Coherence and Raman sideband cooling of a single atom in an optical tweezer (1209.3028v1)

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 $\bar{n}_r = 0.01$ and $\bar{n}_a = 8$ 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.

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

The paper "Coherence and Raman sideband cooling of a single atom in an optical tweezer," authored by J.D. Thompson, T.G. Tiecke, A.S. Zibrov, V. Vuleti, and M.D. Lukin, explores the quantum control capabilities for single atoms trapped in optical tweezer traps. This investigation primarily addresses challenges associated with internal-state decoherence and the realization of Raman sideband cooling to facilitate quantum operations close to the single atom's ground state within an optical trap setting.

Overview of Main Findings

The authors have demonstrated that the use of tightly-focused optical beams in dipole traps, a regime where the paraxial approximation fails, induces considerable internal-state decoherence due to longitudinal polarization components. This effect substantially undermines quantum control fidelity by disrupting coherent state manipulation and diminishing cooling efficiency.

The paper provides a quantitative and experimental examination of how these decoherence effects can be mitigated. By carefully selecting a magnetic bias field, the researchers effectively suppress detrimental polarization gradients, achieving optical trapping with a beam waist as narrow as 900 nm. This strategic use of external magnetic fields allows control over spin coherences and enables the efficient application of Raman sideband cooling.

Numerical Results and Implications

By employing this stabilized system, the authors achieve vibrational occupation numbers of $\bar{n}_r = 0.01$ in the radial direction and $\bar{n}_a = 8$ in the axial direction, reflecting significant advancements in atom cooling performance. These occupation numbers translate to root mean square (rms) sizes of the atomic wavepacket of 24 nm radially and 270 nm axially, illustrating a marked reduction in both spatial volume and phase-space volume—underscoring the high precision of quantum control achieved.

Practical and Theoretical Implications

The outcomes from this research hold considerable promise for advancing hybrid quantum systems. By significantly minimizing the spatial extent of an atom's wavepacket while maintaining low heating rates and reducing decoherence, this work lays the groundwork for intriguing applications in quantum networks and technologies interfacing atoms with solid-state systems. Notably, it highlights the potential for placing atoms in close proximity to surfaces, which is crucial for integrating quantum bits (qubits) into scalable quantum information processing platforms.

Future Directions

The paper ushers in prospects for further experiments that exploit these low heating rates and coherent trapping capabilities to refine quantum gate operations between atoms, investigate atom-surface interactions at the sub-wavelength scale, and support the implementation of scalable quantum computing architectures.

Overall, the research positions optical tweezer traps as a viable tool for high fidelity quantum operations and sets the stage for continued exploration within this domain. The findings suggest routes for optimizing system design and pinpoint the roles of precision magnetic control in mitigating decoherence—a critical step towards practical, robust, and scalable quantum systems.