Quantum-entangled ions tango to the tick of an optical clock

By the mid to late 1990s, advances in atomic spectroscopy had increased the resolution of atomic clocks based on the cesium microwave frequency transition to better than one part in 1015. Now, researchers at the National Institute of Standards and Technology (NIST; Boulder, CO) have mixed quantum entanglement with atomic spectroscopy in a supercooled trap that may ultimately boost the potential resolution of next-generation clocks based on optical frequencies. The technique may also simplify the design of quantum computers.¹

Femtosecond-laser frequency counters, or “gears,” have already been demonstrated for next-generation optical clocks, and the petahertz transition of a single trapped mercury ion has been investigated as a possible optical alternative to the microwave cesium transition frequency of current atomic clocks (see Laser Focus World, May 2004, p. 15). An aluminum ion has also been considered as a potential optical frequency standard because of its reference or “spectroscopy” transition with a resonance quality (Q) factor of on the order of 1017.

The high Q factor of the aluminum spectroscopy transition, however, is just one of four essential criteria necessary for precise atomic spectroscopy. The other three criteria, which aluminum does not possess, are a direct mechanism for laser cooling, a direct and reliable means of initial state preparation, and a direct and efficient method of energy-state detection. The NIST research team appears to have overcome these atomic-spectroscopy deficiencies by pairing aluminum with beryllium, which does fulfill the other three criteria and which is commonly used in research on frequency and time standards.

The researchers confined the aluminum and beryllium ions together in a linear Paul trap. The beryllium ion (directly addressable for cooling, state preparation, and state detection) was designated the “logic” ion, while the aluminum ion (source of the spectroscopic transition) was labeled the “spectroscopy” ion. The Coulomb interaction between the ions coupled their motion such that Doppler laser cooling of the normal modes of the logic ion to near-ground states also sympathetically cooled the spectroscopy ion. Multiple coherent excitations applied in several steps induced resonant oscillations in the spectroscopy ion, mapped the states of the spectroscopy ion onto the logic ion by way of quantum entanglement, and (through repetitive measurements) accurately determined the spectroscopy ion’s transition frequency.

The quantum-entanglement technique might similarly be used to investigate the potential of other atoms, such as boron and helium, for use in optical atomic clocks. In addition, the technique might facilitate information distribution among different ions or atoms in quantum-computing experiments and improve control of ions or atoms in quantum computers by taking advantage of differential sensitivity to specific wavelengths of light.

REFERENCE

1. P.O. Schmidt et al., Science 29, 749 (July 2005).