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Advances in Photonics of Quantum Computing, Memory, and Communication

Modular linear optical circuits for quantum information processing (Conference Presentation)

James C. Gates, Paolo L. Mennea, William R. Clements, Devin H. Smith, Rex H. S. Bannerman, Jelmer J. Renema, W. Steven Kolthammer, Ian A. Walmsley, Peter G. R. Smith

Published Mar 4, 2019• 0 citations
Publisher

Abstract

For over a decade the field of quantum photonics has increasingly looked towards optical integrated platforms to perform more complex and sophisticated experiments. Silica integrated optics is an ideal material for this area, offering low propagation and fibre-coupling losses. To date many of the key on-chip experiments have been carried out in this platform, using bespoke monolithic devices. In this work we propose an alternative approach, implementing a linear network constructed from a number of identical reconfigurable modules. The modules are measured separately to produce an accurate model of the overall network. The cellular nature also allows the replacement of modules that are faulty or substandard. Each module comprises of an array of 10 Mach-Zhender interferometers. Forty thermo-optic phase shifters on each chip allows the control of both the amplitude and phase of the optical field within the devices. By cascading the modules any arbitrary NxN unitary network can be realised. The optical waveguides within the modules are fabricated by direct UV writing, where a scanning focused UV laser beam increases the local refractive index within a photosensitive germanosilicate glass layer. The resulting channel waveguides are engineered to have dimensions that are mode matched to standard optical fibre producing excellent coupling efficiency. Bragg gratings can also be simultaneously produced within the waveguides which greatly assists in the precise characterisation of the phase shifters, coupling ratios and optical losses within the modules. We will present our recent work in this area, demonstrating devices operating at telecom wavelengths for quantum information processing. We present a modular reconfigurable system for on-chip quantum optics experiments with excellent fibre compatibility and low propagation losses, implemented using direct-UV-written silica-on-silicon. The performance of fabricated devices in various configurations is reported.

DOI
10.1117/12.2510301

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Direct UV-written silica-on-silicon provides an attractive platform for quantum optics, offering the key benefits of low propagation losses and excellent optical fibre compatibility. This work has aimed to develop the necessary techniques and components for completely integrated quantum optics experiments to be carried out using this platform, on a larger scale than previously possible. Arrays of matched on-chip photon sources based on birefringence-matched spontaneous four-wave mixing (FWM) are demonstrated, both at 800 nm and in the telecommunications C-band, along with progress towards further integration of these sources. Thermo-optic phase shifters, for use in room temperature quantum circuits, have been optimised via modelling, and a range of alternative modulator technologies have been explored. Further work has been concerned with the development of a modular system for quantum optics, comprising a set of duplicate reconfigurable modules and the necessary drive electronics and software, with the intent of simplifying the task of identifying and quantifying manufacturing imperfections in large integrated experiments. Efforts have also been made to improve the detection efficiency of on-chip transition edge sensor (TES) single photon detectors, including the use of longer absorbers and high reflectors for multiple absorption passes; this has resulted in the demonstration of a photon-number-resolving detector with a Bragg grating enhanced quantum efficiency of 87%. Approaches for further increasing the detection efficiency are considered and a device for on-chip Hong-Ou-Mandel and photon subtraction experiments using these detectors is reported.

