Red-detuned Trap Research Articles

Comparison of single-neutral-atom qubit between in bright trap and in dark trap**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0304502) and the National Natural Science Foundation of China (Grant Nos. 11634008, 11674203, 11574187, and 61227902), and the Fund for Shanxi “1331 Project” Key Subjects Construction, Shanxi Province, China.

A single neutral atom is one of the most promising candidates to encode a quantum bit (qubit). In a real experiment, a single neutral atom is always confined in a micro-sized far off-resonant optical trap (FORT). There are generally two types of traps: red-detuned trap and blue-detuned trap. We experimentally compare the qubits encoded in “clock states” of single cesium atoms confined separately in either 1064-nm red-detuned (bright) trap or 780-nm blue-detuned (dark) trap: both traps have almost the same trap depth. A longer lifetime of 117 s and a longer coherence time of about 10 ms are achieved in the dark trap. This provides a direct proof of the superiority of the dark trap over the bright trap. The measures to further improve the coherence are discussed.

Improved far-field optical trap for cold atoms (molecules) with phase-modulated circular aperture diffraction

We propose a scheme to trap cold atoms (or molecules) by using an improved red- or blue-detuned far-field optical trap, which is formed by an optical system composed of a binary phase plate and a circular aperture illuminated by a plane light wave. We calculate the relative intensity distribution of the far-field trap and study its dependence on the phase $\ensuremath{\varphi}$ of the binary phase plate. Our study shows that the binary phase plate can be used to improve some geometric and optical parameters of the far-field optical trap, and realize the evolution of the far-field optical trap from a red-detuned trap to a blue-detuned one. In particular, when the phase $\ensuremath{\varphi}$ of the binary phase plate is changed from 0 to $\ensuremath{-}\ensuremath{\pi}$, the maximum intensity of the far-field trap and its well depth will be enhanced by $\ensuremath{\sim}4$ times, and the corresponding trapping volume will be increased by $\ensuremath{\sim}8$ times, with respect to a trapping beam obtained from the diffraction by a circular aperture.

Optimized single-beam dark optical trap

We propose a new scheme for constructing a single-beam dark optical trap that minimizes light-induced perturbations of the trapped atoms. The proposed scheme optimizes the trap depth for given trapping laser power and detuning by creating a light envelope with (a) an almost minimal surface area for a given volume and (b) the minimal wall thickness that is allowed by diffraction. The stiffness of the trap’s walls, combined with the large detuning allowed by the efficient distribution of light intensity, yields a low spontaneous photon scattering rate for the trapped atoms. Our trap also optimizes the loading efficiency by maximizing the geometrical overlap between a magneto-optical trap and the dipole trap. We demonstrate this new scheme by generating the proposed light distribution of a single-beam dark trap with a trap depth that is ∼33 times larger than that of existing blue-detuned traps and ∼13 times larger than that of a red-detuned trap with the same diameter, detuning, and laser power. Trapped atoms are predicted to have a decoherence rate that is >200 times smaller than in existing single-beam dark traps and ∼1800 times smaller than in a red-detuned trap with the same diameter, depth, and laser power.