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Sub-Doppler Cooling of Sodium Atoms in Gray Molasses

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Sub-Doppler Cooling of Sodium Atoms in Gray Molasses Sub-Doppler cooling of sodium atoms in gray molasses Giacomo Colzi1;2, Gianmaria Durastante1,∗ Eleonora Fava1, Simone Serafini1, Giacomo Lamporesi1;2, and Gabriele Ferrari1;2 1 INO-CNR BEC Center and Dipartimento di Fisica, Universit`adi Trento, 38123 Povo, Italy and 2 Trento Institute for Fundamental Physics and Applications, INFN, 38123 Povo, Italy (Dated: February 23, 2016) We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using 2 2 the D1 optical transition (3s S1=2 ! 3p P1=2) at 589.8 nm. The technique is applied to samples containing 3 × 109 atoms, previously cooled to 350 µK in a magneto-optical trap, and it leads to temperatures as low as 9 µK and phase-space densities in the range of 10−4. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to 1011 cm−3. PACS numbers: 37.10.De, 32.80.Wr, 67.85.-d I. INTRODUCTION tool in addressing fundamental problems in physics and in devising cold-atom based quantum simulators [11]. Sub-Doppler laser cooling has been a well-established Velocity-selective coherent population trapping and widespread technique since the late 1980s [1]. It (VSCPT) [12] allows one to cool atoms even below 2 2 consists in using light polarization gradients and optical the photon recoil temperature limit Trec = ¯h kL=mkB pumping to cool down atoms below the Doppler tem- (where kL is the wave number and m is the atomic perature limit TD = ¯hΓ=2kB [2, 3], where ¯h is the re- mass), taking advantage of electromagnetically induced duced Planck constant, kB is the Boltzmann constant transparency [13, 14]. Its main limitation is that it relies and Γ is the natural linewidth. This technique, follow- on the diffusion in momentum space, due to spontaneous ing a Doppler precooling stage in a standard magneto- emission, accumulating a relatively small population in a optical trap (MOT), greatly improves the efficiency of velocity-selective dark state. Gray molasses (GM) cool- the cooling process within its velocity capture range, but ing [15{17], on the other hand, consists of polarization fundamentally does not remove the heating effects as- gradient cooling in the presence of VSCPT: dark states sociated with the stochastic nature of photon absorption are given by a coherent superposition of degenerate and spontaneous emission cycles. In addition, the atomic Zeeman sublevels in the ground hyperfine manifold, sample density remains limited both by light-assisted col- while states coupling with the radiation field are shifted lisions [4, 5] and by effective atom-atom repulsive forces in energy according to polarization gradient effects. In caused by reabsorption of scattered photons [6]. Improve- such a scheme atoms are continuously optically pumped ments are obtained in schemes where slowed atoms are into the dark states as they cede kinetic energy to the preserved from the above mentioned effects through se- light field. The velocity-selective mechanism is provided lective trapping into dark states|states that are not cou- by non adiabatic transitions: faster atoms in dark states pled with the light field. Examples include the dark-spot have a higher transition probability to bright states from (DS) -MOT scheme [7] and schemes where the fraction which the cooling cycle reiterates. Dark states exist only of atoms optically pumped in a dark hyperfine state is for jF i ! jF − 1i or jF i ! jF i transition; the cooling dynamically controlled [8]. The use of dark states in mechanism works for blue detunings only, meaning that the achievement of lower and lower temperatures clearly atoms in velocity classes outside the sub-Doppler capture leads to a gain in evaporation efficiency, hence in the fi- range are heated up by Doppler heating. In addition, nal atom number. The advantages offered by dark states stray magnetic fields have to be compensated to preserve can also be exploited in other contexts where spontaneous the cooling efficiency. In the presence of magnetic fields emission still plays a pivotal role, as exemplified by the and for proper light field configurations also a different recent developments in single-site resolved quantum-gas cooling and trapping scheme can be devised. This is microscopy [9, 10]. For this reason, studying techniques referred to as dark optical lattice (DOL) [16, 18]. In that take advantage of atomic dark states is still a promis- the strong magnetic field regime, for which the Zeeman arXiv:1512.07053v2 [cond-mat.quant-gas] 22 Feb 2016 ing field of study. This holds especially true given the energy shift is higher than the polarization gradient [19], fact that