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Optics Letters

Suppressing the dephasing of optically trapped atoms inside a hollow-core fiber

Yitong Song, Wei Li, Xiaobin Xu, Rui Han, Chengchun Gao, Cheng Dai, Ningfang Song

Published Nov 29, 2023• 5 citations
Publisher

Abstract

Rb atoms optically trapped inside a hollow-core fiber. The differential light shift (DLS) for the clock transition caused by the trapping beam is reduced by one order of magnitude through the use of a weak compensation laser beam that is spatially mode-matched to the trapping beam. The coherence of the DLS-compensated system is characterized by microwave Ramsey interferometry, which shows Ramsey fringes with a contrast of over 0.6 at a separation time of 10 ms. The dephasing time, measured by Ramsey spectroscopy at different separation times, reaches tens of milliseconds after DLS cancellation, limited by the residual DLS caused by mode mismatching between the two laser beams. This work paves the way for compact and portable fiber-guided atom interferometers.

DOI
10.1364/ol.507090

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Rb atoms optically trapped inside a hollow-core fiber. The differential light shift (DLS) for the clock transition caused by the trapping beam is reduced by one order of magnitude through the use of a weak compensation laser beam that is spatially mode-matched to the trapping beam. The coherence of the DLS-compensated system is characterized by microwave Ramsey interferometry, which shows Ramsey fringes with a contrast of over 0.6 at a separation time of 10 ms. The dephasing time, measured by Ramsey spectroscopy at different separation times, reaches tens of milliseconds after DLS cancellation, limited by the residual DLS caused by mode mismatching between the two laser beams. This work paves the way for compact and portable fiber-guided atom interferometers.
We demonstrate a fiber-guided atom interferometer in a far-off-resonant trap (FORT) of 100 μK. The differential light shift (DLS) introduced by the FORT leads to the inhomogeneous dephasing of the tightly trapped atoms inside a hollow-core fiber. The DLS-induced dephasing is greatly suppressed in π/2-π-π/2 Doppler-insensitive interferometry. The spin coherence time is extended to 13.4 ms by optimizing the coupling of the trapping laser beam into a quasi-single-mode hollow-core anti-resonant fiber. The Doppler-sensitive interferometry shows a much shorter coherence time, indicating that the main limits to our fiber-guided atom interferometer are the wide axial velocity distribution and the irregular modes of the Raman laser beams inside the fiber. This work paves the way for portable and miniaturized quantum devices, which have advantages for inertial sensing at arbitrary orientations and in dynamic environments.
Differential light shifts (DLS) induced by optical trapping fields fundamentally limit the stability and accuracy of trapped-atom microwave clocks. We demonstrate an in-situ method to cancel DLS by simultaneously interrogating multiple spatially separated atomic ensembles at different trap intensities generated from a common light source. By operating the ensembles at set intensity ratios and performing Ramsey spectroscopy, the intensity-dependent frequency shifts are measured within each experimental cycle and extrapolated to the zero-intensity limit. This approach effectively enables shot-to-shot determination of a DLS-free frequency without requiring magic wavelengths or species-specific cancellation schemes. We validate the method for Rb atoms trapped in time-averaged potentials by introducing controlled variations of the total trap power and show that the extrapolated frequency remains insensitive to these fluctuations. The technique is general and can be extended to other systematic shifts, providing a scalable route toward improved stability and accuracy in compact trapped-atom clocks and related quantum sensors relying on optical dipole traps
Recent demonstrations of light-matter interactions with atoms and molecules confined to hollow waveguides offer great promise for ultralow-light-level applications. The use of waveguides allows for tight optical confinement over interaction lengths much greater than what could be achieved in bulk geometries. However, the combination of strong atom-photon interactions and nonuniformity of guided light modes gives rise to spectroscopic features that must be understood in order to take full advantage of the properties of such systems. We use light-induced atomic desorption to generate an optically dense Rb vapor at room temperature inside a hollow-core photonic band-gap fiber. Saturable-absorption spectroscopy and passive slow-light experiments reveal large ac Stark shifts, power broadening, and transit-time broadening, that are present in this system even at nanowatt powers.
