arXiv subjects

Haijun Kang

Publications and source records attributed to Haijun Kang.

9 recordsLinked to original sources

arXiv2025

Maximal coin-position entanglement and non-Hermitian skin effect in discrete-time quantum walks

A distinctive feature of non-Hermitian systems is the skin effect, which has attracted widespread attention in recent studies. Quantum walks provide a powerful platform for exploring the underlying mechanisms of the non-Hermitian skin effect. Additionally, the generation of hybrid entanglement in quantum walks is recognized as another crucial property. However, the experimentally exploring the influence of skin effect on the evolution of entanglement dynamical in the non-Hermitian system remains a challenge. In this paper, we present a flexible photonic implementation of discrete-time quantum walks over 20 evolution steps using an optimized time-multiplexed loop configuration. Through optimizing the coin parameter, we achieve maximal coin-position entanglement in 20-steps quantum walks. Moreover, we experimentally measure the polarization-averaged growth rates and the evolution of coin-position entanglement for specific coin and loss parameters. We observe the asymmetric Lyapunov exponent profiles and the suppression of entanglement induced by the skin effect in non-Hermitian systems. Interestingly, this entanglement suppression weakens with increasing coin parameters and enhances with increasing loss parameters and evolution steps. Our results demonstrate the potential of quantum walks as a powerful platform for investigating hybrid entanglement properties and skin effect in non-Hermitian systems.

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arXiv2022

Distillation of Gaussian Einstein-Podolsky-Rosen steering with noiseless linear amplification

Einstein-Podolsky-Rosen (EPR) steering is one of the most intriguing features of quantum mechanics and an important resource for quantum communication. For practical applications, it remains a challenge to protect EPR steering from decoherence due to its intrinsic difference from entanglement. Here, we experimentally demonstrate the distillation of Gaussian EPR steering and entanglement in lossy and noisy environments using measurement-based noiseless linear amplification. Different from entanglement distillation, the extension of steerable region happens in the distillation of EPR steering besides the enhancement of steerabilities. We demonstrate that the two-way or one-way steerable region is extended after the distillation of EPR steering when the NLA is implemented based on Bob's or Alice's measurement results. We also show that the NLA helps to extract secret key from insecure region in one-sided device-independent quantum key distribution with EPR steering. Our work paves the way for quantum communication exploiting EPR steering in practical quantum channels.

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arXiv2022

Quantum coherence of an orbital angular momentum multiplexed continuous-variable entangled state

Orbital angular momentum (OAM) multiplexed entangled state is an important quantum resource for high dimensional quantum information processing. In this paper, we experimentally quantify quantum coherence of OAM multiplexed continuous-variable (CV) entangled state and characterize its evolution in a noisy environment. We show that the quantum coherence of the OAM multiplexed CV entangled state carrying topological charges $l=1$ and $l=2$ are the same as that of the Gaussian mode with $l=0$ in a noisy channel. Furthermore, we show that the quantum coherence of OAM multiplexed entangled state is robust to noise, even though the sudden death of entanglement is observed. Our results provide reference for applying quantum coherence of OAM multiplexed CV entangled state in a noisy environment.

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arXiv2021

Experimental demonstration of robustness of Gaussian quantum coherence

Besides quantum entanglement and steering, quantum coherence has also been identified as a useful quantum resource in quantum information. It is important to investigate the evolution of quantum coherence in practical quantum channels. In this paper, we experimentally quantify the quantum coherence of a squeezed state and a Gaussian Einstein-Podolsky-Rosen (EPR) entangled state transmitted in Gaussian thermal noise channel, respectively. By reconstructing the covariance matrix of the transmitted states, quantum coherence of these Gaussian states is quantified by calculating the relative entropy. We show that quantum coherence of the squeezed state and the Gaussian EPR entangled state is robust against loss and noise in a quantum channel, which is different from the properties of squeezing and Gaussian entanglement. Our experimental results pave the way for application of Gaussian quantum coherence in lossy and noisy environments.

