Pawel Masiak, Peter L. Knight
We investigate the degree to which entanglement survives when a correlated pair of two-state systems are copied using either local or non-local processes. We show how the copying process degrades the entanglement, due to a residual correlation between the copied output and the copying machine (itself made of qubits).
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We investigate the problem of copying pure two-qubit states of a given degree of entanglement in an optimal way. Completely positive covariant quantum operations are constructed which maximize the fidelity of the output states with respect to two separable copies. These optimal copying processes hint at the intricate relationship between fundamental laws of quantum theory and entanglement.
In classical information theory one can, in principle, produce a perfect copy of any input state. In quantum information theory, the no cloning theorem prohibits exact copying of nonorthogonal states. Moreover, if we wish to copy multiparticle entangled states and can perform only local operations and classical communication (LOCC), then further restrictions apply. We investigate the problem of copying orthogonal, entangled quantum states with an entangled blank state under the restriction to LOCC. Throughout, the subsystems have finite dimension D. We show that if all of the states to be copi…
In the quantum system, perfect copying is impossible without prior knowledge. But, perfect copying is possible, if it is known that unknown states to be copied is contained by the set of orthogonal states, which is called the copied set. However, if our operation is limited to local operations and classical communications, this problem is not trivial. Recently, F. Anselmi, A. Chefles and M.B. Plenio constructed theory of local copying when the copied set consists of maximally entangled states. They also classified the copied set when it consists of two orthogonal states (New. J. Phys. 6, 164 (…
Recently, Li. \emph{et. al.} [Int. J. Theor. Phys., 48, 2777 (2009)] derived a necessary and sufficient condition for LOCC cloning of a set of bipartite orthogonal partially but equally entangled state. We demonstrates that, the result is based on a wrong observation regarding a set of non-maximally entangled states with equal entanglement. We also provide a simple example in favor of our comment.
We study the degree to which quantum entanglement survives when a three-qubit entangled state is copied by using local and non-local processes, respectively, and investigate iterating quantum copying for the three-qubit system. There may exist inter-three-qubit entanglement and inter-two-qubit entanglement for the three-qubit system. We show that both local and non-local copying processes degrade quantum entanglement in the three-particle system due to a residual correlation between the copied output and the copying machine. We also show that the inter-two-qubit entanglement is preserved bette…
We show that inseparability of quantum states can be partially broadcasted (copied, cloned) with the help of local operations, i.e. distant parties sharing an entangled pair of spin 1/2 states can generate two pairs of partially nonlocally entangled states using only local operations. This procedure can be viewed as an inversion of quantum purification procedures.
In this work,We investigate the problem of secretly broadcasting of three-qubit entangled state between two distant partners. The interesting feature of this problem is that starting from two particle entangled state shared between two distant partners we find that the action of local cloner on the qubits and the measurement on the machine state vector generates three-qubit entanglement between them. The broadcasting of entanglement is made secret by sending the measurement result secretly using cryptographic scheme based on orthogonal states. Further we show that this idea can be extended to …
We focus on the non-locality concerning local copying and local discrimination, especially for a set of orthogonal maximally entangled states in prime dimensional systems, as a study of non-locality of a set of states. As a result, for such a set, we completely characterize deterministic local copiability and show that local copying is more difficult than local discrimination. From these result, we can conclude that lack algebraic symmetry causes extra non-locality of a set.
We investigate how entangled inertial Unruh-DeWitt detectors are affected by interaction with a quantum field using a nonperturbative method. Inertial detectors in a $(3+1)$-dimensional Minkowski spacetime with instantaneous switching ($δ$-switching) experience degradation of their initial entanglement as their coupling strength with a scalar field increases. Somewhat surprisingly, initially separable or weakly entangled detectors can extract mutual information from the vacuum. We also find that entanglement degradation is not reduced if communication via the field is possible; rather this onl…
We study broadcasting of entanglement where we use universal quantum cloners (in general less optimal) to perform local cloning operations. We show that there is a lower bound on the fidelity of the universal quantum cloners that can be used for broadcasting. We prove that an entanglement is optimally broadcast only when optimal quantum cloners are used for local copying. We also show that broadcasting of entanglement into more than two entangled pairs is forbidden using only local operations.
The entanglement of different classes of initially entangled qubit pair is investigated in the presence of short laser pulses of rectangular and exponential shapes with either one or both qubits are excited. For the rectangular pulse, the detuning parameter protects the entanglement from degradation and controls its upper and lower bounds. We show that the upper bounds of entanglement decrease if both particles are excited, but the lower bounds are much better if only one particle is excited. The phenomena of entanglement degradation, sudden- death and -birth are shown for small initial time. …
The distillability of bipartite entangled state as seen by moving observers has been investigated. It is found that the same initial entanglement for a state parameter $α$ and its "normalized partner" $\sqrt{1-α^2}$ will be degraded as seen by moving observer. It is shown that in the ultra relativistic limit, the state does not have distillable entanglement for any $α$.
