LASER COOLING: Single atom performs as a ‘phonon laser’

Researchers in Germany have demonstrated the long-postulated “phonon laser”—a system for the resonant oscillation and coherent amplification of vibrational energy packets in perfect analogy to the light amplification in a laser oscillator.¹

While the suggestion has been around for nearly as long as the laser itself, the phonon laser has proved elusive. The trick, according to the team from the Max Planck Institute for Quantum Optics (MPI; Garching, Germany) is to use a harmonic ion trap as the phonon resonator.

The principal problem in realizing a phonon laser is that most materials have a high density of phonon modes; that is, once vibrational energy is imparted to a given system, there are too many routes for the energy to dissipate, making it difficult to contain and amplify vibrational energy. In an ion trap, however, the ion’s isolation is nearly complete, making the taming of the energy possible (see figure).

Single-ion gain medium

Kerry Vahala, a professor at the California Institute of Technology (Pasadena, California) has a well-established record in the field of microresonators and nanolasers, and joined the team at MPI to lead the research. The “gain medium” of the phonon laser is a single magnesium ion, held in a linear radio-frequency (RF) trap. To get the ion oscillating coherently requires the clever use of two pumping regimes, slightly above and slightly below a resonant transition at around 280 nm.

Laser cooling is a well-known technique for damping the motion, and thus the temperature, of a given atom by pumping it with a frequency slightly detuned toward the red from a transition. The atom can still absorb and radiate the incoming photons, with the energy difference between this pump and the transition being lost as heat. What happens on the other, blue-detuned side of this transition, however, has not been very well explored. Conventional wisdom has it that this blue-side pumping causes heating in a reverse of the laser-cooling phenomenon.

“Several clues in recent experimental work, both in ions and in other subjects, suggested that there was something much more interesting happening in the blue-detuned regime,” said Vahala. “We set out to investigate this regime using the simplest possible experiment involving only one ion.” What Vahala and his colleagues discovered is that the pumping causes the stimulated emission of phonons.

REFERENCE

1. K. Vahala et al., Nature Physics, DOI: 10.1038/nphys1367