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•Dec 1, 2017•1 citation
For over a decade the field of quantum photonics has increasingly looked towards optical integrated platforms to perform more complex and sophisticated experiments. Silica integrated optics is an ideal material for this area, offering low propagation and fibre-coupling losses. To date many of the key on-chip experiments have been carried out in this platform, using bespoke monolithic devices. In this work we propose an alternative approach, implementing a linear network constructed from a number of identical reconfigurable modules. The modules are measured separately to produce an accurate model of the overall network. The cellular nature also allows the replacement of modules that are faulty or substandard. Each module comprises of an array of 10 Mach-Zhender interferometers. Forty thermo-optic phase shifters on each chip allows the control of both the amplitude and phase of the optical field within the devices. By cascading the modules any arbitrary NxN unitary network can be realised. The optical waveguides within the modules are fabricated by direct UV writing, where a scanning focused UV laser beam increases the local refractive index within a photosensitive germanosilicate glass layer. The resulting channel waveguides are engineered to have dimensions that are mode matched to standard optical fibre producing excellent coupling efficiency. Bragg gratings can also be simultaneously produced within the waveguides which greatly assists in the precise characterisation of the phase shifters, coupling ratios and optical losses within the modules. We will present our recent work in this area, demonstrating devices operating at telecom wavelengths for quantum information processing. We present a modular reconfigurable system for on-chip quantum optics experiments with excellent fibre compatibility and low propagation losses, implemented using direct-UV-written silica-on-silicon. The performance of fabricated devices in various configurations is reported.
Direct UV-written silica-on-silicon provides an attractive platform for quantum optics, offering the key benefits of low propagation losses and excellent optical fibre compatibility. This work has aimed to develop the necessary techniques and components for completely integrated quantum optics experiments to be carried out using this platform, on a larger scale than previously possible. Arrays of matched on-chip photon sources based on birefringence-matched spontaneous four-wave mixing (FWM) are demonstrated, both at 800 nm and in the telecommunications C-band, along with progress towards further integration of these sources. Thermo-optic phase shifters, for use in room temperature quantum circuits, have been optimised via modelling, and a range of alternative modulator technologies have been explored. Further work has been concerned with the development of a modular system for quantum optics, comprising a set of duplicate reconfigurable modules and the necessary drive electronics and software, with the intent of simplifying the task of identifying and quantifying manufacturing imperfections in large integrated experiments. Efforts have also been made to improve the detection efficiency of on-chip transition edge sensor (TES) single photon detectors, including the use of longer absorbers and high reflectors for multiple absorption passes; this has resulted in the demonstration of a photon-number-resolving detector with a Bragg grating enhanced quantum efficiency of 87%. Approaches for further increasing the detection efficiency are considered and a device for on-chip Hong-Ou-Mandel and photon subtraction experiments using these detectors is reported.
The emerging field of quantum technologies requires a range of photonic devices, many with demanding specifications. The area of quantum information processing requires scaling in order to increase the complexity of experiments, this has led researchers towards integrated photonic platforms. The key property of these single photon devices is low-loss operation and as a consequence glass and silica devices have been a dominant platform. Whereas atom chip sensors require stable compact systems for converting optical fibre modes to free space beams for atom trapping. In this talk I will present the photonic engineering undertaken at the University of Southampton to fabricate devices for the quantum technology community. The key fabrication approach is a direct UV writing technique, where a UV laser beam is used to define waveguides, splitters/couplers, Bragg gratings and 2D tilted gratings. The fabricated devices include cascaded arrays of Mach-Zehnder interferometers to preform photon-photon interactions, single photon sources via spontaneous four-wave mixing, integrated superconducting single photon detectors and 2D diffraction gratings. In addition to the inscription method I will describe the fabrication of the substrates required for the UV writing process which are generated via Flame Hydrolysis Deposition. I will discuss our most recent work on single photon linear optical networks constructed from a number of identical reconfigurable modules. The modules are characterised separately to produce an accurate model of the network and the cellular approach permits the replacement of modules that are deficient. Each module comprises of an array of 10 Mach-Zehnder interferometers with 40 thermo-optic phase shifters on each chip that control both the amplitude and phase of the optical modes. By concatenating modules any arbitrary N ×N unitary network can be realised. The channel waveguides are engineered to be mode matched to standard optical fibre producing excellent coupling efficiency. I will present our recent work in this area and other photonic devices for the field of quantum technology.
Quantum optics experiments are increasingly taking an integrated format for the benefits of phase stability and scalability. Silica has proved an advantageous material for this work due to its low propagation loss and high-efficiency coupling to optical fibre, and a number of key on-chip experiments have already been carried out with up to four photons using this material [1]. Fabrication imperfections create a number of difficulties in moving to more complex setups since the classical performance of the network must be characterised before an analysis of its quantum properties can be carried out, and this process becomes greatly more challenging as the circuit complexity grows.
We propose and demonstrate a modular architecture for reconfigurable on-chip linear-optical circuits. Each module contains 10 independent phase-controlled Mach-Zehnder interferometers; several such modules can be connected to each other to build large reconfigurable interferometers. With this architecture, large interferometers are easier to build and characterize than with traditional, bespoke, monolithic designs. We demonstrate our approach by fabricating three modules in the form of UV-written silica-on-silicon chips. We characterize these chips, connect them to each other, and implement a wide range of linear optical transformations. We envisage that this architecture will enable many future experiments in quantum optics.
All optical quantum information processing is an important developing field for understanding and exploiting quantum entanglement and offers opportunities in quantum computation, quantum sensing and quantum communication. The presentation will describe work on laser written devices in a silica on silicon platform. The devices can be used to create single photon sources, waveguide circuitry and number resolving detectors. Applications of the technology include boson sampling, quantum teleportation, on-chip number resolving detection and quantum simulation. The use of laser writing allows for rapid prototyping, and by including high quality Bragg gratings, it is possible to provide accurate measurements of device properties including losses and coupler variability. Furthermore, the ability to make accurate measurements of material properties such as dispersion and birefringence allows the creation of tailored devices with optimal properties for identical photon sources.
This thesis presents the development of a platform to fabricate photonics integrated circuits that can be used to scale networks intended for quantum information processing (QIP) experiments. The stringent technical requirements for the transport and manipulation of quantum states of light are discussed with respect to channel waveguides and integrated gratings fabricated in silica-on-silicon through direct UV writing laser processing Tilted gratings are identified as a method to enable polarisation-based applications for this integrated platform. A novel implementation of in-line planar waveguide polarisers based on 45º tilted gratings is presented, demonstrating gratings with polarisation extinction ratio (PER) of 0.25 dB / mm and bandwidth impairments better than 0.3 dB in the C-band. 45º tilted gratings in UV written waveguides are used to create novel polarising coupler architectures with PER of 28.5 dB. The alteration of the material composition of germanesilicate planar core layers is investigated, producing waveguides with birefringence of 4.5 ± 0.2 × 10−4, higher than previously reported for this platform. A process for producing end facet endcaps to extend the platform’s capability for high power applications is also described. These developments offer potential for the scaling of QIP experiments with heralded spontaneous four-wave mixing single-photon sources. Finally, the thesis describes research-based education experiments conducted to inform a wide range of audiences on the importance of photonics technologies. The concept of Photonics, and the underlying science and associated research, has been introduced to 2,952 students from 81 schools in the South of England and over 6,000people in public events.