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Diffractive Optics for Sensing and Networking with Trapped Ions

Valdis Blūms

Published Jan 18, 2021• 0 citations
Publisher

Abstract

In this thesis I present my contribution towards a quantum communication platform using trapped ions, Fresnel optics, and frequency conversion waveguides. I discuss the major components required for this platform, as well as show our high control over trapped ions. I also demonstrate that the high resolution imaging of lens-based optical interconnects enable a new way to monitor trapped ions as 3D sub-attonewton force sensors. The experiments presented here were achieved across two Yb+ ion traps.

DOI
10.25904/1912/4072

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In this thesis I present my contribution towards a quantum communication platform using trapped ions, Fresnel optics, and frequency conversion waveguides. I discuss the major components required for this platform, as well as show our high control over trapped ions. I also demonstrate that the high resolution imaging of lens-based optical interconnects enable a new way to monitor trapped ions as 3D sub-attonewton force sensors. The experiments presented here were achieved across two Yb+ ion traps.
Trapped ion quantum networking aims at distributing quantum information between distant parties. Entanglement links between distant ions are mediated with photonic qubits, necessitating the need for efficient ion-photon interfaces. This thesis reports on progress towards improving the main limitations of creating a practical quantum network by using diffractive optics for high numerical aperture imaging of trapped ions. The diffractive optics are near-planar, patterned structures which are fabricated to collimate a large fraction of ion uorescence, with low aberration, for high resolution imaging. Patterning the central grounding electrode on a chip trap allows the ion to be within less than 100 microns of the surface. Large scale quantum networks will require a large number of ions and ion-photon interfaces. To scale the ion trap system, microfabricated surface traps can be used allowing for the opportunity to etch diffractive mirrors directly onto the trap surface. Collecting ion uorescence with the diffractive mirror, we show the highest ion-single mode optical fibre coupling efficiency, which enhances the entanglement rate of a quantum network. We then build a high resolution UV spectrometer to distinguish between frequency entangled photonic qubits. Frequency qubits offer more robust encoding when compared with polarisation qubits, as a result of energy conservation. Using the spectrometer we demonstrate correlations between the ion's internal state and the frequency state of an entangled photonic qubit. Quantum networks will require local quantum information processing using ion-ion gates to distribute entanglement over the entire network. Typically, the speed of these gates is presently limited to ~10 µs by the secular trap frequency. We demonstrate the first steps towards a fast ion-ion gate, not limited by the secular frequency, based on strong optical forces generated by the coherent, resonant absorption and stimulated emission of light from an ultrafast, pulsed laser. A full-scale quantum network will require many ion trap installations each with their own laser systems. For this challenge we demonstrate simple, robust, and cost-effective methods to lock the 399 nm photoionisation laser, based on polarisation spectroscopy through a hollow cathode lamp, and an opto-mechanical switch which allows locking of multiple lasers to a wavelength meter. Another scalable diffractive optic used is a phase Fresnel lens integrated into a 3D quadrupole ion trap system. The limits of imaging with this lens are tested by demonstrating force sensing at a sub-attoNewton level with super-resolution imaging techniques to find the ion location.
Abstract Quantum networking links quantum processors through remote entanglement for distributed quantum information processing and secure long-range communication. Trapped ions are a leading quantum information processing platform, having demonstrated universal small-scale processors and roadmaps for large-scale implementation. Overall rates of ion–photon entanglement generation, essential for remote trapped ion entanglement, are limited by coupling efficiency into single mode fibers and scaling to many ions. Here, we show a microfabricated trap with integrated diffractive mirrors that couples 4.1(6)% of the fluorescence from a174Yb+ion into a single mode fiber, nearly triple the demonstrated bulk optics efficiency. The integrated optic collects 5.8(8)% of theπtransition fluorescence, images the ion with sub-wavelength resolution, and couples 71(5)% of the collected light into the fiber. Our technology is suitable for entangling multiple ions in parallel and overcomes mode quality limitations of existing integrated optical interconnects.
This thesis describes the use of diffractive mirrors for efficient collection of photons from an ion in a modern surface Paul trap. Ion light collection is one of the important steps of most of the quantum computing and communication protocols using trapped ions and has a direct effect on the performance of these protocols. This is the first demonstration of an integrated-optic for a surface ion trap and a step towards realizing scalable quantum computing and communication using trapped ions. These mirrors are monolithically integrated onto the ground electrode of the trap. The fabrication of the mirrors has been accomplished by patterning diffraction rings on the silicon dioxide substrate of the trap chip and then coating it with aluminium to form a diffractive mirror integrated onto the electrode. We verified that the altered trap is still functional and demonstrated that thesemirrors substantially increase the efficiency of free space ion light collection and ion light single mode bre coupling. The mirror design is aberration free and we demonstrated this by obtaining near Diffraction-limited images of an ion. To measure the collection efficiency of the diffractive mirrors, we performed a single photon generation protocol based on an optical pumping scheme and one chapter of this thesis describes the implementation of this scheme.
The scaling of controlled quantum systems to large numbers of degrees of freedom is one of the long term goals of experimental quantum information science. Trapped-ion systems are one of the most promising platforms for building a quantum information processor with enough complexity to enable novel computational power, but face serious challenges in scaling up to the necessary numbers of qubits. In this thesis, I present both technical and operational advancements in the control of trapped-ion systems and their juxtaposition with photonic modes used for quantum networking. After reviewing the basic physics behind ion trapping, I then describe in detail a new method of implementing Raman transitions in atomic systems using optical frequency combs. Several dierent experimental setups along with simple theoretical models are reviewed and the system is shown to be capable of full control of the qubit-oscillator system. Two-ion entangling operations using optical frequency combs are demonstrated along with an extension of the operation designed to suppress certain experimental errors. I then give an overview of how spatially separated ions can be entangled using a photonic interconnect. Experimental results show that pulsed excitation of trapped ions provide an excellent single photon source that can be used as a heralded entangling gate between macroscopically separated systems. This heralded entangling gate is used to show a violation of a Bell inequality while keeping the detection loophole closed and can be used a source private random numbers. Finally, the coherent Coulomb force-based gates are combined with the probabilistic photon-based gates in a proof of concept experiment that shows the feasibility of a distributed ion-photon network.