Broadband laser cooling of trapped atoms with ultrafast pulses

Abstract

We demonstrate broadband laser cooling of atomic ions in an rf trap using ultrafast pulses from a modelocked laser. The temperature of a single ion is measured by observing the size of a timeaveraged image of the ion in the known harmonic trap potential. While the lowest observed temperature was only about 1 K, this method efficiently cools very hot atoms and can sufficiently localize trapped atoms to produce near diffraction-limited atomic images.PACS numbers: 32.80. Pj, 42.50.Vk Laser cooling of atoms [1,2] has become a cornerstone of modern day atomic physics. Doppler cooling and its many extensions usually involve narrow-band, continuous-wave lasers that efficiently cool atoms within a narrow velocity range (∼ 1 m/s) that corresponds to the radiative linewidth of a typical atomic transition. To increase the velocity capture range, several laser cooling methods were investigated that modulate or effectively broaden a narrow-band laser [3,4,5,6,7,8]. Modelocked pulsed lasers have been used to narrow the velocity distribution of atomic beams within several velocity classes given by the bandwidth of each spectral component of the frequency comb [9,10]. In this letter we report the demonstration of Doppler laser cooling of trapped atoms with individual broadband light pulses from a modelocked laser.To efficiently capture and cool high-velocity atoms, it is necessary to achieve a laser bandwidth large enough to cover the large range of atomic Doppler shifts. For example, Cd + ions used in this experiment are initially created with an average kinetic energy of order 1 eV, which corresponds to an average velocity of about 1300 m/s and a Doppler shift of ∆ D ∼ 36 GHz. Power broadening an atomic transition (saturation intensity I s and natural linewidth γ) would require a laser intensity of2 , which can be prohibitively high. For Cd + (γ/2π ≃ 50MHz, I s ≃ 5000W/m 2 ) this requires I ∼ 10 10 W/m 2 . Modulating a narrow-band laser to generate high bandwidths would allow for significantly less laser power, but it is technically difficult to generate a 100 GHz wide modulation spectrum [4]. On the other hand, an ultrafast laser whose pulse is a few picoseconds long will naturally have a bandwidth in the above range, as well as sufficient intensity to excite the transition.The laser cooling rate depends critically on the photon scatter rate, which for a pulsed laser can be no larger than the laser repetition rate R (about 80 MHz for a typical modelocked laser), given that the atom is excited with unit probability by each pulse. We assume that once excited, the atom decays back to the ground state faster than the time period of the modelocked pulse train 1/R. In this case, the atom has little memory between pulses, or equivalently, the absorption spectrum is a single broad line of width ∆ ∼ 1/τ (τ is the pulse duration) and the frequency comb of spacing R has very little contrast.The equilibrium temperature for broadband pulsed laser cooling of trapped atoms is expected to scale approximately with the laser bandwidth ∆, a...

Laser cooling of atoms and molecules with ultrafast pulses

“…As shown in Fig. 1, the spectrum of such a pulse train is a comb of sharp lines with frequencies ν k = ν car + kν rep , where k is an integer, ν car is the optical carrier frequency, and ν rep is the pulse repetition durations of a few ps [22]. The related "white-light" cooling schemes use an additional CW source near the atomic resonance to achieve temperatures ≪ h∆ν/2 [23,24,25], but this is a difficult requirement in the cases we will consider.…”

Phonon Lasing from Optical Frequency Comb Illumination of Trapped Ions

“…In this case, our model predicts that stable fixed points will exist, with the lowest fixed point energies lying between 2 mK and 2 K, depending upon δ. This suggests that it is possible comb tooth effects were responsible for the anomalously low temperatures that were observed in that work [5].…”

Measuring the temperature and heating rate of a single ion by imaging

“…In order to enable fast and accurate determination of the temperature of a trapped ion at mK temperatures, thermometry by imaging the spatial extent of an ion has been demonstrated [15][16][17][18]. The accuracy of this method was limited only by the imaging resolution and the images' signal-to-noise ratio.…”