Direct Frequency Comb Laser Cooling and Trapping

Ultracold atoms, produced by laser cooling and trapping, have led to recent advances in quantum information, quantum chemistry, and quantum sensors. A lack of ultraviolet narrow-band lasers precludes laser cooling of prevalent atoms such as hydrogen, carbon, oxygen, and nitrogen. Broadband pulsed lasers can produce high power in the ultraviolet, and we demonstrate that the entire spectrum of an optical frequency comb can cool atoms when used to drive a narrow two-photon transition. This multiphoton optical force is also used to make a magneto-optical trap. These techniques may provide a route to ultracold samples of nature’s most abundant building blocks for studies of pure-state chemistry and precision measurement.

Popular Summary

Laser cooling and trapping is used to produce cold, homogeneous samples of atoms for precision measurements in fields ranging from quantum information science to ultracold chemistry since thermal fluctuations are reduced at extremely low temperatures. These techniques are currently limited by the lack of deep ultraviolet continuous-wave lasers; therefore, researchers can only make use of atoms with lower-frequency transitions. Since simple, abundant species such as hydrogen, nitrogen, carbon, and oxygen cannot be studied, ultracold chemistry investigations are confined to working with more chemically exotic species. In contrast to continuous-wave lasers, evenly spaced spectral lines (i.e., an optical frequency comb) can provide high-power ultraviolet radiation, although the process is associated with spreading the optical power over many colors. Here, we demonstrate a new laser-cooling technique that coordinates the effect of all the colors simultaneously by using a type of excitation that matches the structure of these pervasive atoms.

Using an optical frequency comb centered near half the excited 5D-state energy of roughly 10,000,0000 rubidium atoms, we theoretically and experimentally demonstrate that narrow-band laser cooling can be performed on a two-photon transition with all of the comb teeth contributing in parallel so as not to waste power. Despite the complexity of the resulting spectrum, we find that the comb can be used to make a magneto-optical trap for storing and accumulating cold atoms. Our work reveals that two-photon cooling of hydrogen (or antihydrogen) is a realistic possibility, which would provide a new starting point for ultracold chemistry and help to limit systematics in precision measurements.

We expect that our findings will allow the tools of ultracold atoms to be applied to common elements featured in organic chemistry, astrophysics, and biology.

Figure 1

Constructive interference of multiple paths in a two-photon transition driven by a transform-limited optical frequency comb. All pairs of comb teeth whose sum frequency matches the excited state energy interfere constructively to excite atoms. Two example pairs are shown as (a) and (b), and the effective two-level system that results from the sum is shown in (c). Every tooth of the resulting “two-photon comb” of resonant coupling strength $\mathrm{\Omega }$ leverages the full power of all of the optical frequency comb teeth through this massively-parallel constructive interference.

Figure 2

Laser-induced fluorescence spectrum of the $5S\to 5D$ two-photon transition driven by an optical frequency comb. (a) Spectrum from a natural abundance vapor cell and (b) a ${}^{85}\mathrm{Rb}$ cw MOT and collected 420-nm light (inset). Solid curves are theory fitted for (a) Gaussian and (b) Voigt line shapes. These spectra repeat with a period of $f_r=81.14\mathrm{MHz}$ on the horizontal axis. (c) Relevant levels of rubidium. (d)–(g) Absorption images of the atom cloud after free expansion following (d) no ML illumination, (e) ML illumination from the right, (f) left, and (g) both directions, detuned to the red of resonance. Mechanical forces are evident in (e) and (f), and the narrowing of the velocity distribution in the horizontal direction in (g) is the hallmark of cooling.

Figure 3

Detuning dependence of 420-nm fluorescence (top) and the resulting temperature (bottom) of rubidium atoms laser cooled by an optical frequency comb on a two-photon transition. The solid curve is fit for scattering rate, effective linewidth, and detuning offset of data analyzed with the aid of a Monte Carlo technique (data labeled “Constrained”; see Ref. [25]). The same data are also analyzed using a free-expansion model (“Free”), and agree well with the Monte Carlo assisted analysis. Inset: Temperature versus time when the laser detuning is optimized for cooling, giving a minimum temperature of $(57\pm 2)\mu \mathrm{K}$. The time decay of the 420-nm fluorescence shows that atoms leave the interaction region due to transverse motion in $(4.0\pm 0.3)\mathrm{ms}$, but steady-state 1D temperature is reached in $\tau =(1.28\pm 0.09)\mathrm{ms}$. Error bars are statistical over repeated measurements, and do not include the systematic effect of beam mode mismatch, discussed in text.

Figure 4

Phase space (and position space, inset) trajectories for atoms trapped in a two-photon, optical frequency comb MOT. Smooth curves are fits to a damped harmonic oscillator, with fit uncertainties shown as bands. Purple features show the behavior when the ML beam polarizations are intentionally reversed and exhibit an anticonfining force. Error bars are statistical over repeated measurements.