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arXiv:2309.15580 · Physical Review A

Phase-stable traveling waves stroboscopically matched for superresolved observation of trapped-ion dynamics

Florian Hasse, Deviprasath Palani, Robin Thomm, Ulrich Warring, Tobias Schaetz

Published May 13, 2024• 6 citations
arXivHTMLPDF

Abstract

In quantum technologies, it is essential to understand and exploit the interplay of light and matter. We introduce an approach creating and maintaining the coherence of four oscillators: a global microwave reference field, a polarization-gradient traveling-wave pattern of light, and the spin and motional states of a single trapped ion. Our method employs a UV light pattern capable of achieving a gigahertz-modulation bandwidth, here demonstrated in the megahertz regime, allowing for stroboscopic tracking of dynamic changes in phase space. We achieve noise floors of $1.8(2)\phantom{\rule{0.16em}{0ex}}\text{nm}$ for position and $8(2)\phantom{\rule{4.pt}{0ex}}\text{zN}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{s}$ for momentum observables, superresolving variations on timescales $\ensuremath{\lesssim}100$ ns. The implications of our findings contribute to enhancing quantum control and metrological applications.

DOI
10.1103/physreva.109.053105

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In quantum technologies, it is essential to understand and exploit the interplay of light and matter. We introduce an approach creating and maintaining the coherence of four oscillators: a global microwave reference field, a polarization-gradient traveling-wave pattern of light, and the spin and motional states of a single trapped ion. Our method employs a UV light pattern capable of achieving a gigahertz-modulation bandwidth, here demonstrated in the megahertz regime, allowing for stroboscopic tracking of dynamic changes in phase space. We achieve noise floors of 1.8(2)nm1.8(2)\phantom{\rule{0.16em}{0ex}}\text{nm}1.8(2)nm for position and 8(2)zN\textmus8(2)\phantom{\rule{4.pt}{0ex}}\text{zN}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{s}8(2)zN\textmus for momentum observables, superresolving variations on timescales \ensuremath≲100\ensuremath{\lesssim}100\ensuremath≲100 ns. The implications of our findings contribute to enhancing quantum control and metrological applications.
Achieving control over physical systems down to individual quanta is central for advancing our understanding of nature and enabling emerging quantum technologies. Trapped-ion systems provide a powerful platform offering exceptional control over internal and external degrees of freedom and allowing the study of both fundamental and applied aspects of quantum dynamics. This thesis introduces an experimental toolbox, centered on problem-tailored modifications of Ramsey interferometry, for observing and manipulating dynamic processes with high temporal and spatial resolution, showcased in a trapped-ion quantum simulator. Using this versatile approach, we bridge quantum metrology and quantum thermodynamics – from tracking the motional wave packet evolution of a single ion in phase space to observing time-(in)dependent energy-level renormalizations in ultra-strongly coupled open systems. A central innovation of this thesis is the development of a stroboscopic measurement technique that establishes and maintains coherence among four oscillators: a global microwave reference, an ultraviolet polarization-gradient traveling-wave light field, and the spin and motional states of a single trapped ion. This architecture, spanning a phase-stable frequency range of more than nine orders of magnitude, enables superresolved, back-action-free tracking of motional wave packets in phase-space. In this way, we achieve noise floors of 1.8(2) nm for position and 8(2) zN μs for momentum on sub-100-nanosecond timescales. This precision enables quantum-enhanced diagnostics, non-classical state preparation, and fine-grained phase-space control, with broad implications for entanglement generation, quantum interferometry, and quantum sensing. In a complementary study, we exploit the trapped-ion system as a testbed for probing fundamental quantum interactions: We experimentally simulate a strongly coupled open quantum system, where dressed electronic states represent the system and a single motional mode serves as its environment. We observe time-dependent energy shifts up to 15% of the bare transition frequency, induced solely by ultra-strong system-environment coupling. These shifts, anticipated to be a general feature of all quantum platforms across all coupling regimes, are accurately described by the minimal-dissipation ansatz and interpreted as generalized, time-resolved Lamb shifts. Together, the results of this thesis extend the frontier of trapped-ion quantum simulation: The integration of Ramsey interferometry, stroboscopic techniques, and strong-coupling simulations offers a high-resolution window into diverse aspects of quantum dynamics. This work enables advanced studies – including squeezed states, analogues of relativistic effects such as Hawking radiation and the physics of the early universe, and autonomous quantum engines.
We observe the phase space trajectory of an entangled wave packet of a trapped ion with high precision. The application of a spin-dependent light force on a superposition of spin states allows for coherent splitting of the matter wave packet such that two distinct components in phase space emerge. We observe such motion with a precision of better than 9% of the wave packet extension in both momentum and position, corresponding to a 0.8 nm position resolution. We accurately study the effect of the initial ion temperature on the quantum entanglement dynamics. Furthermore, we map out the phonon distributions throughout the action of the displacement force. Our investigation shows corrections to simplified models of the system evolution. The precise knowledge of these dynamics may improve quantum gates for ion crystals and lead to entangled matter wave states with large displacements.
We demonstrate control of the absolute phase of an optical lattice with respect to a single trapped ion. The lattice is generated by off-resonant free-space laser beams, and we actively stabilize its phase by measuring its ac-Stark shift on a trapped ion. The ion is localized within the standing wave to better than 2% of its period. The locked lattice allows us to apply displacement operations via resonant optical forces with a controlled direction in phase space. Moreover, we observe the lattice-induced phase evolution of spin superposition states in order to analyze the relevant decoherence mechanisms. Finally, we employ lattice-induced phase shifts for inferring the variation of the ion position over the 157 μm range along the trap axis at accuracies of better than 6 nm.
In cold atomic systems, fast and high-resolution microscopy of individual atoms is crucial, since it can provide direct information on the dynamics and correlations of the system. Here, we demonstrate nanosecond-scale two-dimensional stroboscopic pictures of a single trapped ion beyond the optical diffraction limit, by combining the main idea of ground-state depletion microscopy with quantum-state transition control in cold atoms. We achieve a spatial resolution up to 175 nm using a NA=0.1 objective in the experiment, which represents a more than tenfold improvement compared with direct fluorescence imaging. To show the potential of this method, we apply it to observe the secular motion of the trapped ion; we demonstrate a temporal resolution up to 50 ns with a displacement detection sensitivity of 10 nm. Our method provides a powerful tool for probing particle positions, momenta, and correlations, as well as their dynamics in cold atomic systems.
Trapped ions provide a highly controlled platform for quantum sensors, clocks, simulators, and computers, all of which depend on cooling ions close to their motional ground state. Existing methods like Doppler, resolved sideband, and dark resonance cooling balance trade-offs between the final temperature and cooling rate. A traveling polarization gradient has been shown to cool multiple modes quickly and in parallel, but utilizing a stable polarization gradient can achieve lower ion energies, while also allowing tailorable light-matter interactions in the sub-wavelength regime. In this Letter, we demonstrate cooling of a trapped ion below the Doppler limit using a phase-stable polarization gradient created using trap-integrated photonic devices. At an axial frequency of 2π×1.45 MHz we achieve ⟨n⟩=1.56±0.07 in 150 μs and cooling rates of ∼0.3 quanta/μs. We examine ion dynamics under different polarization gradient phases, detunings, and intensities, showing reasonable agreement between experimental results and a multilevel model. Cooling is fast and power efficient, with lower average motional Fock state occupation when compared to simulated operation under the corresponding running wave configuration. Our results demonstrate a well-controlled test bed for studying the dynamics of multilevel atomic systems in a phase-stable polarization gradient.