Raman Cooling of a Single Neutral Atom in a Tightly Focused Optical

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Raman Cooling of a Single Neutral Atom in a Tightly Focused Optical Tweezer Centre for Quantum Technologies Syed Abdullah Aljunid1, Jianwei Lee1, Martin Paesold2, Gleb Maslennikov1, Christian Kurtsiefer1 1Department of Physics, National University of Singapore 2ETH, Zurich, Switzerland

Motivation Experimental Sequence Recent experiments have shown that a substantial interaction between a single trapped atom and light can be established by concentrating the light ﬁeld at the location of the atom with a help of a simple lens [1-4]. In State Cooling & Molasses StateRaman Recycle Repeat Check Check if Repeat 87 loading cooling prep transfer pulse transfer & motional detection atom left particular, one can observe a noticeable extinction of a focused beam by a single Rb atom in a tightly focused recycle spectrum 20 ms 20 ms N optical dipole trap. Cooling laser N−1

Repump laser

Probe light UHV chamber Raman beams Laser beams for MOT

Polarizer F1 λ/4 Dichroic F2 MOT coils AL AL mirror

Bz 99/1 BS λ/4 λ/2 100 99 Figure 3: Experimental sequence for Raman sideband cooling. First the atom is loaded into dipole trap 98 97 from Magneto-Optical Trap (MOT) and pre-cooled by molasses beams. Then the atom is optically pumped Probe Transmission 10.0 ± 0.2 beam Atomic fluorescence measurement 96 into |F = 2, m = −2, Ni state by a circularly polarized probe beam. A bias magnetic ﬁeld of 2.1 Gauss is ± F detection 95 7.7 0.2 , , 94 applied to deﬁne quantization axis. Raman beams transfer the atom into |F = 1 mF = −1 N − 1i with a 980 nm σ− σ+ FORT light 93 target Rabi frequency of ∼ 700 Hz thus inducing loss of one motional quanta. Then the atom is recycled D2 D1 transmission (%) 92 back into |F = 2, m = −2i state via spontaneous emission. Transfer and recycle processes are repeated 91 F 90 several times to achieve a net cooling of the atom to the ground state of the trap. The motional spectrum 89 20 30 40 50 60 70 of the atom is obtained by frequency scan of the narrow Raman pulse and subsequent state detection. detuning (MHz)

Figure 1: Setup for transmission measurements together with the experimental results [1] Raman Rabi Oscillations A possibility to coherently transfer atomic population between hyperfine ground states of the atom is essential Temperature Estimation and Trap Parameters for sideband cooling. In Fig. 4 we show the results of such transfer. This allows us to measure two-photon carrier Rabi frequency and estimate the power of the beams that we set for cooling. However, there are technical challenges to be overcome in order to exploit strong focusing. The major one is the atomic motion due to the finite temperature obtained even after laser cooling the atomic sample. This motion leads to displacement of the atom from the focus where one expects a strong interaction (see Fig.2). For these estimates 0.9 the trap frequencies were measured and found to be νt ∼ 56kHz for transverse confinement and νl ∼ 7kHz for longitudinal confinement in a tightly focused Gaussian optical dipole trap beam. 0.8

0.7 300 0.6 250 0.5 200 0.4

150 probability F=2 0.3

100 0.2 lifetime of atom in a trap (ms)

50 0.1 ν l = 7 kHz ν t = 56 kHz 0 10 100 0 modulation frequency (kHz) 0 10 20 30 40 50 time (us)

Figure 2: (Left) Inﬂuence of the ﬁnite atomic temperature on the extinction measurements. (Right) Trap Figure , frequencies determined by parametric resonance heating 4: The probability of transferring atomic population into |F = 1 mF = 1i state versus the duration of the Raman laser pulses. The oscillation reveal the two-photon Rabi frequency of (2π) · 220kHz. The damping constant of the oscillation τd is on the order of 40 microseconds. This constant gives a measure of Experiment the decoherence time that the atom experiences while being tranferred. To bring the atom close to the vibrational ground state of the trap with characterstic frequencies of 20-80 kHz, we implement a Raman Cooling technique similar to the one commonly used in ion traps [5]. Sideband Cooling Results

0.6 before cooling after 20 cooling pulses

0.5

0.4

0.3

probability F=2 0.2

0.1

0 5P -0.15 -0.1 -0.05 0 0.05 0.1 0.15 3 / 2 carrier frequency offset (MHz) 15 GHz State detection beam Figure Dipole trap 5: Resolved motional sidebands of an atom in a trap before and after Raman cooling.

ω r One can determine the mean vibrational quantum number hni in the trapping potential from the ratio of Stokes ω c and anti-Stokes signal ‘intensities’ [5]. From our data we extract hni = 0.55 ± 0.07. We are now optimizing the N sequence to achieve better cooling and possibly extending to 3D cooling of the atom in a tweezer. This result is Probe − 2 important for boosting the eﬃciency of free-space atom-photon coupling. − 1 0 F = 2 1 2 5S X Raman beams 1 / 2 Y References 1 Z Lamb−Dicke factor N−1 0 F = 1 o − 1 4 18 η = 0.083 [1] M. K. Tey, et al., Nature Physics , 924 (2008) [2] M. K. Tey et. al., New J. Phys. 11, 043011 (2009) [3] S.A. Aljunid et al., Phys. Rev. Lett. 103, 153601 (2009) [4] C. Monroe et al., Phys. Rev. Lett. 75, 4011 (1995)