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Fibre-tip microcavities for enhanced ion-photon coupling in scalable quantum information networks

Péter Horák, Nina Podoliak, Hiroki Takahashi, Matthias Keller

Published Sep 20, 2016• 0 citations
OpenAlex

Abstract

Scalable quantum information processing requires efficient coupling between stationary qubits (trapped atoms, ions, NV centres, etc.) via “flying” single-photon qubits. Here we investigate the optimisation and integration of fibre-tip microcavities into radiofrequency ion traps to achieve (i) high-fidelity transfer of ion qubits onto single photons, (ii) low-loss coupling of cavity photons into single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap. Our cavities are fabricated by laser micro-machining the ends of optical fibres and subsequent coating with a thin dielectric mirror, giving rise to cavity mirrors with a Gaussian profile. These cavities allow for small mode volume and thus large ion-photon coupling fulfilling requirement (i), while simultaneously providing intrinsic coupling to optical fibres, requirement (ii), as long as mode matching between the cavity mode and the fibre mode can be achieved at the cavity mirror. Using a semi-analytical numerical method, we show that Gaussian-shaped mirrors can in fact be used to enhance ion-photon coupling beyond what is possible with more common spherical cavity mirrors. Ion traps are notoriously sensitive to the presence of dielectric materials such as cavity mirrors near the trap centre, preventing requirement (iii) from being achieved. We performed an extensive numerical study of five different common ion traps to find the optimum configurations for integrating fibre-tip cavities with minimised effects on the trapping potentials. We discuss these results and present simple and effective design rules for realising optimum ion-cavity based quantum networks.

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Scalable quantum information processing requires efficient coupling between stationary qubits (trapped atoms, ions, NV centres, etc.) via “flying” single-photon qubits. Here we investigate the optimisation and integration of fibre-tip microcavities into radiofrequency ion traps to achieve (i) high-fidelity transfer of ion qubits onto single photons, (ii) low-loss coupling of cavity photons into single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap. Our cavities are fabricated by laser micro-machining the ends of optical fibres and subsequent coating with a thin dielectric mirror, giving rise to cavity mirrors with a Gaussian profile. These cavities allow for small mode volume and thus large ion-photon coupling fulfilling requirement (i), while simultaneously providing intrinsic coupling to optical fibres, requirement (ii), as long as mode matching between the cavity mode and the fibre mode can be achieved at the cavity mirror. Using a semi-analytical numerical method, we show that Gaussian-shaped mirrors can in fact be used to enhance ion-photon coupling beyond what is possible with more common spherical cavity mirrors. Ion traps are notoriously sensitive to the presence of dielectric materials such as cavity mirrors near the trap centre, preventing requirement (iii) from being achieved. We performed an extensive numerical study of five different common ion traps to find the optimum configurations for integrating fibre-tip cavities with minimised effects on the trapping potentials. We discuss these results and present simple and effective design rules for realising optimum ion-cavity based quantum networks.
In many quantum information applications quantum bits (qubits) are stored in single trapped atoms or ions. In particular, single ions held in radiofrequency electromagnetic traps are preferred because of their long achievable decoherence time and record-breaking quantum gate fidelity [1]. However, for scalable quantum information processing many such stationary qubits need to be connected. A preferred way of doing this would be via optical fibres, i.e., by transferring quantum information between stationary qubits with the help of “flying” single-photon qubits. Efficient operation of the quantum network thus requires (i) high-fidelity transfer of an ion qubit onto a single photon, (ii) low-loss coupling of the photon into a single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap.
Constructing a quantum network comprising of matter-based stationary qubits and photonic flying qubits is a key goal towards scalable quantum computing. Individually trapped neutral atoms are amongst the most promising candidates for stationary qubits, while single photons emitted from optical cavities appears to be the best approach to forming flying qubits. This thesis presents a fast and versatile method to trap and transport an array of neutral atoms, with the aid of a spatial light modulator (SLM). We demonstrate trapped 87Rb atoms adapting to the arbitrary and dynamic potential landscapes imposed by the SLM. On top of this we also investigate the inclusion of fibre-tip cavities into the experiment. In these microcavities, the mirrors are replaced by ablated fibre-tip faces, coated in a highly reflective dielectric stack. These have the benefit of small geometries, increasing their scalability; which in turns leads to extremely small mode volumes, allowing for strong coupling with atoms inside the cavity modes. We construct and characterise a high finesse ℱ = 120, 000 fibre-tip cavity constructed from two single mode fibres.
In recent years there has been rapid progress into realising a working universal quantum computer, in particular with the development of chip-based radio frequency ion traps. The next significant leap will come with successfully integrating optical cavities into these ion traps to allow for interaction between remote ions via photons as required for more efficient and scalable quantum networking schemes. Fibre-tip cavities are especially interesting for such applications as they enable highly efficient coupling of photons from the cavity into optical fibres for onward transmission. Here we analyse theoretically and numerically the effects of parallel off-axial misalignment in millimetre scale optical Fabry-Perot cavities. While near-concentric cavity configurations produce the smallest mode waist and thus strongest coupling to a trapped ion, their mode is extremely sensitive to misalignment. Shorter cavities exhibit more robust modes, but at the cost of larger mode waists. For example, for typical experimental parameters (mirror radius of curvature 0.7 mm, mirror diameter 0.140 mm, operation wavelength 850 nm) we find that the cavity lifetime is reduced by a factor 1/e for a misalignment of 0.95 nm for a beam waist of 2.91 um (cavity length of 1.397 mm), which increases to 11.0 nm for a waist of 4.33 um (length of 1.386 mm), and 3.12 um for a waist of 7.38 um (length of 1.273 mm). In the parameter regimes of interest, we derive a simple relation between cavity length, mirror radius, and misalignment sensitivity. Finally, we also consider the effect of mode matching of the misaligned cavity mode with the optical mode of the fibre for efficient cavity to fibre coupling. In conclusion, our model allows us to optimise photon-ion coupling in fibre-tip resonators for quantum information processing in the presence of finite fabrication and alignment tolerances.
We investigate geometries for efficient coupling of single ions to fiber-coupled light fields for applications in quantum sensing, quantum metrology, and quantum information processing. Specifically, we discuss the integration of fiber-tip microcavities into radio-frequency ion traps. The distortions of the trapping fields induced by the presence of the optical fibers are simulated for a range of ion trap geometries and the most promising arrangements are identified. Finally, we investigate the use of fiber-tip microcavities with non-spherical mirrors for enhanced ion-light coupling at the center of the trap by appropriate shaping of the cavity modes.
We study a range of radio-frequency ion trap geometries and investigate the effect of integrating dielectric cavity mirrors on their trapping potential using numerical modelling. We compare five different ion trap geometries with the aim to identify ion trap and cavity configurations that are best suited for achieving small cavity volumes and thus large ion-photon coupling as required for scalable quantum information networks. In particular, we investigate the trapping potential distortions caused by the dielectric material of the cavity mirrors in all 3 dimensions for different mirror orientations with respect to the trapping electrodes. We also analyze the effect of the mirror material properties such as dielectric constants and surface conductivity, and study the effect of surface charges on the mirrors. As well as perfectly symmetric systems, we also consider traps with optical cavities that are not centrally aligned where we find a spatial displacement of the trap centre and asymmetry of the resulting trap only at certain cavity orientations. The best trapcavity configurations with the smallest trapping potential distortions are those where the cavities are aligned along the major symmetry axis of the electrode geometries. These cavity configurations also appear to be the most stable with respect to any mirror misalignment. Although we consider particular trap sizes in our study, the presented results can be easily generalized and scaled to different trap dimensions.