Confining particles in hollow-core photonic crystal fibers has opened up new prospects to scale up the distance and time over which particles can be made to interact with light. However, maintaining long-lived quantum spin coherence and/or transporting it over macroscopic distances in a waveguide remain challenging. Here, we demonstrate coherent guiding of ground-state superpositions of ^{85}Rb atoms over a centimeter range and hundreds of milliseconds inside a hollow-core photonic crystal fiber. The decoherence is mainly due to dephasing from the residual differential light shift from the optical trap and the inhomogeneity of an ambient magnetic field. Our experiment establishes an important step towards a versatile platform that can lead to applications in quantum information networks and a matter wave circuit for quantum sensing.
Coherent manipulation and interaction between atoms and light are the core ingredients in modern quantum science and technology. Various applications such as quantum computation, quantum memories and quantum sensors which are promising in future technologies, are currently under exploration. However, due to the diffraction nature of light, miniaturizing quantum systems involving atom-light interactions has been highly limited. To avoid diffraction, I load cold atoms into a hollow-core fiber (HCF) and demonstrate four major experiments inside the fiber: inertia-sensitive Mach-Zehnder atom interferometer, long-lived quantum spin coherence, light storage and creation of an array of Schr\"odinger cat states. In this thesis, I show the theoretical framework of each experiment and demonstrate experimentally a cold atoms quantum laboratory realized a HCF. Cold 85Rb^{85}\text{Rb}85Rb atoms of temperature ∼\sim∼\SI{6}{\micro\kelvin} are loaded into a Kagome type photonic crystal hollow-core fiber (HCF) through an optical trapping beam coupled into the HCF. I first demonstrate an inertia-sensitive Mach-Zehnder atom interferometer using two counter-propagating Raman beams. However, the short quantum spin coherence time of ∼\sim∼\SI{100}{\micro\second} due to the differential AC stark shift from the trapping beam is a major obstacle to reaching high sensitivity of the optically trapped atom interferometer inside the HCF. By introducing a vector light shift from the dipole trap to cancel the differential scalar light shift, the quantum spin coherence time is increased to hundreds of milliseconds. The long quantum spin coherence time opens up possibilities for various quantum experiments in HCF. Here, I demonstrate light storage using atoms as the storage medium in the HCF. A maximum storage efficiency of \SI{3}{\percent} and storage time of over \SI{20}{\milli\second} is shown in this thesis. With the implementation of dynamical decoupling, the storage time is improved to ∼\sim∼\SI{100}{\milli\second}. Moreover, I create a 1D array of Schr\"odinger cats states in facilitated by an optical lattice in the HCF and study its potential for applications in quantum sensing and quantum simulation in HCF. Both odd and even cat states are validated and their coherence properties are studied. This thesis realized the state-of-the-art quantum experiments that are miniaturized into a HCF. The results signify a promise in future quantum sciences and technologies with compact fiber-based devices.
In our original paper (Altin et al 2011 New J. Phys. 13 065020), we presented the results from a Ramsey atom interferometer operating with an optically trapped sample of up to 10 6 Bose-condensed 87 Rb atoms in the m F = 0 clock states. We were unable to observe projection noise fluctuations on the interferometer output, which we attribute to the stability of our microwave oscillator and background magnetic field. Numerical simulations of the Gross–Pitaevskii equations for our system show that dephasing due to spatial dynamics driven by interparticle interactions accounts for much of the observed decay in fringe visibility at long interrogation times. The simulations show good agreement with the experimental data when additional technical decoherence is accounted for, and suggest that the clock states are indeed immiscible. With smaller samples of 5 × 10 4 atoms, we observe a coherence time of τ = 1.0 +0.5 −0.3 s.