Quantum nanofiber traps hold 4000 atoms, nearing blockade limit

Researchers are now stably trapping approximately 4000 cesium atoms using quantum nanofibers, approaching what is known as the collisional blockade regime and enabling more complex quantum systems. This achievement overcomes a key challenge in the field: efficiently loading atoms into traps a few orders of magnitude smaller than the wavelength of light.

The team employed Λ-enhanced gray molasses (ΛGM) cooling, reaching temperatures of 1μK for cesium atom clouds and demonstrating a sixfold increase in trapped atoms compared to standard loading methods. This technique also extends atom storage time fivefold, even in very shallow traps with a depth of only 24μK.

ΛGM Cooling Substantially Enhances Nanofiber Trap Atom Loading

Maintaining atomic coherence within nanofiber-based traps is now demonstrably improved by a technique that preserves atom-photon coupling strength over extended storage times. Measurements reveal that the application of Λ-enhanced gray molasses (ΛGM) cooling prevents the heating of trapped cesium atoms, a phenomenon that degrades the interaction between the atoms and the guiding nanofiber. Without ΛGM cooling, the atom-photon coupling strength, β, steadily decreases as atoms heat within the trap; however, with ΛGM active during storage, β remains constant, indicating stable temperature maintenance immediately following the initial loading phase.

The ability to stabilize atom temperature is particularly important given the shallow nature of the optical traps used in this work, with a depth of only 24μK. Standard repumping gray molasses cooling (RPGC) proved ineffective in these shallow traps, highlighting the advantage of the ΛGM approach for this specific geometry.

This cooling method relies on measuring the optical energy needed to prepare an ensemble of atoms in a ground state, using a laser resonant with the D2 transition. The team then uses these measurements to estimate the atom-photon coupling strength, β, defined as the probability of an excited atom decaying by emitting a photon into a waveguide. This preservation of coupling strength directly impacts the potential for high-fidelity quantum operations.

The experiment successfully trapped approximately 4000 individual cesium atoms, approaching what researchers term the collisional blockade regime where interactions between atoms limit the number that can be effectively trapped. Achieving this level of occupation is critical for building more complex quantum systems, as it allows for strong interactions between individual atoms and the guided light modes of the nanofiber. The observed flat-topped atom distribution, with a maximum filling factor of about 50%, confirms operation within this collisional blockade regime.

The researchers employed fiber-guided light fields to both trap and cool the atoms. A red-detuned light field creates the primary trap in a standing wave configuration, while a blue-detuned field, also fiber-guided, is used for the ΛGM cooling process. The probe light used to monitor the atoms is detected using a single-photon counting module (SPCM). This configuration allows for precise control over the atomic ensemble and enables the observation of subtle changes in their motional state.

The cooling mechanism itself is based on the principle of electromagnetically induced transparency (EIT). The ensemble becomes transparent to the probe field for transitions that do not alter the atom’s motion, while absorption is maximized for transitions that remove one quantum of motion. By carefully controlling the light fields, the team effectively removes energy from the atoms, counteracting the heating effects of the trap.

Atom loss in these nanofiber traps is driven by heating in the radial direction. The radially decaying intensity profile of the guided light field, however, allows for the detection of radial motional sidebands, providing a sensitive probe of the atoms’ motion. This detailed understanding of the atomic dynamics is important for optimizing trap parameters and maximizing storage times.

Two-Color Dipole Trap Configuration for Cesium Atoms

Efficiently loading atoms into nanoscale optical traps remains a central challenge for building complex quantum systems, but recent work demonstrates a significant improvement using a two-color dipole trap configuration. By combining red- and blue-detuned light fields at wavelengths of and, researchers created traps with a calculated depth of 24μK located approximately 270 nm from the nanofiber surface. This configuration allows for the stable trapping of atoms, approaching what is known as the collisional blockade regime where each trapping site can hold at most one atom due to interatomic interactions.

The experimental setup begins with a three-dimensional magneto-optical trap and molasses configuration using σ⁺ and σ⁻ polarized counter-propagating light fields to prepare a cloud of cold cesium atoms. These atoms are then guided toward a 225-nm-waist-radius nanofiber, where the two-color dipole trap is applied.

Nanofiber Radius and Shallow Trap Depth Specifications

The optical nanofiber used in these experiments possesses a nominal radius of 225 nanometers, a dimension critical for establishing the evanescent field that traps cesium atoms. This precise radius, achieved through tapering an optical fiber, dictates the strength of the light-atom interaction and the resulting trap depth, influencing the number of atoms that can be stably held. Achieving stable trapping of approximately 4000 atoms demonstrates a near approach to the collisional blockade regime, a threshold where atom-atom interactions become significant and limit further loading.

