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Photonics Research

Generation of continuous-variable high-dimensional entanglement with three degrees of freedom and multiplexing quantum dense coding

Hui Guo, Na Liu, Li Zhi, Rongguo Yang, Hengxin Sun, Kui Liu, Jiangrui Gao

Published Sep 27, 2022• 17 citations
Publisher

Abstract

High-dimensional entanglement is a critical foundation for the growing demand for information capacity to implement the high-capacity quantum task. Here, we report continuous-variable high-dimensional entanglement with three degrees of freedom (frequency, polarization, and orbital angular momentum) directly generated with a single type-II optical parametric oscillator (OPO) cavity. By compensating both for dispersion in frequency modes and astigmatism in higher-order transverse modes, the OPO is capable of oscillating simultaneously and outputting thousands of entanglement pairs. The three degrees of freedom high-dimensional entanglement are verified simultaneously possessing frequency comb, spin, and orbital angular momentum entanglement via 14 pairs of Hermite–Gaussian mode correlations measurement. Then, the “space-frequency” multiplexing quantum dense coding communication is also demonstrated by using the entanglement resource. It shows the great superiority of high-dimensional entanglement in implementing the high-capacity quantum task. Apart from an increased channel capacity, it is possible to conduct deterministic high-dimensional quantum protocols, quantum imaging, and especially quantum computing.

DOI
10.1364/prj.469340

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High-dimensional entanglement is a critical foundation for the growing demand for information capacity to implement the high-capacity quantum task. Here, we report continuous-variable high-dimensional entanglement with three degrees of freedom (frequency, polarization, and orbital angular momentum) directly generated with a single type-II optical parametric oscillator (OPO) cavity. By compensating both for dispersion in frequency modes and astigmatism in higher-order transverse modes, the OPO is capable of oscillating simultaneously and outputting thousands of entanglement pairs. The three degrees of freedom high-dimensional entanglement are verified simultaneously possessing frequency comb, spin, and orbital angular momentum entanglement via 14 pairs of Hermite–Gaussian mode correlations measurement. Then, the “space-frequency” multiplexing quantum dense coding communication is also demonstrated by using the entanglement resource. It shows the great superiority of high-dimensional entanglement in implementing the high-capacity quantum task. Apart from an increased channel capacity, it is possible to conduct deterministic high-dimensional quantum protocols, quantum imaging, and especially quantum computing.
To beat the channel capacity limit of conventional quantum dense coding (QDC) with fixed quantum resources, we experimentally implement the orbital angular momentum (OAM) multiplexed QDC (MQDC) in a continuous variable system based on a four-wave mixing process. First, we experimentally demonstrate that the Einstein-Podolsky-Rosen entanglement source coded on OAM modes can be used in a single channel to realize the QDC scheme. Then, we implement the OAM MQDC scheme by using the Einstein-Podolsky-Rosen entanglement source coded on OAM superposition modes. In the end, we make an explicit comparison of channel capacities for four different schemes and find that the channel capacity of the OAM MQDC scheme is substantially enhanced compared to the conventional QDC scheme without multiplexing. The channel capacity of our OAM MQDC scheme can be further improved by increasing the squeezing parameter and the number of multiplexed OAM modes in the channel. Our results open an avenue to construct high-capacity quantum communication networks.
Channel multiplexing quantum communication based on exploiting continuous-variable entanglement of optical modes offers great potential to enhance channel capacity and save quantum resource. Here, we present a frequency-comb-type control scheme for simultaneously extracting a lot of entangled sideband modes with arbitrary frequency detuning from a squeezed state of light. We experimentally demonstrate fourfold channel multiplexing quantum dense coding communication by exploiting the extracted four pairs of entangled sideband modes. Due to high entanglement and wide frequency separation between each entangled pairs, these quantum channels have large channel capacity and the cross talking effect can be avoided. The achieved channel capacities have surpassed that of all classical and quantum communication under the same bandwidth published so far. The presented scheme can be extended to more channels if more entangled sideband modes are extracted.
Continuous variable entanglement is usually set between pairs of optical modes sharing the same geometrical property where distinguishability is demanded to polarization and/or frequency. Thus, the inherent non-local correlation shows-up in quantum quadratures relative to two distinct e.m. modes. In this contribution we will show how the polarization d.o.f. of a pair of entangled modes is coupled to the optical orbital angular momentum giving, at the end, a pair of entangled modes that have orthogonal OAM and polarization. We also show how this experimental scheme can be extended to give more than a pair of entangled modes paving the way to CV entanglement multiplexing.
Spatially structured quantum states, such as orbital angular momentum (OAM) squeezing and entanglement, is currently a popular topic in quantum optics. The method of generating and manipulating spatial quantum states on demand needs to be explored. In this paper, we generated OAM mode squeezed states of −5.4 dB for the L G 0 + 1 mode and −5.3 dB for the L G 0 − 1 mode directly by an optical parametric oscillator (OPO) for the first time. Additionally, we demonstrated that the OAM mode squeezed and entangled states were respectively generated by manipulating the nonlinear process of the OPO by controlling the relative phase of two beams of different modes, thus making two different spatial multimode pump beams. We characterized the Laguerre-Gaussian (LG) entangled states by indirectly measuring the squeezing for the H G 10 ( 45 ∘ ) mode and H G 10 ( 135 ∘ ) mode, and directly measuring the entanglement between the L G 0 + 1 and L G 0 − 1 modes. The effective manipulation of the OAM quantum state provides a novel insight into the continuous variable quantum state generation and construction on demand for high-dimensional quantum information and quantum metrology.
In addition to carrying spin angular momentum (SAM), light can also possess orbital angular momentum (OAM), whose quantum number can, in principle, take any integer value. This property provides a new degree of freedom to expand the capacity of both classical and quantum information. In recent years, generating OAM multiplexed continuous-variable entanglement via four-wave mixing (FWM) has emerged as an effective approach to increasing the capacity of quantum information channels. Consequently, there is a growing demand for precise characterization of the OAM spectrum in such quantum beams. Here we present, to the best of our knowledge, the first experimental demonstration of OAM spectrum measurement in quantum optics using phase-shifting interferometry (PSI). This method enables accurate characterization of the OAM spectrum of both the probe and conjugate beams in OAM multiplexed continuous-variable entanglement generated via the FWM process. These results provide an experimental foundation for the reliable detection and utilization of multiplexed OAM degrees of freedom in quantum communication and quantum information processing.
Entanglement in high-dimensional quantum systems, where one or more degrees of freedom of light are involved, offers increased information capacities and enables new quantum protocols.Here, we demonstrate a functional source of high-dimensional, noise-resilient hyperentangled states encoded in time-frequency and vector vortex structured modes, which in turn carry single-particle entanglement between polarization and orbital angular momentum.Pairing nonlinearity-engineered parametric downconversion in an interferometric scheme with spin-to-orbital-angular-momentum conversion, we generate highly entangled photon pairs at telecom wavelength that we characterize via two-photon interference and quantum state tomography, achieving near-unity visibilities and fidelities.While hyperentanglement has been demonstrated before in photonic qubits, here we present a rich entanglement structure involving spectrally and spatially structured light, where three different forms of entanglement coexist in the same biphoton state.