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arXiv2021

Quantifying quantum coherence of optical cat states

Optical cat state plays an essential role in quantum computation and quantum metrology. Here, we experimentally quantify quantum coherence of an optical cat state by means of relative entropy and l_1 norm of coherence in Fock basis based on the prepared optical cat state at rubidium D1 line. By transmitting the optical cat state through a lossy channel, we also demonstrate the robustness of quantum coherence of optical cat state in the presence of loss, which is different from the decoherence properties of fidelity and Wigner function negativity of the optical cat state. Our results confirm that quantum coherence of optical cat states is robust against loss and pave the way for the application with optical cat states.

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arXiv2021

Deterministic distribution of multipartite entanglement and steering in a quantum network by separable states

As two valuable quantum resources, Einstein-Podolsky-Rosen entanglement and steering play important roles in quantum-enhanced communication protocols. Distributing such quantum resources among multiple remote users in a network is a crucial precondition underlying various quantum tasks. We experimentally demonstrate the deterministic distribution of two- and three-mode Gaussian entanglement and steering by transmitting separable states in a network consisting of a quantum server and multiple users. In our experiment, entangled states are not prepared solely by the quantum server, but are created among independent users during the distribution process. More specifically, the quantum server prepares separable squeezed states and applies classical displacements on them before spreading out, and users simply perform local beam-splitter operations and homodyne measurements after they receive separable states. We show that the distributed Gaussian entanglement and steerability are robust against channel loss. Furthermore, one-way Gaussian steering is achieved among users that is useful for further directional or highly asymmetric quantum information processing.

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arXiv2020

Quantum Noise of Kramers-Kronig Receiver

The Kramers-Kronig (KK) receiver provides an efficient method to reconstruct the complex-valued optical field by means of intensity detection given a minimum-phase signal. In this paper, we analytically show that for detecting coherent states through measuring the minimum-phase signal, while keeping the radial quantum fluctuation the same as the balanced heterodyne detection does, the KK receiver can indirectly recover the tangential component with fluctuation equivalently reduced to 1/3 times the radial one at the decision time, by adopting the KK relations to utilize the information of the physically measured radial component of other time of the symbol period. In consequence, the KK receiver achieves 3/2 times the signal-to-noise ratio of balanced heterodyne detection, while presenting an asymmetric quantum fluctuation distribution depending on the time-varying phase. Therefore, the KK receiver provides a feasible scheme to reduce the quantum fluctuation for obtaining the selected component to 2/ 3 times that of physically measuring the same component of the coherent state. This work provides a physical insight of the KK receiver and should enrich the knowledge of electromagnetic noise in quantum optical measurement.

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arXiv2019

Experimental test of error-disturbance uncertainty relation with continuous variables

Uncertainty relation is one of the fundamental principle in quantum mechanics and plays an important role in quantum information science. We experimentally test the error-disturbance uncertainty relation (EDR) with continuous variables for Gaussian states. Two conjugate continuous-variable observables, amplitude and phase quadratures of an optical mode, are measured simultaneously by using a heterodyne measurement system. The EDR with continuous variables for a coherent state, a squeezed state and a thermal state are verified experimentally. Our experimental results demonstrate that Heisenberg's EDR with continuous variables is violated, yet Ozawa's and Branciard's EDR with continuous variables are validated.

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arXiv2019

Experimental test of error-tradeoff uncertainty relation using a continuous-variable entangled state

Heisenberg's original uncertainty relation is related to measurement effect, which is different from the preparation uncertainty relation. However, it has been shown that Heisenberg's error-disturbance uncertainty relation can be violated in some cases. We experimentally test the error-tradeoff uncertainty relation by using a continuous-variable Einstein-Podolsky-Rosen (EPR) entangled state. Based on the quantum correlation between the two entangled optical beams, the errors on amplitude and phase quadratures of one EPR optical beam coming from joint measurement are estimated respectively, which are used to verify the error-tradeoff relation. Especially, the error-tradeoff relation for error-free measurement of one observable is verified in our experiment. We also verify the error-tradeoff relations for nonzero errors and mixed state by introducing loss on one EPR beam. Our experimental results demonstrate that Heisenberg's error-tradeoff uncertainty relation is violated in some cases for a continuous-variable system, while the Ozawa's and Brainciard's relations are valid.

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