A secret key is needed to securely transfer quantum information from A to B so only the intended receiver can read the message. What if senders added extra “dimensions” to the transmission to make it even more secure?
This method could ensure the message is delivered correctly, no matter who is watching. This is what quantum key distribution (QKD) does. It is a promising way to create secure communication networks, but traditional two-level carriers limit their performance because they have limited capacity and are not very resilient to noise.
Using high-dimensional qudits can overcome these issues. Practical qudit platforms must be highly controllable, compact, and able to propagate robustly, which integrated photonics can provide as a promising solution.
Recent research led by Liang Feng at the University of Pennsylvania and Li Ge at the City University of New York has developed a compact microlaser that securely transmits coded information. This microlaser uses qudits to enhance quantum communication‘s capacity and resilience significantly.
This innovation allows quantum messages to carry more information and be less susceptible to interference, paving the way for future secure, high-dimensional quantum networks.
The research team has miniaturized the large optical setup for generating quantum signals onto a small laser chip. This results in lower energy consumption and a more robust signal than current methods.
Thanks to its sleek design, the team created a portable device that users can carry around without noise.
This new device generates spin-orbit photonic qudits, which encode information in light by manipulating its shape (orbital angular momentum) and twist (polarization). Instead of encoding information in just one property of a photon, the system can multitask, allowing for more complex, high-dimensional encoding.
In other words, the new microlaser system improves on existing QKD methods. Previously, transferring such quantum signals required an entire optical table filled with precise, bulky equipment. Now, the new system fits onto a compact chip, making it suitable for real-world networking applications.
Non-Hermitian physics
The system is built based on the principles of non-Hermitian physics, which guides how energy and information may flow through a system. In conventional Hermitian systems, energy and information behave regularly and rigidly—like a perfectly balanced scale. On the other hand, non-Hermitian systems introduce new degrees of control, allowing energy exchange to be dynamically and conveniently fine-tuned.
Because of this flexibility, the team could generate and manipulate the high-dimensional spin-orbit qudit states in real-time by using the highly compact microlaser. The result was precision-controlled emitted light that ensured stable quantum key transmission and greater efficiency.
Feng said, “We designed a microlaser that can emit four distinct quantum states with perfect spatial and temporal uniformity. This means we don’t have to worry about dephasing effects—light losing synchronicity—or signal loss due to environmental fluctuations.”
The system could promptly transmit quantum keys during experiments over simulated long-distance conditions. It maintained signal integrity across distances equivalent to over 100 kilometers in atmospheric transmission.
In further calculations, the team replaced low-efficiency single-photon avalanche diodes with superconducting nanowire single-photon detectors, and the system was found to push past 500 kilometers, making ground-to-satellite quantum communication a tangible reality.
The researchers also address a major weakness in QKD, multiphoton pulses, weak coherent state propagation, or sophisticated eavesdropping. The team implemented a decoy that introduces random variations in pulse intensity, so some pulses contain the expected number of photons, while others are deliberately weaker or empty.
The team is now seeking to improve the system’s dimensionality further. They want to test it in a practical environment like a fiber network.
Journal Reference:
- Yichi Zhang, Haoqi Zhao, Tianwei Wu, Zihe Gao et al. High-Dimensional Quantum Key Distribution by a Spin-Orbit Microlaser. Physical Review X. DOI: 10.1103/PhysRevX.15.011024
Source: Tech Explorist
