Coherence and Raman Sideband Cooling of a Single Atom in an Optical Tweezer

Abstract

We investigate quantum control of a single atom in an optical tweezer trap created by a tightly focused optical beam. We show that longitudinal polarization components in the dipole trap arising from the breakdown of the paraxial approximation give rise to significant internal-state decoherence. We show that this effect can be mitigated by appropriate choice of magnetic bias field, enabling Raman sideband cooling of a single atom close to its three-dimensional ground state in an optical trap with a beam waist as small as w = 900 nm. We achieve vibrational occupation numbers of nr = 0.01 andna = 8 in the radial and axial directions of the trap, corresponding to an rms size of the atomic wavepacket of 24 nm and 270 nm, respectively. This represents a promising starting point for future hybrid quantum systems where atoms are placed in close proximity to surfaces.Single atoms in "optical tweezer" traps [1] are a promising resource for various applications in quantum science and engineering. They can be individually moved [2], manipulated [3,4], and read-out [5] in a manner similar to trapped ions. At the same time, they may be strongly coupled to photonic [6,7], plasmonic [8], or other solid-state systems [9][10][11], opening a new frontier for the realization of quantum networks and hybrid quantum systems. These intriguing applications require trapping single ultra-cold atoms near surfaces at distances well below an optical wavelength. While this is challenging for ions [12], and magnetically trapped atoms [9,13], it is readily achievable with neutral atoms in optical dipole traps.The collisional blockade regime [1] of optical dipole traps is an attractive starting point for such experiments because it provides a simple way to load and tightly confine single atoms starting with only a conventional optical molasses. In several experiments, it has been found that the temperature of an atom loaded from a molasses into an optical dipole trap in the collisional blockade regime is in the range 30 µK to 180 µK [3,4,7,[14][15][16][17]. This elevated temperature compared to free-space cooling has been identified as a limiting factor in many recent experiments, as the thermal motion reduces the coherence time [3,4,16] and impedes full quantum control [17,18]. Moreover, lower temperatures and a reduction in the spatial extent of the atomic wavepacket are necessary to control the atom at sub-wavelength distances from a surface, or to implement proposed quantum gates using collisional interactions between two ground state atoms [19].One major challenge to laser cooling and quantum control are polarization effects associated with the breakdown of the paraxial approximation in very tightly focused optical dipole traps. Vector light shifts arising from elliptical light polarization [20] are known to be a major obstacle to cooling and manipulating atoms in optical dipole traps. In the paraxial limit, the vector light shift can be eliminated by using a linearly polarized trapping beam. However, in the tightly focused regime, nonpara...

Atom–light interactions in photonic crystals

“…Other opportunities could be tuning to place the atomic resonance within the band gap to induce long-range atom-atom interactions 4,[16][17][18] . By optimizing the power and detuning of the E 1 trap mode, we should be able to achieve stable atomic trapping and ground state cooling 41,50,51 . By applying continuous on-site cooling to Nc1 atoms, we expect to create a 1D atomic lattice with single atoms trapped in unit cells along the APCW, thus opening new opportunities for studying novel quantum transport and many-body phenomena [5][6][7][8][9][10][11][12][13][14][15][16][17][18] .…”

Evaporative cooling of a small number of atoms in a single-beam microscopic dipole trap

“…1). More involved diagnostics will be necessary to study the quantum degenerate regime, such as stimulated Raman spectroscopy [31][32][33], which we leave for future work.…”

Quantum simulators by design: Many-body physics in reconfigurable arrays of tunnel-coupled traps

“…2) The dynamic control over local potential depths can be utilized for Floquet engineering [33,41] with modulation frequencies of up to 10 kHz, fully adjustable modulation amplitudes, and single-site addressability. 3) A promising alternative to the loading schemes starting with BECs arises from the implementation of Raman side-band cooling in individual traps [11,12] with the targeted many-body state assembled atom by atom [15][16][17] out of the low entropy Mott-insulator phase. This facilitates studies of the many-body physics of atomic species, which are not accessible to BEC, or arbitrary mixtures of species.…”

“…Complementing this approach, recent experiments with tightly focused optical tweezers demonstrated efficient trapping and cooling of single atoms to their vibrational ground state [11,12]. In combination with acoustooptic deflectors and spatial light modulators (SLM) this approach has been extended to few-well configurations showing tunnel-coupling [13,14] on one side and the deterministic preparation of larger defect-free 1D and 2D arrays with spacing too large for tunneling on the other side [15][16][17].…”