How can you make something colder? Water can be cooled into ice through contact with the colder air of a freezer, and at a temperature of 77 K (-321° F), contact with liquid nitrogen can quickly cool milk and cream into ice cream. But what if you wanted to make something colder than liquid nitrogen? Cozijn et al. have found a new way to use lasers to cool a collection of 600 beryllium ions to a temperature of just 10 mK (-460° F).

Laser cooling of trapped ions takes advantage of the Doppler effect. The same velocity-dependent effect that produces “Doppler radar” and causes the red and blue shifts of the light from galaxies as the universe expands can be used to remove energy from the trapped ions and cool them. This is possible by making the wavelength of the laser light longer or “redder” than the wavelength of light that a stationary ion would absorb. The wavelength at which this absorption occurs in a stationary particle is called its resonant wavelength, and for beryllium ions resonance occurs for light with a wavelength of 313 nm. Just as the siren from an ambulance is “blue-shifted” to a higher frequency (shorter wavelength) as it approaches an observer, as the ions move around in the trap, the light from the laser will be “blue-shifted” toward resonance for those ions that are moving toward the laser. Regardless of the movement of the ion and the wavelength of light that it absorbs as a result of its motion, the wavelength of light that the beryllium ions re-emit is always 313 nm. This means that the motion of the ions in the trap combined with the Doppler effect allows the ions to absorb a lower energy photon than they will re-emit, and this difference in energy results in a net loss of energy or “Doppler cooling” of the trapped ions.

In order to produce the 313 nm wavelength light used in the laser cooling, Cozijn et al. use a technique known as second harmonic generation (SHG). This sensitive nonlinear interaction can occur in specially designed crystals and is also called “frequency doubling” because it effectively combines two 626 nm wavelength photons to produce a photon with twice the frequency (or half the wavelength).

In contrast to previous laser cooling work with beryllium ions which used a dye laser as the source for the 626 nm wavelength light for SHG, the authors built an external cavity diode laser (ECDL). There are many potential advantages to the use of diode lasers for this application including a significantly smaller footprint compared to dye lasers and the ability to narrow the laser linewidth by extending the lasing cavity and intentionally introducing optical feedback. Diode lasers use semiconductors as their gain medium, and the wavelength of the light depends on the electronic properties of the semiconductor material and its temperature. Previous implementation of diode lasers for laser cooling of beryllium ions had been limited by the difficulty in controllably creating appropriate parameters for the generation of light at a wavelength of 626 nm. At room temperature their diode laser chip produces 636 nm light, but through the construction of a two-stage thermoelectric cooling process, they were able to cool the chip to -31° C (-24° F) and tune the wavelength of the emitted light to 626 nm. Ultimately it was the development of this technique for controllably cooling the laser diode that enabled the generation of light at the appropriate wavelength to cool the trapped beryllium ions.

Laser cooling of trapped ions has already offered many advances in the areas of precision spectroscopy, quantum-logic optical clocks, and quantum information processing. The authors have clearly demonstrated that hundreds of trapped beryllium ions can be efficiently laser cooled to 10 mK through SHG of 626 nm light produced by an ECDL cooled to -31° C. Their technique has the exciting potential to increase the feasibility of future experiments with trapped ions.

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