A quantum memory for orbital angular momentum photonic qubits (1308.0238v2)

Abstract: Among the optical degrees of freedom, the orbital angular momentum of light provides unique properties, including mechanical torque action with applications for light manipulation, enhanced sensitivity in imaging techniques and potential high-density information coding for optical communication systems. Recent years have also seen a tremendous interest in exploiting orbital angular momentum at the single-photon level in quantum information technologies. In this endeavor, here we demonstrate the implementation of a quantum memory for quantum bits encoded in this optical degree of freedom. We generate various qubits with computer-controlled holograms, store and retrieve them on demand. We further analyse the retrieved states by quantum tomography and thereby demonstrate fidelities exceeding the classical benchmark, confirming the quantum functioning of our storage process. Our results provide an essential capability for future networks exploring the promises of orbital angular momentum of photons for quantum information applications.

• The paper demonstrates a quantum memory for OAM-encoded qubits using cold cesium atoms and dynamic EIT protocols, achieving state fidelity above 92%.
• It employs Laguerre-Gaussian modes from spatial light modulators and quantum state tomography to accurately retrieve and verify stored photonic states.
• The work offers promising implications for secure, high-capacity quantum networks and paves the way for enhanced memory time and scalability in quantum systems.

Quantum Memory for Orbital Angular Momentum Photonic Qubits: An Overview

The research presented in the paper explores the development of a quantum memory system capable of storing and retrieving quantum bits encoded with orbital angular momentum (OAM) of light. OAM, a property of light with unique characteristics, provides significant potential for enhanced data encoding in quantum information systems. As quantum technologies advance, the ability to utilize OAM at the single-photon level has garnered considerable interest. This paper makes a noteworthy contribution by demonstrating the implementation of quantum memory for OAM-encoded qubits, employing a large ensemble of cold cesium atoms along with dynamic electromagnetically-induced transparency (EIT) protocols.

Experimental Setup and Methodology

The experimental setup involves encoding OAM qubits using Laguerre-Gaussian (LG) modes, which possess a helical phase structure. These are generated using computer-controlled spatial light modulators. The system employs a magneto-optical trap (MOT) containing cold cesium atoms, where the qubits are stored through the EIT protocol. The qubit field is resonant with a designated transition in the cesium atoms, and a control field aids in the storage process by inducing transparency in the medium. The research notably achieves fidelities of retrieved states exceeding the classical benchmark, thus demonstrating the quantum nature of the memory process.

Quantum state tomography is leveraged to analyze the retrieved photonic states, verifying the integrity of the storage by reconstructing the density matrices of stored qubits. This ensures that the retrieval process retains high fidelity, with values surpassing $92\%$ as per experimental results, and reaching $98\%$ with background noise correction.

Implications and Future Directions

Practically, the implementation of such a quantum memory could significantly benefit future quantum networks, which demand reliable storage of high-dimensional quantum information. The paper suggests that their current device can outperform classical memory protocols, thereby possessing potential utility in secure quantum communication systems and long-distance quantum networks.

Theoretically, this research advances our understanding of how OAM can be leveraged for high-capacity and high-dimensional quantum information processing. It provides a solid foundation for exploring quantum networking protocols and applications that exploit the vast potential of OAM as a carrier of quantum information.

The limitations noted around memory time, currently constrained by the finite optical depth and inhomogeneous broadening, can be addressed through further advancements such as enhanced cooling techniques and better atom trapping methods. Future research could also include improving memory efficiency and storage time, possibly using alternate trapping mechanisms or rare earth-doped crystals, where longer storage durations and scalability can be achieved.

Moreover, the potential to store OAM entanglement between distant nodes and extending storage capabilities to higher OAM values are intriguing future avenues. These could benefit from advancements in OAM sorting and manipulation techniques, as higher-dimensional OAM states offer greater information density and secure communications.

In conclusion, the implementation of a quantum memory for OAM photonic qubits as outlined in the paper represents a significant step toward realizing practical quantum communication and computing architectures. The integration of higher OAM values and improving quantum memory time are imperative for the progress and scalability of quantum technologies based on such principles.