High-Dimensional Intra-City Quantum Cryptography with Structured Photons (1612.05195v1)

Abstract: Quantum key distribution (QKD) promises information-theoretically secure communication, and is already on the verge of commercialization. Thus far, different QKD protocols have been proposed theoretically and implemented experimentally [1, 2]. The next step will be to implement high-dimensional protocols in order to improve noise resistance and increase the data rate [3-7]. Hitherto, no experimental verification of high-dimensional QKD in the single-photon regime has been conducted outside of the laboratory. Here, we report the realization of such a single-photon QKD system in a turbulent free-space link of 0.3 km over the city of Ottawa, taking advantage of both the spin and orbital angular momentum photonic degrees of freedom. This combination of optical angular momenta allows us to create a 4-dimensional state [8]; wherein, using a high-dimensional BB84 protocol [3, 4], a quantum bit error rate of 11\% was attained with a corresponding secret key rate of 0.65 bits per sifted photon. While an error rate of 5\% with a secret key rate of 0.43 bits per sifted photon is achieved for the case of 2-dimensional structured photons. Even through moderate turbulence without active wavefront correction, it is possible to securely transmit information carried by structured photons, opening the way for intra-city high-dimensional quantum communications under realistic conditions.

• The paper introduces a novel 4-dimensional QKD protocol using structured photons, achieving a QBER of 11% and a secret key rate of 0.65 bits per sifted photon.
• The experiment validates high-dimensional QKD over a 0.3 km free-space urban link, highlighting its resilience against moderate atmospheric turbulence.
• The study integrates classical image encryption to showcase practical applications and lays the groundwork for scalable, robust quantum networks.

High-Dimensional Intra-City Quantum Cryptography with Structured Photons

The research paper titled "High-Dimensional Intra-City Quantum Cryptography with Structured Photons" presents an advanced experimental implementation of quantum key distribution (QKD) utilizing high-dimensional quantum states encoded in structured photons. The paper demonstrates the realization of a single-photon QKD system operating over a free-space link of 0.3 km in urban Ottawa. The experiment leverages the spin and orbital angular momentum (OAM) of photons to establish secure communication channels.

The key contribution of this work lies in the experimental validation of high-dimensional QKD beyond laboratory settings. The authors employ a 4-dimensional BB84 protocol enabled by structured photons formed through combinations of polarization and OAM states. Specifically, they achieve a quantum bit error rate (QBER) of 11% for 4-dimensional states with a secret key rate of 0.65 bits per sifted photon. For comparison, 2-dimensional structured photons yield a QBER of 5% with a secret key rate of 0.43 bits per sifted photon.

These results are significant in illustrating the robustness and potential for increasing secure data transmission rates with high-dimensional quantum states, especially in turbulent urban environments without the need for active wavefront correction. The use of structured photons, characterized by their vectorial nature and complex intensity profiles, offers enhanced tolerance against noise and the ability to encode more information per photon.

The experimental setup involves a heralded single-photon source, state preparation by the sender (Alice), and state measurement by the receiver (Bob) using $q$-plates and wave plates for mode generation and selection. The system effectively maintains optical alignment and reduces divergence over the link. The authors additionally integrate a classical image encryption and decryption scheme to highlight the practical applications of their high-dimensional QKD implementation.

The paper underscores the essential role of high-dimensional quantum systems in advancing secure intra-city quantum communication networks. The resilience to moderate atmospheric turbulence positions structured photons as a promising candidate for future free-space quantum networks, potentially enabling applications such as intra-city quantum teleportation.

In terms of future implications, the paper suggests pathways for enhancing quantum network scalability and robustness, driving the development of longer-distance quantum communication links. This research also lays foundational groundwork for further exploration into combining multiple photonic degrees of freedom for optimized QKD.

Overall, the demonstrated approach effectively harnesses the properties of structured photons within a real-world setting, opening avenues for practical quantum communication solutions in complex environments.