The nanofiber’s geometry, therefore, is not merely a structural detail but a fundamental parameter governing the system’s capacity for building complex quantum systems. For all results in this paper, the red-detuned trapping field is set to a power of and is launched from both ends of the nanofiber to create a standing-wave dipole trap.

The careful selection of wavelengths and power levels minimizes unwanted atomic heating and maximizes the efficiency of atom capture near the nanofiber surface. The shallow trap depth of only 24μK presents a significant challenge, rendering conventional rapid polarization-gradient cooling (RPGC) ineffective at extending atom storage times. However, the implementation of EIT-assisted cooling, with guided beams totaling a few hundred pW, overcomes this limitation.

This approach yields a fivefold increase in the atom storage time and allows us to preserve the comparatively high atom-photon interaction strength observed immediately after loading over very long storage times. While the current trap depth of 24μK is insufficient to explore regimes requiring U≳ℏΓ_D1, where U is the trap depth and ${\Gamma }_{D}1$ is the natural linewidth of the D1 transition, the demonstrated principles clear the way for future investigations into collective radiative phenomena.

Implementing ΛGM in nanophotonic cold-atom interfaces could be important for investigating collective radiative phenomena that require high filling factors, such as selective radiance. This work establishes a foundation for exploring more complex quantum systems and manipulating light-matter interactions at the nanoscale.

Optical Depth Measurement Reveals 4000-Atom Capture

This achievement relies on EIT-assisted cooling implemented with the same optical beams used for Λ-enhanced gray molasses (ΛGM) cooling, a technique particularly effective in the extremely shallow traps currently employed. The trap depth, calculated at 24μK, proved insufficient for conventional rapid polarization-gradient cooling (RPGC), highlighting the efficacy of the alternative approach. The two-photon detuning, experimentally controlled by varying ${\Delta }_{1}P,P$ is a key parameter in the cooling process.

Following 150 milliseconds of ΛGM cooling, the atomic temperature was found to be 1. 9±0. 1μK, as detailed in the Supplemental Material. This precise temperature control is essential for maximizing atom occupancy and minimizing losses from the shallow traps. The team estimates the total number of trapped atoms using a technique previously described in Ref., allowing for a precise determination of the local filling factor within the ensemble.

Beyond a certain point, no further increase in the number of loaded single atoms occurs; increasing the size of the atomic cloud would be necessary to achieve an even larger optical depth. The team determined the Rabi frequency ${\Omega }_{1}P,EIT,C$ at the maximum optical density, depicted by green data points finding that the data fit a prediction for a trap frequency of 46±2kHz.

Calculated trap frequencies in the experiment are 55 kHz radially, 32 kHz azimuthally, and 81 kHz axially. The overall rate of EIT cooling, the researchers explain, is determined by how rapidly the dark state changes as the atom moves, or more quantitatively, by the position-dependence of the ratio between the Rabi frequencies of the control and probe field.

EIT-Cooling Preserves Atom-Photon Interaction Strength

Maintaining strong interaction between atoms and photons is essential for building complex quantum systems, and a recent advance demonstrates a method for preserving this interaction even when atoms are held in exceptionally shallow traps. Experiments reveal that EIT-assisted cooling not only extends atom storage time fivefold, but also maintains the initial atom-photon coupling strength over extended periods, a critical factor for sustained quantum operations. The team employed a sophisticated cooling sequence beginning with a standard magneto-optical trap, followed by Λ-enhanced gray molasses (ΛGM) cooling and then EIT-assisted cooling using fiber-guided light.

This approach uses the unique properties of a Λ system, where two ground states are coupled to an excited state, to achieve cooling via the D1 transition. Specifically, the researchers used guided beams to perform EIT-cooling on atoms trapped near the nanofiber, a technique that proved successful in counteracting the heating effects that typically degrade atom-photon interaction.

Measurements of the atom-light coupling strength, denoted as β, confirmed this preservation; a non-cooled ensemble exhibited a decreasing β over time, while the EIT-cooled ensemble maintained its initial value of approximately 1%. Analysis of optical density (OD) measurements further supports these findings.

The team observed an extended storage time, reaching a time constant of when EIT-cooling was applied, compared to a passive storage time of without cooling, and a slight increase in OD when the cooling beams were activated. This suggests that EIT-cooling actively recovers the initial coupling strength by reducing atomic motion within the shallow traps.

This preservation is important for enabling long-duration quantum operations and exploring collective radiative phenomena within these nanophotonic cold-atom interfaces. The work demonstrates a pathway toward realizing more complex quantum systems by overcoming a key challenge in efficiently loading and maintaining atoms in extremely small, shallow traps, and opens new avenues for investigating collective radiative effects.

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