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Optica

Self-healing of multipartite entanglement in optical quantum networks

Xutong Wang, Jing Wen Fu, Shengshuai Liu, Yao Wei, Jietai Jing

Published Jun 6, 2022• 17 citations
Publisher

Abstract

Multipartite entanglement serves as an essential resource for constructing quantum networks and makes it possible to realize multi-user quantum information protocols outperforming their classical counterparts. Unfortunately, multipartite entanglement is fragile when distributed in complex environments. Therefore, it is urgent to address the issue of multipartite entanglement decoherence caused by complex environments. Here we demonstrate the self-healing of multipartite continuous-variable (CV) entanglement after an obstruction. In our experiment, the tripartite entanglement destroyed by the obstruction-introduced noise and loss can self-heal after free-space propagation of a certain distance due to the self-healing property of a Bessel–Gaussian (BG) beam. We show that the BG beam provides a more robust mode basis for free-space CV quantum entanglement distribution than the widely used fundamental Gaussian beam under similar conditions. In addition, we also demonstrate the self-healing of five sets of tripartite entanglement and 10 sets of bipartite entanglement in orbital-angular-momentum-multiplexed optical quantum networks. Our results pave the way for constructing obstruction-tolerant high-capacity CV optical quantum networks.

DOI
10.1364/optica.458939

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Multipartite entanglement serves as an essential resource for constructing quantum networks and makes it possible to realize multi-user quantum information protocols outperforming their classical counterparts. Unfortunately, multipartite entanglement is fragile when distributed in complex environments. Therefore, it is urgent to address the issue of multipartite entanglement decoherence caused by complex environments. Here we demonstrate the self-healing of multipartite continuous-variable (CV) entanglement after an obstruction. In our experiment, the tripartite entanglement destroyed by the obstruction-introduced noise and loss can self-heal after free-space propagation of a certain distance due to the self-healing property of a Bessel–Gaussian (BG) beam. We show that the BG beam provides a more robust mode basis for free-space CV quantum entanglement distribution than the widely used fundamental Gaussian beam under similar conditions. In addition, we also demonstrate the self-healing of five sets of tripartite entanglement and 10 sets of bipartite entanglement in orbital-angular-momentum-multiplexed optical quantum networks. Our results pave the way for constructing obstruction-tolerant high-capacity CV optical quantum networks.
A quantum communication network can be constructed by distributing a multipartite entangled state to space-separated nodes. Entangled optical beams with highest flying speed and measurable brightness can be used as carriers to convey information in quantum communication networks. Losses and noises existing in real communication channels will reduce or even totally destroy entanglement. The phenomenon of disentanglement will result in the complete failure of quantum communication. Here, we present the experimental demonstrations on the disentanglement and the entanglement revival of tripartite entangled optical beams used in a quantum network. We experimentally demonstrate that symmetric tripartite entangled optical beams are robust in pure lossy but noiseless channels. In a noisy channel, the excess noise will lead to the disentanglement and the destroyed entanglement can be revived by the use of a correlated noisy channel (non-Markovian environment). The presented results provide useful technical references for establishing quantum networks.
Abstract Multipartite entanglement is the crucial requirement for quantum information processing, quantum computation, quantum communication and quantum sensing. Therefore, the generation and characterization of such entanglement are primary goals in quantum technologies. Here we theoretically study the multipartite continuous-variable quadrature entanglement generated by three symmetric cascaded nonlinear optical systems. Such cascaded systems allow us to study the bipartite, tripartite and quadripartite entanglement. We use different continuous-variable entanglement criteria to characterize the entanglement. We observe that only a few bipartite combinations show the entanglement out of all possible combinations, while most tripartite combinations are entangled. Interestingly such a system depicts a full quadripartite entanglement. Moreover, we show that, unlike the quadrature sum-variance entanglement criterion, the negative partial transpose criterion can show the entanglement in most cases. The present comprehensive analysis of the generation and characterization of the continuous-variable entanglement with cascaded systems would pave the way to realizing a complex quantum network.
We demonstrate the experimental generation of orbital angular momentum (OAM) multiplexed multipartite entanglement with cascaded four-wave mixing processes in a continuous variable (CV) system. In particular, we implement the simultaneous generation of 9 sets of OAM multiplexed tripartite entanglement over 27 Laguerre-Gauss (LG) modes, as well as 20 sets of OAM multiplexed bipartite entanglement over 40 LG modes, which show the rich entanglement structure of the system. In addition, we also generate tripartite entanglement of three types of coherent OAM superposition modes. Such OAM multiplexed multipartite entanglement opens the avenue to construct CV parallel quantum network for realizing parallel quantum information protocols.
Quantum entanglement shared by more than two partites is the essential bases for developing quantum communication networks and quantum computation. The multipartite entanglement of continuous variables can be generated by distributing or combining squeezed states of light using linear optics. We experimentally obtained a tripartite entangled state of bright optical field by means of a nondegenerate optical parametric amplifier (NOPA) and optical beam-splitters. The NOPA is operated in the state of parametric deamplification by locking the relative phase between the pump laser and the injected seed wave to (2n+1)π, so the generated Einstein-Podolsky-Rosen (EPR) beam for continuous variables is an entangled state of light with the quantum correlation of phase quadratures and the quantum anticorrelation of amplitude quadratures [1]. The output EPR beam consists of two frequency-degenerate optical modes with horizontal and vertical polarizations which are distributed to three parties with two linear beam-splitters. We have theoretically and experimentally demonstrated the three parties are in a tripartite entangled state with three-mode “position” (amplitude quadrature) correlation and relative “momentum” (phase quadrature) correlation [2]-[4]. The obtained GHZ-like state is a fully inseparable tripartite continuous-variables state violating the three inequalities used for the criterion of the inseparability [5][6]. The controlled dense coding quantum communication is experimentally accomplished by exploiting the tripartite entanglement. The three entangled modes are sent to a sender (Alice), a receiver (Bob) and a controller (Claire), respectively. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity completed under Claire’s help is always larger than that without her help.
Entanglement in bipartite continuous-variable systems is investigated in the presence of partial losses such as those introduced by a realistic quantum communication channel, e.g., by propagation in an optical fiber. We find that entanglement can vanish completely for partial losses, in a situation reminiscent of so-called entanglement sudden death. Even states with extreme squeezing may become separable after propagation in lossy channels. Having in mind the potential applications of such entangled light beams to optical communications, we investigate the conditions under which entanglement can survive for all partial losses. Different loss scenarios are examined, and we derive criteria to test the robustness of entangled states. These criteria are necessary and sufficient for Gaussian states. Our study provides a framework to investigate the robustness of continuous-variable entanglement in more complex multipartite systems.
Entanglement in bipartite continuous-variable systems is investigated in the presence of partial losses, such as those introduced by a realistic quantum communication channel, e.g. by propagation in an optical fiber. We find that entanglement can vanish completely for partial losses, in a situa- tion reminiscent of so-called entanglement sudden death. Even states with extreme squeezing may become separable after propagation in lossy channels. Having in mind the potential applications of such entangled light beams to optical communications, we investigate the conditions under which entanglement can survive for all partial losses. Different loss scenarios are examined and we derive criteria to test the robustness of entangled states. These criteria are necessary and sufficient for Gaussian states. Our study provides a framework to investigate the robustness of continuous-variable entanglement in more complex multipartite systems.