PORTLAND, Ore. A strontium-based timekeeper providing up to 50 percent better accuracy could serve as the next-generation atomic clock.
By controlling collisions between neutral strontium atoms, the new atomic clock is said to be accurate to within one second in 300 million years, according to its inventor, Jun Ye of a joint institute formed by the U.S. National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.
Standard atomic clocks are based on microwave frequency carriers, but the next-generation candidates for the new standard atomic clock will operate at higher optical frequencies in order to gain accuracy.
Atomic clocks based on clouds of cold atoms--cesium in the current standard--are confined by laser beams directed at right angles to each other to push atoms together. This method cools atoms to near absolute zero.
For the cesium standard, vertical lasers toss the atoms upward into a microwave cavity where they are then turned off. The cesium atoms then fall back to the bottom of the cavity and the process repeats itself at precisly one second intervals to attain an accuracy of +/-1 second every 80 million years.
Last year, a quantum clock that looses or gains 1 second every 1 billion years was demonstrated by NIST scientist Till Rosenband, who cooled aluminum atoms with the laser. However, the aluminum atoms could not be directly synchronized. Instead, a more complex scheme coupled the quantum states of the aluminum ions with a nearby beryllium atoms, synchronizing a laser to its vibration frequency to achieve an indirect optical clock with the accuracy of aluminum ions' natural vibrational mode.
The new strontium approach needs no indirect measurements.
The Consultative Committee for Time and Frequency at the Bureau International des Poids et Mesures (Paris) will decide which of several candidates becomes the new standard.
Ye's improved strontium-based clock uses fermions, in contrast to bosons, which are thought to occupy only a single quantum state and location at a given time, thus forbidding collisions. Ye's research group observed what appeared to be occasional collisions that disturbed their clock's performance.
To solve the mystery of why fermions were having seemingly forbidden collisions, Ye used lasers to cool 2,000 strontium atoms stacked in levels of optical traps, each with about 30 atoms per well. Detailed observations revealed that two atoms in the same well, but located some distance apart, were subjected to slight variations in laser pulses used to boost the atoms between energy levels, thus exciting them unevenly and enabling collisions.
The scientists also discovered that by exciting the atoms to about halfway between their ground and excited states, the collision-related shifts in the clock frequencies could be zeroed out. By optimizing these adjustments, researchers were able to recduce the corrections needed to compensate for occasional collisions, thereby increasing clock accuracy.
The techniques can also be theoretically applied to atomic clocks based on bosons, which unlike fermions can exist in the same place and energy state and thus are subject to collisions. The new strontium lattice technique also could be useful in future quantum computers, according to Ye, as well as for increasing understanding of quantum phenomena like superconductivity.