Twisted Photons: New Quantum Perspectives in High Dimensions (1708.06101v1)

Abstract: Quantum information science and quantum information technology have seen a virtual explosion world-wide. It is all based on the observation that fundamental quantum phenomena on the individual particle or system-level lead to completely novel ways of encoding, processing and transmitting information. Quantum mechanics, a child of the first third of the 20th century, has found numerous realizations and technical applications, much more than was thought at the beginning. Decades later, it became possible to do experiments with individual quantum particles and quantum systems. This was due to technological progress, and for light in particular, the development of the laser. Hitherto, nearly all experiments and also nearly all realizations in the fields have been performed with qubits, which are two-level quantum systems. We suggest that this limitation is again mainly a technological one, because it is very difficult to create, manipulate and measure more complex quantum systems. Here, we provide a specific overview of some recent developments with higher-dimensional quantum systems. We mainly focus on Orbital Angular Momentum (OAM) states of photons and possible applications in quantum information protocols. Such states form discrete higher-dimensional quantum systems, also called qudits. Specifically, we will first address the question what kind of new fundamental properties exist and the quantum information applications which are opened up by such novel systems. Then we give an overview of recent developments in the field by discussing several notable experiments over the past 2-3 years. Finally, we conclude with several important open questions which will be interesting for investigations in the future.

• The paper demonstrates that employing twisted photons with orbital angular momentum significantly boosts information encoding capacity.
• It presents innovative experimental methods enabling robust quantum teleportation and the generation of multi-mode entangled states.
• The research highlights improved security in quantum key distribution with higher error thresholds and resistance to cloning attacks.

Review of "Twisted Photons: New Quantum Perspectives in High Dimensions"

The paper "Twisted Photons: New Quantum Perspectives in High Dimensions" by Erhard et al. presents a detailed exploration into the burgeoning field of high-dimensional quantum systems, with a specific emphasis on the Orbital Angular Momentum (OAM) of photons. The paper highlights the potential advantages and recent advancements in employing these multi-level quantum states (qudits) over traditional binary-based qubits in quantum information science.

The core motivation of the research described in the paper is the inherent capacity limitations associated with qubits, the most common quantum information entities composed of two-level systems. The investigation pivots towards the utilization of OAM states of photons—quantum systems with potentially infinite discrete levels—offering a radically enhanced information encoding capacity. This innovation leverages high-dimensional quantum systems to provide demonstrable benefits in various domains such as increased information capacity, heightened robustness in quantum cryptography, and improved threshold resilience against eavesdropping and cloning attacks.

The authors elucidate several salient advantages provided by these high-dimensional quantum states. Crucially, OAM's power arises from its promise of larger violations of local realism, surpassing the limitations imposed by conventional Bell-type inequalities for qubits. Improved fidelity in cloning resistance and higher tolerable error thresholds in quantum key distribution (QKD) mark significant strides towards robust high-dimensional quantum communication frameworks. Additionally, the ability to avoid monitoring signal disturbance in QKD protocols via robust quantum state transitions represents another pivotal advantage, further demarcating high-dimensional quantum systems as a fertile territory for exploration and application.

Quantum teleportation emerges as another focal point, as the paper discusses its application to multiple degrees of freedom, overcoming earlier constraints that limited teleportation to singular, binary quantum features. Notably, the research delineates an experimental leap forward through simultaneous teleportation of both polarization and OAM degrees of freedom, demonstrating a sophisticated understanding and control of multi-level quantum states.

Furthermore, the paper provides insight into recent experimental efforts bridging the theoretical promise with practical realizations. The intricacies of generating entangled states in higher dimensions are unpacked, revealing innovative experimental strategies, such as the use of non-linear optical processes, to create multi-mode entanglement. Recent results are highlighted, including groundbreaking intra-city distribution of ququart states over free space, affirming promising applications in the real world.

Looking ahead, the authors suggest several challenges that necessitate attention for the maturation of high-dimensional quantum systems. These include refining methods for the long-distance transmission of high-dimensional OAM states, developing unitary transformations, and achieving robust quantum teleportation. Indeed, investigating efficient methods for arbitrary transformations and two-qudit quantum gates remain as vital pursuits to further extend the envelope of quantum computation and communications.

In conclusion, Erhard et al.'s paper sketches a comprehensive landscape of high-dimensional quantum systems using twisted photons, underscoring their transformative implications for the field of quantum information technology. The discussion is compelling and analytical, enriching the reader's understanding of the current state and future potential of OAM in quantum science. The ongoing integration of theoretical and experimental studies within this space is expected to enhance the trajectory of quantum innovation, and the insights provided here will undoubtedly fuel future research and technological breakthroughs.