of Leibniz Universität Hannover, QUEST’s partner institutions are the Laser Zentrum Hannover (LZH), the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) with the gravitational wave detector GEO600, the Physikalisch-Technische Bundesanstalt Braunschweig and the Centre of Applied Space Technology and Microgravity (ZARM) at University of Bremen.