Strong atom-field coupling for Bose-Einstein condensates in an optical cavity on a chip (0706.1390v2)

Abstract: An optical cavity enhances the interaction between atoms and light, and the rate of coherent atom-photon coupling can be made larger than all decoherence rates of the system. For single atoms, this strong coupling regime of cavity quantum electrodynamics (cQED) has been the subject of spectacular experimental advances, and great efforts have been made to control the coupling rate by trapping and cooling the atom towards the motional ground state, which has been achieved in one dimension so far. For N atoms, the three-dimensional ground state of motion is routinely achieved in atomic Bose-Einstein condensates (BECs), but although first experiments combining BECs and optical cavities have been reported recently, coupling BECs to strong-coupling cavities has remained an elusive goal. Here we report such an experiment, which is made possible by combining a new type of fibre-based cavity with atom chip technology. This allows single-atom cQED experiments with a simplified setup and realizes the new situation of N atoms in a cavity each of which is identically and strongly coupled to the cavity mode. Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field. This gives rise to a controlled, tunable coupling rate, as we confirm experimentally. We study the heating rate caused by a cavity transmission measurement as a function of the coupling rate and find no measurable heating for strongly coupled BECs. The spectrum of the coupled atoms-cavity system, which we map out over a wide range of atom numbers and cavity-atom detunings, shows vacuum Rabi splittings exceeding 20 gigahertz, as well as an unpredicted additional splitting which we attribute to the atomic hyperfine structure.

• The paper demonstrates a novel experimental setup that achieves uniform strong coupling of each atom in a BEC to the cavity mode using fibre-based Fabry-Perot and atom chip technology.
• The setup enables precise deterministic control of atom position, yielding vacuum Rabi splittings over 20 GHz with minimal atomic heating.
• The collective coupling strength scales as √N, underscoring the potential for high-fidelity quantum interfaces through efficient atom-photon interactions.

Strong Atom-Field Coupling for Bose-Einstein Condensates in an Optical Cavity on a Chip

The paper presents an experimental setup that achieves strong atom-field coupling for Bose-Einstein condensates (BECs) within an optical cavity, leveraging fibre-based technology and atom-chip integrations. This configuration facilitates simplified experiments and introduces a novel situation where each atom in a cavity is strongly and identically coupled to the cavity mode.

Atomic physics and cavity quantum electrodynamics (cQED) have long been areas of intense research due to their profound implications on quantum information processing and quantum-optical interfaces. The pursuit of enhancing atom-photon interactions surpassing decoherence rates has been a critical focus. The experiment described here advances this field by effectively combining a fibre-based Fabry-Perot cavity with atom chip technology, thereby achieving strong coupling for BECs in a cavity mode.

Methodology and Experimental Setup

The experimental setup involves a novel fibre-based Fabry-Perot (FFP) cavity mounted on an atom chip, achieving unprecedented single-atom peak coupling rates through a reduced mode volume and high mirror curvature. The cavity's design is critical, employing concave mirror surfaces on optical fiber tips, enhancing atom-cavity coupling while maintaining their distance from any material surfaces to prolong coherent trapping times. The FFP cavity configuration presents an advantage in simplicity and robustness over microtoroidal or microsphere cavities.

In the experiment, a BEC of Rb atoms is positioned in an optical lattice within the cavity field by controlling magnetic and optical potentials. The setup allows precise manipulation of atomic position and coupling strength due to the small spatial spread of the BEC and the cavity's properties. Various laser beams are used within the cavity, including a probe beam that is tuned to resonance with atomic transitions and an off-resonant beam creating a lattice for control over the atom-field coupling rate.

Key Findings and Implications

The experimental results report several critical insights:

• Controlled Coupling Rate: The setup allows deterministic control over the BEC's position, demonstrating that atoms can be localized within a single cavity antinode. The BEC's ability to be placed precisely within the cavity mode offers controlled, tunable coupling rates, reducing or eliminating measurable heating in strongly coupled scenarios. The paper confirmed this experimentally by noting minimal heating under specific coupling regimes.
• Vacuum Rabi Splitting: The spectrum of the atom-cavity system displays vacuum Rabi splittings exceeding 20 GHz. An additional, unexpected splitting linked to the atomic hyperfine structure illustrates complex underlying interactions and offers potential pathways for further investigation.
• Atom-Number Scaling: The collective coupling strength scales as √N, with results indicating a high degree of consistency for both thermal and quantum degenerate ensembles. The realization of the collective interaction demonstrates the potential for a "superatom" behavior where the ensemble's collective coupling is enhanced.
• Quantum Information Processing: This system holds promise as a quantum interface, essential for memory qubit-to-flying qubit transitions where high cooperativity and low decoherence rates are critical. The inherent atom-cavity interaction dynamics might increase the fidelity of such quantum processes.

Future Directions

The integration of BECs within a strong-coupling cavity as depicted in the paper opens multiple avenues for future research. Potential areas include examining the effects of quantum degeneracy on atom-light interactions, probing collective quantum phases within optical lattices, and enhancing quantum memory capacities. The cavity's inherent fibre coupling may serve as a foundation for more integrated photonic systems, advancing optical quantum computing paradigms.

Significantly, these findings illustrate the foundational elements necessary for exploring quantum information processing's full potential, promoting further research into fiber-based optical cavities' efficacy for complex quantum systems. As the field continues to progress, enhancements in cavity design and atom positioning will likely yield new discoveries, fostering a deeper understanding of light-matter interactions within quantum technology spheres.