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arXiv:2203.00877 · Journal of Physics B Atomic Molecular and Optical Physics

Chiral-coupling-assisted refrigeration in trapped ions

Chi‐Chih Chen, Yi-Cheng Wang, Chun-Che Wang, H. H. Jen

Published Mar 23, 2023• 5 citations
arXivHTMLPDF

Abstract

Abstract Trapped ions can be cooled close to their motional ground state, which is imperative in implementing quantum computation and quantum simulation. Here, we theoretically investigate the capability of light-mediated chiral couplings between ions to enable a superior cooling scheme exceeding the single-ion limit of sideband cooling. Under asymmetric driving, the target ion manifests the chiral-coupling-assisted refrigeration at the price of heating others, where its steady-state phonon occupation outperforms the lower bound set by a single ion. We further explore the optimal operation conditions for the refrigeration where a faster rate of cooling can still be sustained. Under an additional nonguided decay channel, a broader parameter regime emerges to support the superior cooling and carries over into the reciprocal coupling, suppressing the heating effect instead. Our results present a tunable resource of collective chiral couplings which can help surpass the bottleneck of the cooling procedure and open up new possibilities in applications of trapped-ion-based quantum computation and simulation.

DOI
10.1088/1361-6455/acc709

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Abstract Trapped ions can be cooled close to their motional ground state, which is imperative in implementing quantum computation and quantum simulation. Here, we theoretically investigate the capability of light-mediated chiral couplings between ions to enable a superior cooling scheme exceeding the single-ion limit of sideband cooling. Under asymmetric driving, the target ion manifests the chiral-coupling-assisted refrigeration at the price of heating others, where its steady-state phonon occupation outperforms the lower bound set by a single ion. We further explore the optimal operation conditions for the refrigeration where a faster rate of cooling can still be sustained. Under an additional nonguided decay channel, a broader parameter regime emerges to support the superior cooling and carries over into the reciprocal coupling, suppressing the heating effect instead. Our results present a tunable resource of collective chiral couplings which can help surpass the bottleneck of the cooling procedure and open up new possibilities in applications of trapped-ion-based quantum computation and simulation.
Cooling down a trapped ion into its motional ground state is a central step for trapped-ion-based quantum information processing. State-of-the-art cooling schemes often work under a set of optimal cooling conditions derived analytically using a perturbative approach, in which the sideband coupling is assumed to be the weakest of all the relevant transitions. As a result, the cooling rate is severely limited. Here we propose to use a quantum control technique powered with automatic differentiation to speed up the classical cooling schemes. We demonstrate the efficacy of our approach by applying it to find the optimal cooling conditions for classical sideband cooling and electromagnetically induced transparency cooling schemes, which are in general beyond the weak-sideband-coupling regime. Based on those numerically found optimal cooling conditions, we show that faster cooling can be achieved while at the same time a low average phonon occupation can be retained.
Abstract Cooling the trapped atoms toward their motional ground states is key to applications of quantum simulation and quantum computation. By utilizing nonreciprocal couplings between two atoms, we present an intriguing dark-state cooling scheme in Λ-type three-level structure, which is shown superior than the conventional electromagnetically-induced-transparency cooling in a single atom. The effective nonreciprocal couplings can be facilitated either by an atom–waveguide interface or a free-space photonic quantum link. By tailoring system parameters allowed in dark-state cooling, we identify the parameter regions of better cooling performance with an enhanced cooling rate. We further demonstrate a mapping to the dark-state sideband cooling under asymmetric laser driving fields, which shows a distinct heat transfer and promises an outperforming dark-state sideband cooling assisted by collective spin–exchange interactions.
