Integrated quantum photonics

Introduction

Quantum photonics is the science of generating, manipulating and detecting light in regimes where it is possible to coherently control individual quanta of the light field (photons). Quantum photonics is generally recognised as being fundamental field in exploring quantum phenomena. Quantum photonics is also expected to play a central role in advancing future technologies - such as Quantum Information Processing (QIP). Photons are particularly attractive carriers of quantum information due to their low decoherence properties, light-speed transmission and ease of manipulation. Single photons were used for the very first violations of Bells inequality, and are employed within many highly successful proof-of principle demonstrations of emerging quantum technologies.

The most common implementation of quantum photonics employs linear optical components such as beamsplitters, mirrors and wave-plates. Such realizations of quantum photonics have been used to implement Linear Optical quantum Computation (LOQC).

However, such experiments suffer from several optical scaling problems as the size of the experiments are increased.

1. Stability - Large numbers of linear optical components can introduce non-coherent phase changes.
2. Experiment size - Linear optical components are typical large, requiring
3. Manufacturability - Devices built using linear optical components can present problems in mass-manufacturing

These problems are especially pertinent for LOQC applications of quantum photonics.

Integrated Quantum Photonics is an approach that addresses these problems by employing implementations of linear optical components that can be built on-chip using variations on existing fabrication technologies. Such an integrated approach to quantum photonics inherently ensures phase stability (with all components being within the same chip substrate), allowing high quality non-classical interference to be achieved. Being based on well-developed fabrication techniques, the elements employed in Integrated Quantum Photonics are more readily miniaturisable, and products based on this approach can be manufactured using existing production methodologies.

History

Linear optics, as a platform for quantum computation specifically, began to show a marked rise in activity after the publication of the seminal theory paper of Knill-Laflamme-Milburn, which demonstrated the feasibility of linear optical quantum computers using post-selection, feed-forward and number resolving detection. Following this there were several experimental proof-of-principle demonstrations of two-qubit gates performed in bulk optics. It soon became clear that integrated optics could provide a powerful enabling technology for this emerging field. Early experiments in integrated optics demonstrated the feasibility of the field via demonstrations of high-visibility non-classical and classical interference in simple components based on linear optics such as directional couplers (DCs) and Mach–Zehnder interferometers. These components were then be used to fabricate more complicated circuits such as multi-photon entangling gates as well as core components used for fully reconfigurable quantum circuits, where reconfigurability can achieved using both thermal and electro-optic phase shifters.

Another area of research in which integrated optics will prove pivotal in its development is Quantum communication and has been marked by extensive experimental development demonstrating, for example, quantum key distribution (QKD), quantum teleportation, quantum relays based on entanglement swapping, and quantum repeaters.

Since the birth of integrated quantum optics experiments have ranged from technological demonstrations, for example integrated sources and integrated detectors, to fundamental tests of nature, new methods for quantum key distribution, and the generation of new quantum states of light. It has also been demonstrated that a single integrated device is sufficient to implement the full field of linear optics.

As the field has progressed new quantum algorithms have been developed which provide short and long term routes towards the demonstration of the superiority of quantum computers over their classical counterparts. Cluster state computation is now generally accepted as the approach that will be used to develop a fully fledged quantum computer. Whilst development of quantum computer will require the synthesis of many different aspects of integrated optics, boson sampling seeks to demonstrate the power of quantum computation via technologies readily available and is therefore a very promising near term algorithm to doing so. In fact shortly after its proposal there were several small scale experimental demonstrations of the boson sampling algorithm

Materials

Control over photons can be achieved with integrated devices that can be realised in different material platforms such as Silica, Silicon, Gallium arsenide, Lithium niobate and Indium phosphide.

Silica

Two methods for using silica:

1. Flame hydrolosis and another method - more like traditional lithography etching and depositing different materials.

2. Direct write - only uses single material and laser (use computer controlled laser to damage the glass and user lateral motion and focus to write paths with required refractive indices to produce waveguides). This method has the benefit of not needing a clean room. Most common method now. Some challenges, but promising. Also starting to write heaters using lasers (Roberto osellame).

Silicon

Using traditional lithography - same recipes used in electronics. Structures are different from modern electonic ones. But techniques scale. Silicon has a really high refractive index. Makes nice interface with glass which has lower refractive index. This allows waveguides made form silicon and glass to be small and have tight bends, which allows you to make denser systems.

Fabrication

Conventional fabrication technologies are based on photolitographic processes, which enable strong miniaturization and mass production. In quantum optics applications a relevant role has been played also by the direct inscription of the circuits by femtosecond lasers or UV lasers; these are serial fabrication technologies, which are particularly convenient for research purposes, where novel designs have to be tested with rapid fabrication turnaround. Femtosecond laser written quantum circuits have proven particularly suited for the manipulation of the polarization degree of freedom and for building circuits with innovative three-dimensional design. Quantum information is encoded on-chip in either the path, polarisation, time bin or frequency state of the photon, and manipulated using active integrated components in a compact and stable manner.

Waveguides

Channels

Directional coupler

Produces a fixed phase

Active components

Thermo-optic effect – slowly varying changes ,

Electro-optic effect - Injecting carriers in PN. Can’t be done in QM domain yet