28 September 2011

Quantum Computers

A team of scientists from the National Institute of Standard and Technology (NIST), located in Boulder, USA successfully entangled ions using microwave fields for the first time in the science history. The team has implemented a method that could be important for the realization of an integrated quantum computer with trapped ions.

So what is the entanglement process? It is a concept of quantum physic in which we know from the probability rules in mathematics, if tow coins are tossed simultaneously, each of them alone would show a random pattern of head or tails (independent events), but in quantum physic, the two coins could be manipulated such that, the results of two events will be the same (if one of them shows heads, the other one will too, and vice versa). In quantum this is called an "entangled state" of tow 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. We can use this concept for building a "Quantum computer". This device can be used to solve certain problems in physics, mathematics and cryptography much faster than a classical supercomputer can do.

Read more about Microwave quantum logic gates for trapped ions here.

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

Ions or 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. These operations can be done using miniaturized microwave electronics routinely used e. g. in cell phones, 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.

The team found that this process works 76% of the time. The new experiment has one important advantage than the laser-based approach that has been developed for several years: 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. 

"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.

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.