Tandem Ions May Lead The Way To Better Atomic Clocks

As reported in the July 29 issue of Science,* the unusual tandemtechnique involves use of a single beryllium ion to accurately sensethe higher-frequency vibrations of a single aluminum ion. The NISTgroup used ultraviolet lasers to transfer energy from the aluminum’svibrations to a shared “rocking” motion of the pair of ions, and thendetected the magnitude of the vibrations through the beryllium ion. Thenew technique solves a long-standing problem of how to monitor theproperties of an aluminum ion, which cannot be manipulated easily usingstandard laser techniques.

The tandem approach might be used to make an atomic clock based onoptical frequencies, which has the potential to be more accurate thantoday’s microwave-based atomic clocks. It may also allow simplifieddesigns for quantum computers, a potentially very powerful technologyusing the quantum properties of matter and light to represent 1s and 0s.

“Our experiments show that we can transfer information back andforth efficiently between different kinds of atoms. Now we are applyingthis technique to develop accurate optical clocks based on singleions,” said Till Rosenband of NIST’s laboratories in Boulder, Colo.

Today’s international time and frequency standards measure naturallyoccurring oscillations of cesium atoms that fall within the frequencyrange of microwaves, about 9 billion cycles per second. By contrast,optical frequencies are about 100,000 times higher, or about onequadrillion cycles per second, thus dividing time into smaller units.Aluminum may offer advantages over other atoms, such as mercury, beingconsidered for optical atomic clocks.

Building a clock based on aluminum ions has been impractical untilnow because this atom fails to meet three of four requirements. It doesoscillate between two different energy states at a stable, opticalfrequency that can be used as a clock reference. However, aluminumcannot be cooled with existing lasers, and its quantum state isdifficult to prepare and detect directly. The Science paper describeshow beryllium—a staple of NIST research on time and frequency standardsas well as quantum computing—can fulfill these three requirements whilethe aluminum acts as a clock.

In the NIST experiments, the two ions were confined close togetherin an electromagnetic trap. The beryllium ion was laser cooled andslowed to almost absolute zero temperature, which helped to cool theadjacent aluminum ion. Then the scientists used a different laser toplace the aluminum ion in a special quantum state called a“superposition,” in which, due to the unusual rules of quantum physics,the ion is in both of its clock-related energy levels at once. Morelaser pulses were used to convert this clock state into a rockingmotion, which—because of the physical proximity of the two ions and theinteraction of their electrical charges—was shared by the berylliumion. As the two ions rocked together in a coordinated fashion,scientists applied two additional laser beams to convert this motioninto a change in energy level of the beryllium ion, which was thendetected.

When the information is transferred between the two ions, they arebriefly “entangled,” another unusual phenomenon of quantum physics inwhich the properties of physically distinct particles are correlated. Alogic operation borrowed from quantum computing was used to transferthe aluminum’s quantum state to the beryllium. Logic operations aresimilar to “if/then” statements in which the outcome depends on theinitial state. For instance, if the aluminum’s original state was atthe lowest energy level, then no information was transferred. But ifthe original state was at a higher level, then energy was transferredto the beryllium in a proportional amount.

By repeating the experiment many times, with different laserfrequencies creating a variety of superposition states in the aluminum,scientists could determine its “resonant” or characteristic frequencyextremely accurately. This is the frequency of an internal vibration ofthe aluminum atom, which can be used as the “ticks” of an atomic clock.

The tandem technique could be used to investigate the potential ofvarious atoms, such as boron and helium, for use in optical atomicclocks, according to the paper. The technique also could be used inquantum computing experiments to distribute information betweendifferent types of ions or atoms. Because different atoms respond todifferent frequencies of light, this could improve control of ions oratoms within a potential future quantum computer. Information aboutNIST research in this field is available at http://qubit.nist.gov.

The work described in Science was supported in part by the Office ofNaval Research and the Advanced Research and DevelopmentActivity/National Security Agency.

As a non-regulatory agency, NIST develops and promotes measurement,standards and technology to enhance productivity, facilitate trade andimprove the quality of life.

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*P.O. Schmidt, T. Rosenband, C. Langer, W.M. Itano, J.C. Bergquist ,D.J. Wineland. Spectroscopy using quantum logic. Science. July 29, 2005.