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arXiv:2312.11196 · Optica

Extending the coherence time limit of a single-alkali-atom qubit by suppressing phonon-jumping-induced decoherence

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

Published Sep 12, 2024• 12 citations

Abstract

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.

DOI: 10.1364/optica.529577

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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 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.

Abstract We analyse in detail various multi-level Rydberg excitation schemes which can be used to suppress motional dephasing in optically trapped Cesium atom qubits in the context of quantum information processing. To explore the dephasing mechanism on a quantitative level, we analyse the time evolution of the atom’s wavefunction, under the application of a composite pulse sequence ( π − 4 π − π ), taking into account controllable experimental parameters like the Rabi frequency and temperature. Our study determines the characteristics of a three level system that can be used with this pulse sequence for eliminating motional dephasing. Such systems can hence be directly employed in state-of-the-art quantum simulation and quantum computation experiments to reduce the trap induced decoherence. This paves the way towards the realization of large scale quantum information processing architecture using optically trapped alkali atom qubits.

We experimentally investigate the coherence properties of a qubit stored in the Zeeman substates of the

{5}^{\phantom{\rule{0.16em}{0ex}}2}{S}_{1/2},\phantom{\rule{0.222222em}{0ex}}F=1

hyperfine ground level of a single optically trapped

{}^{87}

Rb atom. Larmor precession of a single atomic spin-1 system is observed by preparing the atom in a defined initial spin state and then measuring the resulting state after a programmable period of free evolution. Additionally, by performing quantum-state tomography, maximum knowledge about the spin coherence is gathered. By using an active magnetic field stabilization and without application of a magnetic guiding field, we achieve transverse and longitudinal dephasing times of ${T}_{2}^{*}=75\ensuremath{-}150\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ and

{T}_{1}>0.5\phantom{\rule{0.28em}{0ex}}\mathrm{ms},

respectively. We derive the light-shift distribution of a single atom in the approximately harmonic potential of a dipole trap and show that the measured atomic spin coherence is limited mainly by residual position- and state-dependent effects in the optical trapping potential. The improved understanding enables longer coherence times, an important prerequisite for future applications in long-distance quantum communication and computation with atoms in optical lattices, or for a loophole-free test of Bell's inequality.

As an important candidate for quantum simulation and quantum computation, a microscopic array of single atoms confined in optical dipole traps is advantageous in controlled interaction, long coherence time, and scalability of providing thousands of qubits in a small footprint of less than 1 mm2. Recently, several breakthroughs have greatly advanced the applications of neutral atom system in quantum simulation and quantum computation, such as atom-by-atom assembling of defect-free arbitrary atomic arrays, single qubit addressing and manipulating in two-dimensional and three-dimensional arrays, extending coherence time of atomic qubits, controlled-NOT (C-NOT) gate based on Rydberg interactions, high fidelity readout, etc.In this paper, the experimental progress of quantum computation based on trapped single neutral atoms is reviewed, along with two contributions done by single atom group in Wuhan Institute of Physics and Mathematics of Chinese Academy of Sciences. First, a magic-intensity trapping technique is developed and used to mitigate the detrimental decoherence effects which are induced by light shift and substantially enhance the coherence time to 225 ms which is 100 times as large as our previous coherence time thus amplifying the ratio between coherence time and single qubit operation time to 105. Second, the difference in resonant frequency between the two atoms of different isotopes is used to avoid crosstalking between individually addressing and manipulating nearby atoms. Based on this heteronuclear single atom system, the heteronuclear C-NOT quantum gate and entanglement of an Rb-85 atom and an Rb-87 atom are demonstrated via Rydberg blockade for the first time. These results will trigger the quests for new protocols and schemes to use the double species for quantum computation with neutral atoms. In the end, the challenge and outlook for further developing the neutral atom system in quantum simulation and quantum computation are also reviewed.

Abstract Quantum states with long-lived coherence are essential for quantum computation, simulation and metrology. The nuclear spin states of ultracold molecules prepared in the singlet rovibrational ground state are an excellent candidate for encoding and storing quantum information. However, it is important to understand all sources of decoherence for these qubits, and then eliminate them, in order to reach the longest possible coherence times. Here, we fully characterise the dominant mechanisms for decoherence of a storage qubit in an optically trapped ultracold gas of RbCs molecules using high-resolution Ramsey spectroscopy. Guided by a detailed understanding of the hyperfine structure of the molecule, we tune the magnetic field to where a pair of hyperfine states have the same magnetic moment. These states form a qubit, which is insensitive to variations in magnetic field. Our experiments reveal an unexpected differential tensor light shift between the states, caused by weak mixing of rotational states. We demonstrate how this light shift can be eliminated by setting the angle between the linearly polarised trap light and the applied magnetic field to a magic angle of arccos(1/√3)≈55°. This leads to a coherence time exceeding 6.9 s (90% confidence level). Our results unlock the potential of ultracold molecules as a platform for quantum computation.

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.