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Single mode fibre based interaction of trapped ions enhanced by optical cavities

Dean T. Clarke

Published Sep 12, 2019• 0 citations
OpenAlex

Abstract

Quantum technology has the potential to change the way we solve many problems in our daily lives. Perhaps the most ambitious quantum technology in development is quantum computers. In the past decade significant progress has been made in the number of qubits within a quantum computer, and this trend seems to be increasing. In this work we look at one particular architecture for quantum computing; the iontrap quantum computer. In this thesis we investigate the integration of this type of quantum computer with fibre tip optical cavities. We numerically analyse the effect of misalignment of these cavities and attempt to find the optimal cavity parameters for integration into the ion-trap quantum computer. This work should provide a useful framework for optimal integration of cavities, and the tolerances to misalignment. This should allow for future ion-trap quantum computer designs to be more efficient and help increase the number of qubits, paving the way to develop a useful universal quantum computer.

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Alignment requirements of Fabry-Perot microresonators for ion trap quantum information processing (Conference Presentation)

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•Jun 16, 2013•0 citations
Quantum technology has the potential to change the way we solve many problems in our daily lives. Perhaps the most ambitious quantum technology in development is quantum computers. In the past decade significant progress has been made in the number of qubits within a quantum computer, and this trend seems to be increasing. In this work we look at one particular architecture for quantum computing; the iontrap quantum computer. In this thesis we investigate the integration of this type of quantum computer with fibre tip optical cavities. We numerically analyse the effect of misalignment of these cavities and attempt to find the optimal cavity parameters for integration into the ion-trap quantum computer. This work should provide a useful framework for optimal integration of cavities, and the tolerances to misalignment. This should allow for future ion-trap quantum computer designs to be more efficient and help increase the number of qubits, paving the way to develop a useful universal quantum computer.
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 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.
The coupling of a single ion to an optical cavity is a promising route towards scalable quantum technologies. This study presents the design, initial construction and simulations of two different ion traps for quantum computation with novel capabilities. The first system is an end cap system that utilises Fabry Perot cavities formed by the facets of fibres. The small mode volumes (cavity length ~ 300 µm) of the fibre cavities are ideal to achieve high cooperativity and coupling strength. This trap features the novel ability to mechanically adjust the position of the ion by perturbing/distorting the trapping potential with grounded side electrodes. A capability crucial to maximising the ion-cavity coupling. The design incorporates a number of improvements over the previous iteration by Takahashi et al. such as increased optical access, increased mode matching and improved mechanical stability of the cavity. To verify and test the design modifications; Pound-Drever-Hall cavity locking was set up. The test set up failed to lock to an error signal due to a mechanically unstable translation stage. The other design presented is a linear micro trap intended to trap strings of ions and does not use fibre based cavities. This design features multiple different regions reserved for storage, computation and communication and aims to implement a shuttling scheme that will allow the selective loading of single ions from a reservoir of ions into the desired regions. The fabrication process would utilise methods in microfabrication. Simulations of the trapping dynamics were used to optimise the electrode geometries and splitting protocol.
Combining optical cavities with trapped atomic ions is a powerful tool to investigate the interaction of light with matter on the most fundamental scale: a single atom with a single particle of light. Recent advances have not only demonstrated the emission of single photons, ion-photon entanglement and ion-ion entanglement but paved the way to use these systems in modern quantum technologies. Ion-cavity systems are now one of the most promising technologies for realising a scalable quantum computer and quantum networks. While the integration of optical cavities and ion traps is challenging, new techniques and technologies made it possible to demonstrate important building blocks for such systems.
Recent experimental progress in quantum information processing with trapped ions have demonstrated most of the fundamental elements required to realize a scalable quantum computer. The next set of challenges lie in realization of a large number of qubits and the means to prepare, manipulate and measure them, leading to error-protected qubits and fault tolerant architectures. The integration of qubits necessarily require integrated optical approach as most of these operations involve interaction with photons. In this paper, we discuss integrated optics technologies and concrete optical designs needed for the physical realization of scalable quantum computer.
The controlled emission and absorption of single photons is an important enabling technology in the fields of quantum communication, cryptography and computing. We have realised a novel single photon source, based on a miniature ‘endcap’ ion trap with integrated optical fibres. To minimise distortion of the trapping field the fibres are tightly integrated and recessed within the two hollow cylindrical rf electrodes of the trap. This allows us to bring the fibres to within approximately 300 µm of the trapped ion. With the fibres in place we are able to collect the ion’s fluorescence using no further optics. In this thesis the ion trap is fully characterised, and the quantum nature of the light demonstrated in the results of a variant on the Hanbury Brown-Twiss photon-correlation experiment. The scheme will ultimately be extended to implement a coherent ion-photon interface through strong coupling cavity-QED. Towards this end, an ultra-high-finesse cavity has been designed and fabricated by laser-machining and coating surfaces with a range of radii of curvature on the end facets of the fibres. To improve the stability and precision manipulation of the cavity in-vacuum, an entirely new trap has been designed and built. Finally, the current status of the fibre cavity and the outlook for the experiment are presented.