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arXiv:1405.5571 · Physical Review A

Two-mode coupling in a single-ion oscillator via parametric resonance

Dylan J Gorman, Philipp Schindler, Sankaranarayanan Selvarajan, Nikos Daniilidis, Hartmut Häffner

Published Jun 27, 2014• 26 citations
arXivHTMLPDF

Abstract

Atomic ions, confined in radio-frequency Paul ion traps, are a promising candidate to host a future quantum information processor. In this article, we demonstrate a method to couple two motional modes of a single trapped ion, where the coupling mechanism is based on applying electric fields rather than coupling the ion's motion to a light field. This reduces the design constraints on the experimental apparatus considerably. As an application of this mechanism, we cool a motional mode close to its ground state without accessing it optically. As a next step, we apply this technique to measure the mode's heating rate, a crucial parameter determining the trap quality.

DOI
10.1103/physreva.89.062332

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Atomic ions, confined in radio-frequency Paul ion traps, are a promising candidate to host a future quantum information processor. In this article, we demonstrate a method to couple two motional modes of a single trapped ion, where the coupling mechanism is based on applying electric fields rather than coupling the ion's motion to a light field. This reduces the design constraints on the experimental apparatus considerably. As an application of this mechanism, we cool a motional mode close to its ground state without accessing it optically. As a next step, we apply this technique to measure the mode's heating rate, a crucial parameter determining the trap quality.
In this work, we present a novel method to couple any two vibrational modes of a single trapped ion, allowing energy to be swapped between the two modes. We use the scheme to perform ground state cooling and heating rate measurements of vibrational modes without direct optical access. This lessens experimental design constraints in trapped ion experi- ments, particularly in surface trap apparatus where optical access can be difficult.We use a single ion as an electric-field noise sensor to study noise processes originating on the metallic surfaces of microfabricated ion traps. We show that realistic models of surface noise predict a specific polarization of the electric-field fluctuations relative to the trap geometry. In contrast, technical noise sources predict a different polarization direction and magnitude which can be inferred by electrostatic simulation of the trapping electrodes. We show that, by comparing heating rates of the two radial modes of a single trapped ion, one can determine whether technical noise sources are a significant contribution to heating. This is an important test for experiments aimed at studying surface noise effects. We also study dephasing due to surface noise, in which the electric potential curvature due to surface noise sources disturbs the phase of the ion motion. We measure the dephasing time for trapped ion motion. Using a noise model featuring dipolar noise sources, we probe the power spectrum of surface noise effects. These measurements, especially if repeated in a trap with smaller ion-electrode distances, may yield new insights as to the physical origin of surface noise effects.We demonstrate a two-ion quantum simulation of vibrationally-assisted energy transfer, an important phenomenon in biochemical energy transfer. We show that the quantum simulator performs well when benchmarked against exact numerical simulation. We believe that our approach can be scaled to more complicated systems beyond the reach of classical simulation, and discuss several methods for extending the simulation.
We investigate high frequency motional states of trapped atomic ions. Trapped ions in rf traps are confined by an approximate harmonic potential and exhibit quantum motional states that mediate essential techniques in quantum computing, simulation, networking, and precision measurement. However, motional state decoherence mechanisms, heating and dephasing, are broadly limiting: reduced two-qubit gate fidelities; lower fidelity and lifetime of highly nonclassical bosonic states; long laser cooling times; and large recoil heating rates. These also challenge the scalability of increasingly sophisticated protocols. We propose high motional frequency ion trapping as an operating regime that addresses these challenges and reshapes the design landscape for quantum information experiments and quantum control techniques. We report an experimentally motivated investigation of realizing this high-frequency regime and discuss the consequences for laser cooling, motional state coherence, fidelity and lifetime of nonclassical bosonic states, and scalability of experimental runtimes. We report clear design trajectories for ion traps to reach high motional frequency, a new limiting mechanism for laser cooling at these high frequencies, and more than an order-of-magnitude speedup in experimental duty cycles with larger speed ups possible for quantum error correction protocols. Taken together, high motional frequency ion trapping has broad implications for the future of quantum information experiments.
We present a new method of spin-motion coupling for trapped ions using microwaves and a magnetic field gradient oscillating close to the ions' motional frequency. We demonstrate and characterize this coupling experimentally using a single ion in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave field and the oscillating magnetic field gradient. Using this method, we perform resolved-sideband cooling of a single motional mode to its ground state.
We report on ground-state laser cooling of single 111Cd+^{111}\mathrm{Cd}^{+}111Cd+ ions confined in radio-frequency (Paul) traps. Heating rates of trapped ion motion are measured for two different trapping geometries and electrode materials, where no effort was made to shield the electrodes from the atomic Cd\mathrm{Cd}Cd source. The low measured heating rates suggest that trapped 111Cd+^{111}\mathrm{Cd}^{+}111Cd+ ions may be well suited for experiments involving quantum control of atomic motion, including applications in quantum information science.
We present two methods for characterization of motional-mode configurations that are generally applicable to the weak- and strong-binding limit of single or multiple trapped atomic ions. Our methods are essential to realize control of the individual as well as the common motional degrees of freedom. In particular, when implementing scalable radio-frequency trap architectures with decreasing ion-electrode distances, local curvatures of electric potentials need to be measured and adjusted precisely, e.g., to tune phonon tunneling and control effective spin-spin interaction. We demonstrate both methods using single 25Mg+^{25}\text{Mg}^{+}25Mg+ ions that are individually confined $40\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ above a surface-electrode trap array and prepared close to the ground state of motion in three dimensions.
Cooling the motion of trapped ions to near the quantum ground state is crucial for many applications in quantum information processing and quantum metrology. However, certain motional modes of trapped-ion crystals can be difficult to cool due to weak or zero interaction between the modes and the cooling radiation, typically laser beams. We overcome this challenge by coupling a mode with weak cooling radiation interaction to one with strong cooling radiation interaction using parametric modulation of the trapping potential, thereby enabling indirect cooling of the former. In this way, we demonstrate near-ground-state cooling of motional modes with weak or zero cooling radiation interaction in multi-ion crystals of the same and mixed ion species, specifically 9^99Be+^++-9^99Be+^++, 9^99Be+^++-25^{25}25Mg+^++, and 9^99Be+^++-25^{25}25Mg+^++-9^99Be+^++ crystals. This approach can be generally applied to any Coulomb crystal where certain motional modes cannot be directly cooled efficiently, including crystals containing molecular ions, highly-charged ions, charged fundamental particles, or charged macroscopic objects.