Tunable Lattice-Nanofiber Platform

• The paper demonstrates a tunable lattice achieved via an 'optical accordion' method, modulating the lattice period from 0.88 to 1.5 μm for precise atom trapping.
• It achieves efficient atom–photon coupling by trapping approximately 1300 atoms about 220 nm from the nanofiber, resulting in a coupling efficiency of around 1.8%.
• The system integrates with optical tweezers to enable reconfigurable control of atomic ensembles, advancing scalable quantum networking and simulation.

A tunable lattice–nanofiber system is an integrated quantum optical platform in which ultracold atoms are trapped in periodic potentials (optical lattices) in the vicinity of a subwavelength-diameter optical nanofiber. The core objective is to achieve precise control over the atomic array geometry and depth while maximizing the efficiency of atom–photon coupling via the fiber’s evanescent field. This hybrid approach enables efficient interfacing of large atomic ensembles (∼1300 atoms) with fiber-guided light, offers tunable lattice periods spanning subwavelength to micron scales (0.88–1.5 μm), and is adaptable to scalable quantum information architectures and photonic networks (Han et al., 26 Sep 2025).

1. Creation and Tuning of the Optical Lattice-Nanofiber System

The tunable lattice is implemented by projecting an optical interference pattern (“optical accordion” approach) onto the nanofiber. Two converging beams form an interference pattern that is imaged via a 4f stationary system, with a custom binary phase grating controlling the phase and angular content. The lattice period $d_\text{lat}$ varies from 0.88 μm to 1.5 μm, covering a broad range tunable by shifting the phase grating or adjusting projection conditions.

The general form for the lattice period is

$d_\text{lat} = \frac{q \lambda_0}{2 n_\text{eff}},$

where $q$ is a diffraction order, $\lambda_0$ the atomic resonance wavelength, and $n_\text{eff}$ the effective index of the nanofiber mode. This tuning is critical for satisfying or detuning Bragg-type conditions for collective scattering, enabling both superradiant and subradiant photon-mediated regimes.

Atoms are loaded from a MOT into this potential and trapped $\sim220$ nm above the fiber surface due to the combined effect of the optical lattice and the evanescent field. Reflections from the nanofiber enhance the spatial modulation of the potential, yielding robust confinement in all degrees of freedom.

2. Atom–Photon Coupling Near the Nanofiber

The proximity of the trapped atoms to the nanofiber ($\sim220$ nm) maximizes their overlap with the evanescent field of the guided optical mode, increasing the spontaneous emission rate into the fiber. For the closest lattice site, the coupling efficiency (fraction of emission into the guided mode) is approximately 1.8%. For ensembles of $\sim$1300 atoms distributed in periodic lattice sites, atom–photon coupling is collectively enhanced, enabling phenomena such as Bragg reflection of the probe or superradiant and subradiant emission.

Such atomic arrays provide efficient, loss-minimized interfaces between matter and guided photons, facilitating access to regimes required for quantum optical networking and distributed quantum memory.

3. Trap Engineering and Confinement

The total trapping potential is composed of the projected lattice intensity $U_\text{lattice}(z)$, the attractive van der Waals interaction $U_\text{vdW}(z)$ near the dielectric fiber, and the geometric modifications from reflection. The combined potential produces spatial maxima at the desired distance from the fiber ($\sim220$ nm), with a typical trap depth $U_0 \simeq k_B \times 0.5$ mK.

Atoms are tightly confined along the lattice, transversely by the evanescent field, and axially by the periodicity of the lattice. The motional frequencies along the fiber axis are inversely related to the lattice period: $$f_\text{ax} = \frac{1}{d_\text{lat}} \sqrt{\frac{U_0}{2M}},$$ where $M$ is the atomic mass. As $d_\text{lat}$ increases, axial confinement softens. Experimentally, $f_\text{ax}$ is measured by parametric heating—modulating the lattice beam at $2f_\text{ax}$ and probing escape from the traps.

The degree of atomic localization determines the Debye–Waller factor

$f_\text{DW} = \exp(-4k^2 \sigma_\text{ax}^2),$

where $k$ is the wavevector and $\sigma_\text{ax}$ the spatial spread, which in turn sets the amplitude for coherent scattering and Bragg reflection.

4. Tunability and Control of the Lattice Spacing

The haLLMark of this platform is its variable lattice period, achieved without altering the trapping laser’s wavelength. The “optical accordion” method employs projection of a phase grating with variable spatial period onto the nanofiber, ensuring mechanical and optical flexibility in the execution of different periodicities over a 0.88–1.5 μm range.

Such tunability is essential to:

• Match the lattice to Bragg or anti-Bragg conditions for collective photon scattering,
• Investigate different regimes of atomic wavefunction overlap and collective effects,
• Control the quantum optical response, such as the formation of atomic mirrors or engineered subradiance.

The method is readily applicable to other nanophotonic platforms and is not constrained to a particular atom type, fiber diameter, or resonance.

5. Integration with Optical Tweezers and Scalability

The system supports seamless integration with optical tweezers, enabling both the collective trapping of large ensembles and the arbitrary rearrangement and addressing of single atoms. Specifically:

• The standard lattice provides global, scalable atom–photon interfaces needed for high optical depth, quantum gates, or mirrors.
• Optical tweezers (“optical tweezing”) can be overlaid to select, move, or address single atoms, thereby uniting collective and single-qubit control.

This dual-access capability is central for advanced quantum simulation and computational schemes, offering the flexibility to engineer both highly entangled ensembles and reconfigurable small arrays.

6. Applications in Quantum Networking and Collective Phenomena

The nanofiber serves not only as the trapping and interfacing element but as a direct optical conduit to standard fiber networks, supporting efficient quantum networking protocols. The intrinsic connectivity and the ability to tune the atomic array periodicity facilitate:

• Quantum memory elements,
• Quantum repeaters,
• Distributed entanglement,
• Studies of long-range photon-mediated interactions,
• Exploration of collective emission (superradiant and subradiant states) as a function of lattice detuning.

The platform allows optimized paper of photon-mediated long-range interactions, the realization of quantum optical mirrors, and scalable integration into larger photonic architectures.

7. Experimental Performance, Stability, and Outlook

Demonstrated trapping of approximately 1300 atoms with lifetimes of $\sim$15 ms in a variable-period lattice establishes the practical viability. The architecture is robust against changes in the lattice period, capable of preserving both the tight atom–fiber coupling and three-dimensional motional confinement.

Limitations include finite trap lifetimes due to heating and light scattering, as well as possible effects from inhomogeneities in the trapping potential introduced by surface roughness or imperfections in the fiber or projection system.

Future extensions suggest straightforward adaptation to new atomic species, integration with photonic bandgap structures, and implementation of tailored, time-dependent interactions for quantum simulation. The engineered tunable lattice–nanofiber platform uniquely supports scalable quantum networks, collective radiative phenomena, and hybrid interfaces marrying waveguide QED with atom- and photon-based quantum information processing (Han et al., 26 Sep 2025).