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arXiv:2502.20794 · Chinese Physics Letters

Enhancing the Coherence Time of a Neutral Atom by an Optical Quartic Trap

Haobo Chang, Zhuangzhuang Tian, Xin Lv, Mengna Yang, Zhihui Wang, Qi Guo, Pengfei Yang, Pengfei Zhang, Gang Li, Tiancai Zhang

Published Sep 29, 2025• 0 citations
arXivHTMLPDF

Abstract

Abstract The coherence time of an optically trapped neutral atom is a crucial parameter for quantum technologies. We found that optical dipole traps with higher-order spatial forms inherently offer lower decoherence rates compared to those with lower-order spatial forms. We formulated the decoherence rate caused by the variance of the differential energy shift and photon jumping rate. Then, we constructed blue-detuned harmonic and quartic optical dipole traps, and experimentally investigated the coherence time of a trapped single cesium atom. Specifically, we demonstrated a significant improvement in the coherence time of a single atom by employing a quartic trap with the same characteristic trap potential and size as its harmonic counterpart—achieving an enhancement from 315 ms to 474 ms. The experimental results qualitatively verified our theory. Our approach provides a novel method to enhance the coherence time of optically trapped neutral atoms.

DOI
10.1088/0256-307x/42/11/110303

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Abstract The coherence time of an optically trapped neutral atom is a crucial parameter for quantum technologies. We found that optical dipole traps with higher-order spatial forms inherently offer lower decoherence rates compared to those with lower-order spatial forms. We formulated the decoherence rate caused by the variance of the differential energy shift and photon jumping rate. Then, we constructed blue-detuned harmonic and quartic optical dipole traps, and experimentally investigated the coherence time of a trapped single cesium atom. Specifically, we demonstrated a significant improvement in the coherence time of a single atom by employing a quartic trap with the same characteristic trap potential and size as its harmonic counterpart—achieving an enhancement from 315 ms to 474 ms. The experimental results qualitatively verified our theory. Our approach provides a novel method to enhance the coherence time of optically trapped neutral atoms.
A single neutral atom is one of the most promising candidates to encode a quantum bit (qubit). In a real experiment, a single neutral atom is always confined in a micro-sized far off-resonant optical trap (FORT). There are generally two types of traps: red-detuned trap and blue-detuned trap. We experimentally compare the qubits encoded in “clock states” of single cesium atoms confined separately in either 1064-nm red-detuned (bright) trap or 780-nm blue-detuned (dark) trap: both traps have almost the same trap depth. A longer lifetime of 117 s and a longer coherence time of about 10 ms are achieved in the dark trap. This provides a direct proof of the superiority of the dark trap over the bright trap. The measures to further improve the coherence are discussed.
In the fields of quantum metrology and quantum information processing with the system of optically trapped single neutral atoms, the coherence time of a qubit encoded in the electronic states is regarded as one of the most important parameters. A longer coherence time is always pursued for higher precision of measurement and quantum manipulation. The coherence time is usually assumed to be merely determined by the relative stability of the energy between the electronic states, and the analysis of the decoherence was conducted by treating the atom motion classically. We proposed a complete description of the decoherence of a qubit encoded in two ground electronic states of an optically trapped alkali atom by adopting a full description of the atomic wavefunction. The motional state, i.e., the phonon state, is taken into account. In addition to decoherence due to the variance of the differential light shift (DLS), a new, to our knowledge, decoherence mechanism, phonon-jumping-induced decoherence (PJID), was discovered and verified experimentally. The coherence time of a single-cesium-atom qubit can be extended to T 2 ≈20s by suppressing both the variances of DLS and PJID by trapping the atom in a blue-detuned bottle beam trap (BBT) and preparing the atom in its three-dimensional motional ground states. The coherence time is the longest for a qubit encoded in an optically trapped single alkali atom. Our work provides a deep understanding of the decoherence mechanism for single atom qubits and thus provides a new way to extend the coherence time limit. The method can be applied for other atoms and molecules, opening up new prospects for high-precision control of the quantum states of optically trapped atoms or molecules.
Abstract The fidelity of the gate operation and the coherence time of neutral atoms trapped in an optical dipole trap are figures of merit for the applications. The motion of the trapped atom is one of the key factors which influences the gate fidelity and coherence time. The motion has been considered as a classical oscillator in analyzing the influence. Here we treat the motion of the atom as a quantum oscillator. The population on the vibrational states of the atom are considered in analyzing the gate fidelity and decoherence. We show that the fidelity of a coherent rotation gate is dramatically limited by the temperature of a thermally trapped atom. We also show that the dephasing between the two hyperfine states due to the thermal motion of the atom could rephase naturally if the differential frequency shift is stable and the vibrational states do not change. The decoherence due to the fluctuations of the trap laser intensity is also discussed. Both the gate fidelity and coherence time can be dramatically enhanced by cooling the atom into vibrational ground states and/or by using a blue-detuned trap. More importantly, we propose a ‘magic’ trapping condition by preparing the atom into specific vibrational states.
We demonstrate that the coherence of a single mobile atomic qubit can be well preserved during a transfer process among different optical dipole traps (ODTs). This is a prerequisite step in realizing a large-scale neutral atom quantum information processing platform. A qubit encoded in the hyperfine manifold of an ^{87}Rb atom is dynamically extracted from the static quantum register by an auxiliary moving ODT and reinserted into the static ODT. Previous experiments were limited by decoherences induced by the differential light shifts of qubit states. Here, we apply a magic-intensity trapping technique which mitigates the detrimental effects of light shifts and substantially enhances the coherence time to 225±21 ms. The experimentally demonstrated magic trapping technique relies on the previously neglected hyperpolarizability contribution to the light shifts, which makes the light shift dependence on the trapping laser intensity parabolic. Because of the parabolic dependence, at a certain "magic" intensity, the first order sensitivity to trapping light-intensity variations over ODT volume is eliminated. We experimentally demonstrate the utility of this approach and measure hyperpolarizability for the first time. Our results pave the way for constructing scalable quantum-computing architectures with single atoms trapped in an array of magic ODTs.
This thesis presents experiments concerning the preparation and manipulation of single neutral atoms in optical traps. The experimental setup as well as the properties of the optical dipole trap are described. The examination and control of all trapping parameters and heating effects is a prerequisite for the future realization of quantum gates. A magneto-optical trap captures and cools down a few cesium atoms. By efficiently detecting their fluorescence, we are able to determine their exact number. They are then transferred without loss into a standing-wave optical dipole trap. The temperature of the atoms in this trap is measured using two methods which are devised to work with small numbers of atoms: By adiabatically lowering the trap depth, an energy-selective loss of atoms is obtained, which yields the energy distribution of the atoms in the trap. Alternatively, the temperature is inferred from the size of the cloud and from the oscillation frequencies of the atoms in the dipole trap. The oscillation frequencies are determined using resonant and parametric excitation. Various intrinsic as well as technical heating mechanisms in the dipole trap are examined experimentally and theoretically. Finally, the setup of a miniature ultra-high finesse optical resonator is presented, which will be used to couple two trapped atoms via the exchange of a photon. The optical resonance frequency of the resonator is successfully stabilized to the atomic transition by an electronic servo loop.
Single neutral atoms are interesting candidates for serving as qubits, the elementary objects of quantum information processing. We give an overview of experimental methods which allow us to prepare, detect, and manipulate individual neutral atoms with excellent efficiency in an optical dipole trap. We have measured long coherence times for atomic quantum state superpositions and identified the relevant decoherence mechanisms. As an application we have constructed a neutral atom quantum register from a string of atoms which can be selectively prepared in arbitrary quantum states.