[Paper Review] Laser-cooled atoms inside a hollow-core photonic-crystal fiber

Michal Bajcsy, Sebastian Hofferberth+6 more|DSpace@MIT (Massachusetts Institute of Technology)|Apr 27, 2011

Cold Atom Physics and Bose-Einstein Condensates1 references3 citations

TL;DR

This paper demonstrates the efficient loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber (PCF), where they are trapped by a far-detuned optical dipole trap and probed by a resonant guided beam. The method achieves an optical depth of ~180—six times higher than previous work—enabling strong atom-photon interactions for quantum optics applications at the few-photon level.

ABSTRACT

We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of ~30,000 rubidium atoms, which creates a medium with optical depth ~180 inside the fiber. Compared to our earlier study this represents a six-fold increase in maximum achieved optical depth in this system.

Motivation & Objective

• To develop a robust method for loading laser-cooled atoms into a hollow-core photonic-crystal fiber for quantum optics experiments.
• To achieve high optical depth in a confined atomic ensemble within the fiber to enable strong light-matter interactions.
• To characterize the trapped atoms' temperature, lifetime, and spatial distribution for use in quantum information and nonlinear optics.
• To enable both trapping and probing of atoms using light guided through the same fiber, facilitating integrated photonic-atom systems.
• To overcome limitations of previous capillary-based systems by using a single-mode PCF with low-loss, single-mode guidance of both trapping and probe beams.

Proposed method

• Atoms are initially cooled in a magneto-optical trap (MOT) and then transferred into a fiber-coupled far-detuned optical dipole trap formed by a Gaussian beam focused into the hollow core of the PCF.
• The dipole trap is generated by a 1064 nm laser beam tightly focused into the PCF core, creating a transverse potential that confines atoms radially.
• A weak resonant probe beam at 780 nm is coupled into the same PCF to probe the atomic ensemble, with light guided through the hollow core via photonic bandgap effects.
• Loading efficiency is optimized by adjusting the MOT size, trap beam intensity, and timing of the dipole trap activation relative to MOT release.
• The radial temperature and cloud size are extracted by measuring the recapture efficiency after releasing the atoms from the fiber trap and re-capturing them in the MOT.
• The optical depth is calculated from the measured absorption of the probe beam, with the radial density profile assumed to be Gaussian.

Experimental results

Research questions

• RQ1 What is the maximum achievable optical depth in a laser-cooled atomic ensemble confined within a hollow-core photonic-crystal fiber?
• RQ2 How does the loading efficiency of laser-cooled atoms into the PCF depend on experimental parameters such as MOT size and trap beam intensity?
• RQ3 What are the temperature and spatial distribution of the atoms after transfer into the fiber-based dipole trap?
• RQ4 Can both the trapping and probing light be efficiently guided through the same PCF while maintaining high atom-light interaction strength?
• RQ5 To what extent is the loading process adiabatic, and how does the phase space density of the atomic cloud evolve during transfer?

Key findings

• The most efficient loading procedure results in the capture of approximately 30,000 rubidium atoms within the fiber core.
• The system achieves an optical depth of ~180, representing a six-fold increase over the previous maximum reported in similar systems.
• The temperature of the trapped atomic cloud is measured to be ~1.6 mK, with a radial cloud size of ~2.2 μm, indicating a high degree of transverse confinement.
• The phase space density of the atomic cloud in the fiber is ~10⁻⁷, matching the peak phase space density of the original MOT, confirming an adiabatic transfer process.
• The lifetime of the trapped atoms exceeds 100 ms, enabling sufficient time for optical probing and quantum experiments.
• The radial density profile of the atomic cloud is well described by a Gaussian distribution, consistent with the assumptions used in the optical depth and temperature analysis.