We study the evolution of OAM states in fibers, showing that they can be maintained over km-lengths with purity levels exceeding 94%. This suggests a novel means for encoding information in fiber-based quantum encryption links.
Going beyond two-state qubits, $q\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}s$ based on quantum states of high dimension constitute a rich resource in quantum information, and their exploitation will play a prominent role in next-generation technologies. Generation and manipulation of qudits have improved strongly over the last decades; their reliable transmission between remote locations remains the central challenge. The authors use an air-core fiber supporting orbital angular momentum (OAM) modes to faithfully transmit qudits. Four OAM quantum states and their superpositions are created, propagated over a 1.2-km fiber, and detected. Moreover, three quantum-key-distribution protocols are implemented.
Quantum communication consists of the reliable transmission of quantum states among several parties and looks toward the long-term goal of a Quantum Internet. Quantum Internet could open up a whole universe of new applications, spanning from fundamental physics and secure communications to remote quantum computing. Multiple degrees of freedom can be used for the distribution of the quantum states, such as polarization, frequency, time and space. In particular, Orbital Angular Momentum (OAM) of light is one of the most promising thanks to its unbounded nature, albeit challenging to manipulate. OAM has been largely investigated both for classical and quantum communications, allowing for enhanced data rate in classical links and determining a high-dimensional basis for quantum communication. Here, we report our recent results related to quantum states encoded in the orbital angular momentum of light, proving the capability of preparing, manipulating, transmitting and measuring high-dimensional quantum states through a multimode fibre.
Quantum cryptography provides the inherent security for transmitting confidential information across free space or a fiber link. However, a high secure-key rate is still a challenge for a quantum-cryptography system. High-dimensional quantum cryptography, which can tolerate much higher channel noise, is a prospective way to share a higher secure-key rate between legitimate users, and has received substantial attention over the last decade. In particular, orbital angular momentum (OAM) can provide an abundant resource for high-dimensional quantum cryptography. Furthermore, combining spin angular momentum (SAM) with OAM can increase the encoding alphabet. Here we verify a prepare-and-measure quantum-cryptography scheme based on four-dimensional SAM-OAM hybrid states over kilometer-scale ring-core fibers. The measured quantum-bit error rates are $4.3\mathrm{%}$ for 4 km of fiber and $16.3\mathrm{%}$ for 25 km of fiber. The scheme simplifies the process of state preparation and measurement, with a compact and scalable setup.