Simultaneous Dual-Species Laser Cooling Using an Optical Frequency Comb

Simultaneous dual-species laser cooling using an optical frequency comb D. Buhin,1 D. Kovaˇcić,1 F. Schmid,2 M. Kruljac,1 V. Vulić,1 T. Ban,1 and D. Aumiler1 1Institute of Physics, Bijeniˇckacesta 46, 10000 Zagreb, Croatia 2Max-Planck-Institut fürQuantenoptik, 85748 Garching, Germany (Dated: June 3, 2020) We demonstrate 1D simultaneous laser cooling of 87Rb and 85Rb atoms using an optical frequency comb. By adjusting the pulse repetition frequency and the offset frequency, the frequency comb spectrum is tuned to ensure that two distinct frequency comb modes are simultaneously red- detuned from the cooling transitions, one mode for each species. Starting from a pre-cooled cloud of 85;87Rb atoms at above-Doppler temperatures, we show simultaneous cooling of both species down to the Doppler temperature using two counter-propagating σ+/σ−-polarized beams from the frequency comb. The results indicate that simultaneous dual-species frequency comb cooling does not affect the cooling characteristics of individual atomic species. The results of this work imply that several atomic species could be cooled simultaneously using a single frequency comb source. This comb-based multi-channel laser cooling could bring significant advances in multi-species atom interferometers for space applications and in the study of multi-species interactions. PACS numbers: 37.10.De, 37.10.Vz Optical frequency combs (FCs) are unique light sources with applications ranging from metrology [1] and high- resolution spectroscopy [2,3] to quantum communication and processing [4]. In the time domain, the output of a FC is a train of phase-stable ultrashort pulses of typically high peak power. This allows for efficient frequency conversion and other non-linear interactions. The spectrum of a FC consists of a series of equally spaced narrow spectral lines. Owing to these unique properties, FCs have been proposed as potential light sources for laser cooling of atoms with strong cycling transitions in the vacuum ultraviolet (VUV) [5{7]. This part of the spectrum, and thus some of the most prevalent atomic species, has so far remained inaccessible to laser cooling since generating continuous wave (CW) laser radiation in the VUV is extremely challenging. Cooling of atoms and ions using FCs has recently been demonstrated. Jayich et al. [7] achieved FC Doppler cooling of pre-cooled rubidium atoms on the two-photon transition at 778 nm. Davila-Rodriguez et al. [6] showed FC Doppler cooling of trapped magnesium ions on a single-photon transition in the UV. Ip et al. [8] demonstrated loading, cooling and crystallization of hot ytterbium ions, and Santićetˇ al. [9] demonstrated cooling of rubidium atoms on a single-photon transition at 780 nm. A recent theoretical work by our group [10] investigated simultaneous laser cooling in multiple cooling channels using a FC and has shown that simultaneous cooling of 40K, 85Rb, and 87Rb can be achieved using a single FC by appropriate selection of comb parameters, i.e. pulse repetition frequency and offset frequency. In this Rapid Communication, we demonstrate simultaneous laser cooling of 85Rb and 87Rb atoms using an optical frequency comb. To our knowledge, this is the first demonstration of FC cooling in multiple cooling channels simultaneously. We believe that the application of FC multi-channel cooling could bring significant advances in multi-species atom interferometers (AIs). Simultaneous dual-species AIs pave the way for future ground and space experiments dedicated to testing the weak equivalence principle, also known as the universality of free fall [11{13]. Current design for space applications is based on the 85Rb/87Rb dual-species interferometer which employs four amplified diode laser modules at 780 nm, offset locked to the rubidium spectroscopy referenced frequency doubled Telecom laser, for the simultaneous laser cooling and coherent manipulation of atoms [14]. Multi-species AIs offer extended dynamic measurement ranges [15] which could increase the sensitivity and resolution of the instruments. To our knowledge, three- (and more than three) species AIs have so far not been demonstrated, most likely due to the complexity of the laser systems required. In this context, we believe that the application of frequency combs with multi-species cooling capabilities, potentially utilizing the developing chip-based microresonator FC technology [16{18], could lead arXiv:2006.01547v1 [physics.atom-ph] 2 Jun 2020 to a breakthrough in the development of multi-species AIs for space applications. Dual- and multi- species magneto-optical traps (MOTs) are an experimental tool for investigating atomic interactions. They are a starting point for the production of quantum degenerate mixtures [19{22] as well as for the formation of heteronuclear cold molecules [23, 24]. FC