Multimode integrated time-frequency quantum photonics

Quantum physics is at the centre of a technological revolution that promises to transform information and communications technologies (ICT) from secure communications, enhanced precision in measurement, high-capacity simulation and computation beyond that capable using only classical resources. These quantum-enhanced technologies will significantly increase the capacity to securely transmit and process the ever-rising volumes of data on which the global economy is based. The ability to simulate complex systems, both quantum and classical, could allow creation of new functional materials ranging from designer drugs to solar cells with improved efficiency. Furthermore, improved sensitivity in measurements opens paths to probe fundamental physics previously not accessible, as well as more practical applications such as low-light-level spectroscopy. Light will play a central role in many of these quantum technologies such as spectroscopy, imaging, communications, and computation.

When addressing individual photons, information is most often encoded as one quantum bit (or qubit) of information per transmitted photon. However, this severely limits the information processing capacity of current approaches to small sizes. One recent approach to increase the complexity of quantum optical systems that can be achieved utilizes integrated quantum photonics, in which photons are generated, guided and manipulated on silicon chips. However, these techniques still rely on encoding information as qubits and do not take advantage of the rich structure of individual photons. This project will take a new approach to encoding an increased amount of information per photon, and thus allow significantly increased system complexity to be achieved.

To increase the information capacity of single photons for integrated photonics this project will focus on encoding information in the time-frequency state of individual photons, in which the temporal shape and colour of the photon carry the information. This approach parallels methods used in classical ICT such as wavelength- and time-division multiplexing where information is encoded in the wavelength (colour) or arrival time of light pulses. This new approach is made possible by recent developments at Oxford in fibre-based photon sources and integrated quantum photonics, which allow deterministic frequency control of single photons. By moving to this regime of increased information capacity per photon on an integrated platform, the project aims to enable significant advances in quantum-enhanced technologies. Further afield the techniques for low-light-level time-frequency sensing developed during the project could be utilized in a diverse range of applications from spectroscopy to optical network diagnostics

Quantum physics underlies nearly all observed phenomena and is becoming increasingly important to both pure and applied research, with the promise of improved performance for certain tasks such as precision measurement, lithography, and information-communications technologies (ICT). Indeed, quantum principles underpin many of the technological advances that enable much of today's globally linked electronic commerce. From semiconductor devices to optical telecommunications networks, we are surrounded by the fruits of quantum labour. Future technological developments are likely to emerge from breakthroughs in harnessing quantum properties of physical systems. Furthermore, understanding the role that quantum physics plays in naturally occurring processes, such as photosynthesis and other biochemical processes, is at the fore of current physics research. Such highly-exploratory enquiry could lead to significant insight to efficient light-harvesting mechanisms and potentially lead to bio-inspired designs for solar cells.

As in classical applications, light will play a key role to many quantum technologies such as imaging, spectroscopy, and ICT. To realize the potential quantum enhancement offered by such optical quantum technologies requires improved abilities to create, manipulate, and characterize non-classical states of light. The proposed research is centred on the advancement of integrated-optics based methods to generate, coherently manipulate, and measure time-frequency (TF) states of one and two photons. Moving to the TF domain opens new directions for research by significantly increasing the dimensionality of the accessible photonic state space, allowing systems of greater complexity to be harnessed experimentally for unparalleled processing power. Beyond the direct impact to quantum technologies, the research results of this work will likely find applications in areas where low-light sensing is a central tool. The techniques developed during this project to completely characterize the TF state of light at the single-photon level will be applicable to researchers in disciplines such as chemistry, biology, condensed matter physics, and semiconductor device research, who would benefit from the enhanced spectroscopic capabilities enabled by these approaches.