What is the technical breakthrough identified in the paper?
(EA) The breakthrough is the integration of photonic crystal surface emitting lasers (PCSELs), which efficiently generate waves with orbital angular momentum (OAM) at 1310nm, and the use of highly sensitive avalanche photodiodes (APDs) for reliable photon detection. This combination significantly improves sensitivity when manipulating light levels, which is crucial for quantum communication systems.
This research achieved ~95% of spin-to-orbital angular conversion efficiency using PCSELs and APD’s for OAM generation and detection. Shaped like a corkscrew pattern, the OAM beam’s spatial phase profile can theoretically achieve an infinite number of spiral turns in one wavelength. The number of spiral turns in one wavelength shows an OAM state. Information can be modulated onto different OAM patterns or states, enhancing communication capacity.
Once light is transformed into an OAM wave, its polarisation handedness changes. At the detection site, the polarisation becomes opposite. If light is initially optimised for horizontal polarisation without any OAM beam generated, once generated, the OAM beam is directed to a Quarter Wave Plate (QWP). Change in polarisation handedness maximises vertical polarisation. The ratio of vertical polarisation to total power determines spin-to-orbital angular conversion efficiency.
This enhanced quantum communication system provides greater efficiency and reliability in data transmission. More sensitive OAM waves make communication under low light conditions possible. Power requirements reduce and the communication network range increases. Using higher-dimensional states can improve device security, increase data transfer speed and enhance information capacity per photon.
What was CSA Catapult’s role in this research?
Using an OAM concept established by E. Karimi et al. we implemented a new setup in the lab to circularly polarise light, which passes through a Q plate and generates the first order OAM beam.
Using Vector Photonics’s PCSELs with commercial detectors, around 86% conversion efficiency was achieved. The detectors were replaced with highly sensitive APDs which measure near-noise levels of responsivity, and the spin-to-orbital angular conversion efficiency increased to ~95%. This resulted in an overall increase in the system’s capability to detect conversion efficiency at low light levels.
CSA Catapult supported the optimisation of the PCSELs and APD integration with a UK-based supply chain. The laser source (PCSEL) and detectors (APDs) were manufactured in the UK, and the OAM system was assembled and integrated in the UK, building a commercial pathway for applications that will benefit the UK economy.
How is this research unique and how will it help accelerate highly secure quantum communication networks?
This research is unique because it uses PCSELs for generating OAM beams. Improvements in PCSEL fabrication, high-power, and circular beams can pave the way for efficient OAM generation with high state purity and APDs.
Advancements in this technology will provide exceptional sensitivity in low-light detection, enabling long-distance quantum communication.
By improving the generation and detection of OAM states, this system accelerates the development of highly secure quantum communication networks with greater data throughput and robustness.
Challenges with maintaining OAM state purity in free space and atmospheric effects still need to be addressed. However, the improved sensitivity demonstrated by APDs makes this system a promising step forward for secure, long-range quantum communications.
How will this technology benefit our customers?
This research will help develop systems that use spatial light modulators and meta materials to create higher-dimensional states. The next step is to modulate, capture, and decode the information and apply these methods to real-world scenarios.
For example, in free-space communication, robust testing of how the system transmits and receives information could help mitigate issues like turbulence and signal distortion. Understanding communication protocols will also help optimise system components and increase the number of communication states.
What type of applications will this technology help deliver?
This collaborative research with Vector Photonics and Phlux Technology, aims to accelerate highly secure quantum communication networks, Quantum Key Distribution (QKD) systems, and long-distance communication networks.
The sensitivity of APDs and the efficiency of OAM generation can play a crucial role in advancing fields like quantum computing, advanced cryptography, and secure satellite communications.
To find out more about the latest advancements in optical communications visit ECOC 2024.