The quest for a quantum system best satisfying the stringent requirements of a quantum information processor has made tremendous progress in many fields of physics. In the last decade, trapped ions have been established as one of the most promising architectures to accomplish the task. Internal states of an ion which can have extremely long coherence time can be used to store a quantum bit, and therefore allow many gate operations before the coherence is lost. Entanglement between multiple ions can be established via Coulomb interactions mediated by appropriate laser fields. Entangling schemes usually require the ions to be initialized to near their motional ground state. The interaction of fluctuating electric fields with the motional state of the ion leads to heating and thus to decoherence for entanglement generation limiting the fidelity of quantum logic gates. Effective ground state cooling of trapped ion motion and suppression of motional heating are thus crucial to many applications of trapped ions in quantum information science. In this thesis, I describe the implementation and study of several components of a Cadmium-ion-based quantum information processor, with special emphasis on the control and decoherence of trapped ion motion. I first discuss the building and design of various ion traps that were used in this work. I also report on the use of ultrafast laser pulses to photoionize and load cadmium ions in a variety of rf Paul trap geometries. A detailed analysis of the photoionization scheme is presented, along with its dependence on controlled experimental parameters. I then describe the implementation of Raman sideband cooling on a single trapped 111 Cd + ion to the ground state of motion, where a ground state population of 97% was achieved. The efficacy of this cooling technique is discussed with respect to different initial motional state distributions and its sensitivity to the presence of motional heating. I also present an experiment where the motion of a single trapped 112 Cd + ion is sympathetically cooled by directly Doppler cooling a 114 Cd + ion in the same trap. The implications of this result are relevant to the scaling of a trapped ion quantum computer, where the unwanted motion of an ion crystal can be quenched while not affecting the internal states of the qubit ions. (Abstract shortened by UMI.)
Resolved sideband cooling provides a crucial step in subrecoil cooling the trapped atoms toward their motional ground state, which is essential in atom-based quantum technologies. Here we present an enhanced dark-state sideband cooling in trapped atoms utilizing photon-mediated dipole-dipole interactions among them. By placing the atoms at the magic interparticle distances, we manifest an outperformed cooling behavior in the target atom, which surpasses the limit that a single atom permits. We further investigate various atomic configurations in a multiatom setup with a laser detuning and different light polarization angles, where multiple magic spacings can be identified and a moderate improvement in cooling performance is predicted as the number of atoms increases. Our results provide insights to subrecoil cooling of atoms with collective and light-induced long-range dipole-dipole interactions and pave the way toward implementing genuine quantum operations in multiple quantum registers.
We develop a unified many-body theory of two-photon dark-state laser cooling, the workhorse for preparing trapped ions close to their motional quantum ground state. For ions with a ΛΛΛ level structure, driven by Raman lasers, we identify an ion-number-dependent crossover between weak and strong coupling where both the cooling rate and final temperature are simultaneously optimized. We obtain simple analytic results in both extremes: In the weak coupling limit, we show a Lorentzian spin-absorption spectrum determines the cooling rate and final occupation of the motional state, which are both independent of the number of ions. We also highlight the benefit of including an additional spin dependent force in this case. In the strong coupling regime, our theory reveals the role of collective dynamics arising from phonon exchange between dark and bright states, allowing us to explain the enhancement of the cooling rate with increasing ion number. Our analytic results agree closely with exact numerical simulations and provide experimentally accessible guidelines for optimizing cooling in large ion crystals, a key step toward scalable, high-fidelity trapped-ion quantum technologies.
Abstract We report on a comparative analysis of quenched sideband cooling in trapped ions. We introduce a theoretical approach for time-efficient simulation of the temporal cooling characteristics and derive the optimal conditions providing fast laser cooling into the ion’s motional ground state. The simulations were experimentally benchmarked with a single 172Yb+ ion confined in a linear Paul trap. Sideband cooling was carried out on a narrow quadrupole transition, enhanced with an additional clear-out laser for controlling the effective linewidth of the cooling transition. Quench cooling was thus for the first time studied in the resolved sideband, intermediate and semi-classical regime. We discuss the non-thermal distribution of Fock states during laser cooling and reveal its impact on time dilation shifts in optical atomic clocks.