highly degenerate quantum gases shifted rapidly it consists in lifting the Zeeman degeneracy such that from being the object of study to being an experimental dark states correspond to mF eigenstates uncoupled to the light field due to its local polarization. GM cooling was achieved experimentally in the 1990s 133 87 on Cs [20{23] and on Rb [24] atoms using the D2 ∗ present address: Institut f¨urQuantenoptik und Quanteninforma- transition (nS1=2 ! nP3=2). The transverse cooling of a ¨ 52 tion, Osterreichische Akademie der Wissenschaften, 6020 Inns- Cr atomic beam [25] was also reported. The D1 transi- bruck, Austria tion (nS1=2 ! nP1=2) was already used in the context of 2 a b δ Δ F=2 λ/2 189 MHz Laser 3P1/2 EOM AOM AOM Source 1771.6 +200 -200 F=1 590 nm To the experiment AOM To +80 589.8 nm PZT λ/2 λ/4 AOM Na Vapor F=2 +80 1771.6 MHz Lock-In + PID 3S1/2 F=1 FIG. 1. (a) Sodium D1 level structure. Cooler (blue, solid) and repumper (red, dashed) transitions are shown. (b) Sketch of the D1 laser source. 87 85 DOLs with Rb [19] and Rb [26]. This choice is ben- jFe = 2i cooler transition frequency using modulation- eficial due to the absence of off-resonant excitations on transfer saturated absorption spectroscopy on a heat- the blue side of the hyperfine manifold of interest and to pipe containing sodium vapors. The repumper laser the more resolved energy spectrum. More recently, GM frequency, corresponding to jFg = 1i ! jFe = 2i is ob- cooling was demonstrated on 85Rb [27], 40K [28, 29], 7Li tained generating frequency sidebands in the laser field 39 6 [30], K [31, 32], and Li [29, 33] using the D1 transition, by means of an electro-optic modulator (EOM) driven and on metastable 4He [34]. at about 1771:6 MHz, the hyperfine splitting frequency In this article we report on an experimental charac- of the electronic ground state. After two acousto-optic terization of GM cooling of 23Na atoms exploiting the modulators (AOMs) acting as fast switches, a polariza- D1 transition jFg = 2i ! jFe = 2i. The level structure tion maintaining optical fiber is used to deliver the laser is shown in Fig. 1(a). In the following section (Sec. II) light on the optical table hosting the atomic cell. the reader will find the description of the D1 laser source. Subsequently (Sec. III) the description of the experimen- The aforementioned experimental configuration en- tal procedure and its characterization will be provided, ables us to control all the parameters relevant to GM before drawing the conclusions in the last section (Sec. cooling. Both the cooler detuning ∆ and the total in- IV). tensity IC + Irep (cooler and repumper intensity, respec- tively) delivered to the atoms are tuned by changing the frequency and amplitude of the RF signals driving the two AOMs before the optical fiber. The relative intensity II. EXPERIMENTAL APPARATUS Irep=IC and the Raman detuning δ between repumper and cooling can be tuned by changing the amplitude and The D1 laser source employed for GM cooling is frequency of the microwave field driving the EOM. We sketched in Fig. 1(b). The main 590 nm laser source deliver up to 200 mW of power to the atoms starting comprises a 1180 nm master oscillator diode (Innolume from a single collimated beam of 4:7 mm waist (e−2 ra- GC-1178-TO-200) mounted in Littrow configuration [35] dius). The corresponding intensity is about 90 Isat in ± and capable of emitting 50 mW, a Raman fiber ampli- units of the D2 saturation intensity for σ polarized light 2 fier (RFA) pumped with an ytterbium fiber laser capable Isat = 6:26 mW=cm . This beam is superimposed on the of emitting up to 8 W of power while maintaining the one generated by the D2 laser source that is used for spectral properties of the master oscillator beam, and a standard MOT and bright molasses (BM) cooling (for a resonant second harmonic generation stage based on a detailed description, see Ref. [37]). The two copropa- 15 mm LiB3O5 nonlinear crystal mounted in a bow-tie gating beams are then split into six independent beams cavity stabilized by means of polarization spectroscopy and sent on the atoms along three orthogonal directions, [36]. counterpropagating in pairs with σ+ − σ− polarizations. Operating the RFA at 5 W we obtain 2:2 W of power The atoms can be loaded in a DS-MOT using an addi- at 590 nm. The laser frequency is tuned by acting on tional D2 repumper beam that is sent through a circular the master laser, and is stabilized on the jFg = 2i ! obstacle. Imaging the obstacle on the atoms results in a 3 a b c 100 100 100 75 75 75 50 50 50 25 25 25 Capture efficiency (%) 0 0 0 160 60 120 K) µ 45 90 120 60 80 30 30 40 15 Temperature ( 0 0 0 0 1 2 3 1.5 3.0 4.5 6.0 25 50 75 Pulse time τ (ms) Δpulse/Γ Ic/Isat FIG.
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