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Optics Letters

Spatially multiplexed orbital-angular-momentum-encoded single photon and classical channels in a free-space optical communication link

Yongxiong Ren, Cong Liu, Kai Pang, Jiapeng Zhao, Yinwen Cao, Guodong Xie, Long Li, Peicheng Liao, Zhe Zhao, Moshe Tur, Robert W. Boyd, Alan Eli Willner

Published Nov 21, 2017• 27 citations
Publisher

Abstract

We experimentally demonstrate spatial multiplexing of an orbital angular momentum (OAM)-encoded quantum channel and a classical Gaussian beam with a different wavelength and orthogonal polarization. Data rates as large as 100 MHz are achieved by encoding on two different OAM states by employing a combination of independently modulated laser diodes and helical phase holograms. The influence of OAM mode spacing, encoding bandwidth, and interference from the co-propagating Gaussian beam on registered photon count rates and quantum bit error rates is investigated. Our results show that the deleterious effects of intermodal crosstalk effects on system performance become less important for OAM mode spacing Δ≥2 (corresponding to a crosstalk value of less than -18.5 dB). The use of OAM domain can additionally offer at least 10.4 dB isolation besides that provided by wavelength and polarization, leading to a further suppression of interference from the classical channel.

DOI
10.1364/ol.42.004881

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We experimentally demonstrate spatial multiplexing of an orbital angular momentum (OAM)-encoded quantum channel and a classical Gaussian beam with a different wavelength and orthogonal polarization. Data rates as large as 100 MHz are achieved by encoding on two different OAM states by employing a combination of independently modulated laser diodes and helical phase holograms. The influence of OAM mode spacing, encoding bandwidth, and interference from the co-propagating Gaussian beam on registered photon count rates and quantum bit error rates is investigated. Our results show that the deleterious effects of intermodal crosstalk effects on system performance become less important for OAM mode spacing Δ≥2 (corresponding to a crosstalk value of less than -18.5 dB). The use of OAM domain can additionally offer at least 10.4 dB isolation besides that provided by wavelength and polarization, leading to a further suppression of interference from the classical channel.
In this paper, a free-space quantum key distribution (QKD) system based on BB84 protocol is studied assuming that the transmitted quantum bits are multiplexed spatially based on orthogonal orbital angular momentum (OAM) carrying beams. The performance of such a system is estimated in terms of quantum bit error rate, outage capacity, and effective data rate taking into account the impact of error correction and privacy amplification. Based on the simulation of OAM-carrying beam propagating through atmospheric channels impaired by turbulence, the crosstalk among the multiplexed channels are investigated and its effect on the performance of the QKD system is evaluated. The presented results provide insight into the efficient design of such multiplexed QKD systems particularly in terms of the choice of the optimal average photon number per pulse and the multiplexing mode set.
The orbital angular momentum (OAM) of photons offers a suitable support to carry the quantum data of multiple users. We present two optical setups that send the information of nnn quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength, and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges nnn channels into the same link, which transmits nnn independent photons. The second solution, the OAM multiplexer, uses controlled-not (CNOT\mathrm{CNOT}CNOT) gates to transfer the information of nnn optical channels to a single photon. Additional applications of the multiplexer circuits, such as quantum arithmetic, as well as connections to OAM sorting are discussed.
We experimentally demonstrate a 10 Mbit/s free-space quantum communication link using data encoding on orthogonal Laguerre–Gaussian (LG) modes with the same azimuthal index but different radial indices. Data encoding on two LGℓp modes (i.e., for ℓ=0, we encode [“0”, “1”] as [p=0, p=1], and for ℓ=1, we encode [“0”, “1”] as [p=0, p=1]) is demonstrated by employing directly modulated laser diodes and helical phase holograms. The quantum symbol error rate (QSER) of <5% is achieved at an encoding rate of 10 Mbit/s. Moreover, the influence of the circle radius (R) of the receiver phase pattern on registered photon rates and QSERs is investigated. Our results show that a receiver phase pattern whose R does not match the beam size of the LG modes would induce higher cross talk between the two encoded quantum branches.
Recently,the free space optical communication system based on orbital angular momentum( OAM) states multiplexing has become one of the hot topics in optical communications and quantum optics,because OAM states are new dimensional information carriers. An efficient orbital-angular-momentum-multiplexed free-space optical communication scheme is proposed,where the efficient orbital angular momentum states separation method is adopted. With this OAM separation method,different orbital angular momentum states can be obtained simultaneously from the superposition OAM state beam,which greatly saves time and the expensive devices for demodulating the information from the information-carrying OAM states. Numerical simulation results show that the proposed scheme can reduce time and the number of the devices at the receiver for the information retrieving. The bit error rate of the system can be less than 10- 4when the signal-to-noise ratio of the system is greater than 23 dB. Therefore,this OAMmultiplexed free-space optical communications scheme is proved to be feasible.
Orbital angular momentum (OAM) of photons provides a promising approach to enhance the capacity, robustness, and functionality of quantum communication systems in optical fibers. With many twist states available, OAM enables high-dimensional encoding, significantly increasing data rates and noise tolerance. This review explores sources, channels, and system architectures for OAM-based quantum communication in fibers. It compares free-space and integrated emitters, discussing in-fiber pair generation and mode conversion pathways that produce fiber-ready single photons and entangled states. For the channels, it details design principles and trade-offs for step-index and ring-core fibers that minimize intermodal coupling and reviews milestones in kilometer-scale single-photon transmission and high-dimensional entanglement transport. On the system side, the review examines coupling, multiplexing, and detection techniques—including mode (de)multiplexers, lanterns, and programmable sorters—and discusses transmission-matrix-based control methods to mitigate dispersion and bending effects. The review concludes with practical guidance and open challenges, such as improving mode selectivity and stability under real-world conditions; developing compact, efficient detectors for many modes; and integrating co-designed error correction schemes. These directions aim to bridge laboratory demonstrations with the deployment of scalable, high-dimensional fiber networks that harness OAM to significantly boost capacity, resilience, and functionality.