multi-channel cooling demonstrated in this work could greatly reduce the complexity of multi-species MOT experimental systems by replacing a series of CW lasers (independent cooling and repumper lasers are usually required for each species) with a single frequency comb source where different modes within the comb spectrum can serve as cooling as well as repumper lasers. One notable simplification of the experiment involves replacing a large number of feedback loops (one feedback loop is required to stabilize each individual CW laser) with only two feedback loops that can stabilize and phase-lock all lines within the FC spectrum. FC cooling would therefore allow cooling of different atomic species by highly phase-coherent frequency comb modes 2 which could bring new insights into the physics of heteronuclear cold collisions and molecules formation [10, 25]. Our apparatus consists of a dual-species MOT in which ≈ 1 · 106 85Rb atoms and ≈ 3 · 106 87Rb atoms are simultaneously loaded from a background vapor in a stainless steel chamber. The MOT relies on four independent frequency-stabilized CW lasers arranged in a standard six-beam configuration, which together with a quadrupole magnetic field creates the trapping potential for both species. To ensure that the 85Rb and 87Rb MOTs are well overlapped, the cooling beams for both species are delivered through a single optical fiber. A detailed description of the experimental setup and the loading characteristics of the dual-species MOT can be found in the Supplemental Material. In typical experimental conditions, we obtain two clouds of cold 85Rb and 87Rb atoms with temperatures in the range of 200-300 µK. The clouds typically have 1/e2 radii of ≈ 0.8 mm, and their centers of mass overlap to within 5 % of their radii. Such overlapped pre-cooled clouds of 85Rb and of 87Rb atoms represent the initial sample for all measurements presented in this work. The FC is generated by frequency doubling an Er:fiber mode-locked laser (TOPTICA FFS) operating at 1550 nm with a nominal repetition rate frep = 80:5 MHz. A repetition rate tuning range of 50 kHz can be achieved by adjusting the laser cavity length with the help of an integrated stepper motor and a piezo transducer. The frequency-doubled spectrum used in the experiment is centered around 780 nm with a FWHM of about 5 nm and a total power of 68 mW. The spectrum of the FC consists of a series of sharp lines, i.e. comb modes [26]. The optical frequency of the n-th comb mode is given by fn = n · frep + f0, where frep is the laser repetition rate and f0 is the offset frequency. In our experiment, we actively stabilize frep and fn by giving feedback to the cavity length and pump power of the mode-locked laser, thus indirectly fixing f0. The n-th comb mode, fn, is phase-locked to a frequency-shifted CW 87 0 reference laser (ECDL, Moglabs CEL002), which is locked to the Rb j5S1=2; F = 2i ! j5P3=2; F = 3i transition. The frequency shift of the CW reference laser is achieved by an acousto-optic modulator (AOM) in a double pass configuration. frep is stabilized to a low-noise synthesizer which is referenced to a rubidium frequency standard. A detailed description of the FC stabilization scheme is presented in our recent paper [9]. In order to achieve simultaneous cooling of two atomic species, two distinct modes within the comb spectrum must be simultaneously red detuned from the cooling transitions of the atomic species [10]. Careful tailoring of the FC spectrum, i.e. choosing the appropriate frep and f0, is therefore crucial for the successful realization of FC cooling. In our experiment, the repetition rate is fixed during the measurements and set to frep = 80:495 MHz, while f0 is scanned by adjusting the heterodyne beat frequency between the CW reference laser and the n-th comb mode, fn. This way it 87 0 is possible to control the detuning of the n-th comb mode with respect to the Rb j5S1=2; F = 2i ! j5P3=2; F = 3i transition. The heterodyne beat frequency can be continuously changed over a range of 5-30 MHz by changing the frequency of the local oscillator, so four separate scans with different CW reference laser frequency shifts are performed and subsequently merged to fully scan the n-th comb mode frequency by one frep. We start the investigation of simultaneous interaction of the FC with cold 85Rb and 87Rb atoms by measuring the FC radiation pressure force. The experimental setup and the measurement sequence used are similar to the ones described in our recent work [9]. A single circularly polarized FC beam is sent through an AOM for fast switching, and is then directed to the center of the dual-species MOT. The total power of the FC beam on the atoms is 10 mW and the beam size (FWHM) is 2.7 mm, resulting in the power and intensity per comb mode of about 0.3 µW and 3.6 µW/cm2, respectively.

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