Nanophotonic hybrid electro-optic modulator for quantum information processing

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

Previous few years have seen an explosive growth in the field of integrated photonics. Integrated photonics is on its way to revolutionize a number of application areas, for example, data centers, high-performance computing and sensing. It is also a prime candidate as a scalable platform for quantum information processing due to the fast propagation speeds of photons, robustness against decoherence and various degrees of freedom. The optical quantum computer is most likely to be based on one-way quantum computer proposed by Raussendorf and Briegel. This model requires qubits to be initialized in a highly entangled cluster state and the quantum computation to be carried out by a sequence of single-qubit measurements with classical feedforward of their outcomes. This scheme can be implemented by a universal set of quantum logic operations in the form of single-qubit rotations and nontrivial two-qubit gates. One of the essential elements to realise fast active switching required for one-way quantum computing is the optical modulator. Modulation can be produced through the use of the thermo-optic effect and optical microelectromechanical systems (MEMS) based structures. However, these approaches are limited to very low speed applications and do not work as well at low temperatures.The current approach is to develop optical modulators based on Pockel's 'electro-optic' effect.

In the past few years, a huge amount of research has been focussed towards silicon photonics in order to realise silicon-based electro-optic modulator. The key driving force behind silicon photonics is the ability to use CMOS like fabrication resulting in high-volume production at low cost. Silicon, however, has a number of shortcomings as a photonic material. In its basic form, it is not an ideal material in which to produce light sources, optical modulators or photodetectors. Additionally, silicon is affected by two-photon absorption (TPA) at telecom wavelengths, causing increased losses in single-photon sources. TPA also poses a problem for fast optical switching utilizing Kerr effect. An alternative route to achieving high-speed modulation is to introduce other materials with strong electro-optic effects to the silicon platform. Considerable efforts have been made towards integrating a variety of materials with silicon, including III-V semiconductors, plasmonics, graphene, polymers and ferroelectric materials like LiNbO3 and BaTiO3. In this proposal, we propose an electro-optic modulator based on ferroelectric materials, particularly Lithium Niobate (LiNbO3).

We have chosen to make the first device with Lithium Niobate. For decades, lithium niobate (LN) has been the preferred material for high-performance electro-optic modulators due to its wide bandgap (high transparency) and large second order electro- optic coefficient (30 pm/V). In contrast to Si and InP, index of refraction in LN changes linearly with an applied electrical field at femtosecond timescale. Also, Lithium Niobate devices work well at low temperatures. The first step of the research is to come up with a design of the electro-optic modulator. The standard design of an electro-optic modulator is based on a Mach-Zehnder interferometer in which the active arm is formed by the waveguide structure. The proposed waveguide structure for our device consists of amorphous silicon deposited on LiN substrate and a layer of SiN or SiO2 as an index matching layer. While there are a lot of standard reference designs and PDKs for SOI, for Si on LN, we are essentially in uncharted territory. The idea is to use simulation tools like Lumerical to optimise the design parameters of the structure. The solutions to following design problems will be explored:
Efficient grating couplers
Low loss & ultra-low power phase shifters
Design of resonant modulators

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R51245X/1 01/10/2017 30/09/2021
2107515 Studentship EP/R51245X/1 18/09/2017 30/09/2021 Ankur Khurana