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Optical Interconnects for Trapped-Ion Quantum Transmitters

Mojtaba Ghadimi

Published Jan 1, 2017• 0 citations
Publisher

Abstract

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.

DOI
10.25904/1912/2045

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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.
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.
Quantum information processing (QIP) promises to radically change the outlook for secure communications, both by breaking existing cryptographic protocols and offering new quantum protocols in their place. A promising technology for QIP uses arrays of atomic ions that are trapped in ultrahigh vacuum and manipulated by lasers. Over the last several years, work in my research group has led to the demonstration of a monolithically integrated, scalable optical interconnect for trapped-ion QIP. Our interconnect collects single photons from trapped ions using a diffractive mirror array, which is fabricated directly on a chip-type ion trap using a CMOS-compatible process. Based on this interconnect, we have proposed an architecture that couples trapped ion arrays with photonic integrated circuits to achieve compatibility with current telecom networks. Such tightly integrated, highly parallel systems open the prospect of long-distance quantum cryptography.
Ion trap systems allow the faithful storage and manipulation of qubits encoded in the energy levels of the ions, and can be interfaced with photonic qubits that can be transmitted to connect remote quantum systems. Single photons transmitted from two remote sites, each entangled with one quantum memory, can be used to entangle distant quantum memories by interfering on a beam splitter. Efficient remote entanglement generation relies upon efficient light collection from single ions into a single mode fiber. This can be realized by integrating an ion trap with an optical cavity and employing the Purcell effect for enhancing the light collection. Remote entanglement can be used as a resource for a quantum repeater for provably secure long-distance communication or as a method for communicating within a distributed quantum information processor. I present the integration of a 1 mm optical cavity with a micro-fabricated surface ion trap. The plano-concave cavity is oriented normal to the chip surface where the planar mirror is attached underneath the trap chip. The cavity is locked using a 780 nm laser which is stabilized to Rubidium and shifted to match the 369 nm Doppler transition in Ytterbium. The linear ion trap allows ions to be shuttled in and out of the cavity mode. The Purcell enhancement of spontaneous emission into the cavity mode would then allow efficient collection of the emitted photons, enabling faster remote entanglement generation.