The entanglement between two modes of the free scalar and Dirac fields as seen by two relatively accelerated observers has been investigated. It is found that the same initial entanglement for an initial state parameter $α$ and its "normalized partner" $\sqrt{1-α^{2}}$ will be degraded by the Unruh effect along two different trajectories except for the maximally entangled state, which just shows the inequivalence of the quantization for a free field in the Minkowski and Rindler coordinates. In the infinite acceleration limit the state doesn't have the distillable entanglement for any $α$ for t…
For carrying out many quantum information protocols entanglement must be established in advance between two distant parties. Practically, inevitable interaction of entangled subsystems with their environments during distribution and storage will result in degradation of entanglement. Here we investigate the decoherence of two-qubit entangled states in the local amplitude noise. We show that there exists a set of partially entangled states that are more robust than maximally entangled states in terms of the residual quantum correlation measured by concurrence, fully entangled fraction, and quan…
Entanglement degradation under local dissipative Landau-Zener noise
本研究探讨耗散Landau-Zener噪声下纠缠退化，发现耦合方向及驱动速度影响纠缠保持，横向耦合和快驱动更有利。
The recent surface plasmon entanglement experiment [E. Altewischer et al., Nature (London) 418, 304 (2002)] is theoretically analyzed. The entanglement preservation upon transmission in the non-focused case is found to provide information about the interaction of the biphoton and the metallic film. The entanglement degradation in the focused case is explained in the framework of a fully multimode model. This phenomenon is a consequence of the polarization-selective filtering behavior of the metallic nanostructured film.
The entanglement behavior of two classes of multi-qubit system, GHZ and GHZ like states passing through a generalized amplitude damping channel is discussed. Despite this channel causes degradation of the entangled properties and consequently their abilities to perform quantum teleportation, one can always improve the lower values of the entanglement and the fidelity of the teleportrd state by controlling on Bell measurements, analyzer angle and channel's strength. Using GHZ-like state within a generalized amplitude damping channel is much better than using the normal GHZ-state, where the deca…
Entangled photon pairs are key to many novel applications in quantum technologies. Semiconductor quantum dots can be used as sources of on-demand, highly entangled photons. The fidelity to a fixed maximally entangled state is limited by the excitonic fine-structure splitting. This work demonstrates that, even if this splitting is absent, the degree of entanglement cannot reach unity when the excitation pulse in a two-photon resonance scheme has a finite duration. The degradation of the entanglement has its origin in a dynamically induced splitting of the exciton states caused by the laser puls…
The influence of losses in the interferometric generation and the transmission of continuous-variable entangled light is studied, with special emphasis on Gaussian states. Based on the theory of quantum-state transformation at absorbing dielectric devices, the amount of entanglement is quantified by means of the relative-entropy measure. Upper bounds of entanglement and the distance to the set of separable Gaussian states are calculated. Compared with the distance measure, the bounds can substantially overestimate the entanglement. In particular, they do not show the drastic decrease of entang…
We experimentally realize a high-intensity three-photon Greenberger-Horne-Zeilinger (GHZ) entanglement source directly following the proposal by Rarity and Tapster [J. G. Rarity and P. R. Tapster, Phys. Rev. A 59, R35 (1999)]. The threefold coincidence rate can be more than 200 Hz with a fidelity of 0.811, and the intensity can be further improved with moderate fidelity degradation. The GHZ entanglement is characterized by testing the Bell-Mermin inequality and using an entanglement witness operator. To optimize the polarization-entangled source, we theoretically analyze the relationship betwe…
Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots
本文研究双光子激发与声子对量子点纠缠光子对的影响，发现声子即使在零温也通过纯退相干和辅助过程降低纠缠度，且纠缠度随温度或脉冲时长增加而下降，并受精细结构分裂影响。
We address the degradation of continuous variable (CV) entanglement in a noisy channel focusing on the set of photon-number entangled states. We exploit several separability criteria and compare the resulting separation times with the value of non-Gaussianity at any time, thus showing that in the low-temperature regime: i) non-Gaussianity is a bound for the relative entropy of entanglement and ii) Simon' criterion provides a reliable estimate of the separation time also for nonGaussian states. We provide several evidences supporting the conjecture that Gaussian entanglement is the most robust …
We analyse decoherence properties of entangled coherent states (ECSs) due to channel losses, and compare to the performance of bi-photon Bell states. By employing the concept of "entanglement of formation" (EOF), the degradation of fidelity and degree of entanglement are calculated. Two situations are considered: (1) asymmetric and (2) symmetric noise channel. In the first case, only the lower bound of EOF is given. However, in the latter case, we have obtained an explicit expression of concurrence (and then EOF) and found surprisingly our result is just incompatible with that of [Phys. Rev. A…
Entanglement Degradation in Causal Diamonds
本文研究因果钻石视界对二体纠缠的影响，证明惯性观测者 Alice 与寿命受限的钻石观测者 Dave 共享的最大纠缠态会因因果视界而发生退化。
The influence of losses in the transmission of continuous-variable entangled light through linear devices such as optical fibers is studied, with special emphasis on Gaussian states. Upper bounds on entanglement and the distance to the set of separable Gaussian states are calculated. Compared with the distance measure, the bounds can substantially overestimate the entanglement and thus do not show the drastic decrease of entanglement with increasing mean photon number, as does the distance measure. In particular, it shows that losses give rise to entanglement saturation, which principally limi…
Radiative Processes of Entangled Detectors in Rotating Frames
该研究利用一阶微扰理论分析了旋转参考系中与有质量标量场作用的纠缠Unruh-DeWitt探测器的辐射过程，探讨了纠缠态寿命及纠缠的提取与退化。
The distribution of entangled states between distant parties in an optical network is crucial for the successful implementation of various quantum communication protocols such as quantum cryptography, teleportation and dense coding [1-3]. However, owing to the unavoidable loss in any real optical channel, the distribution of loss-intolerant entangled states is inevitably inflicted by decoherence, which causes a degradation of the transmitted entanglement. To combat the decoherence, entanglement distillation, which is the process of extracting a small set of highly entangled states from a large…
Entangled Unruh-DeWitt detectors amplify quantum coherence
本研究采用非微扰方法探究纠缠Unruh-DeWitt探测器与量子场的相互作用，发现其可在降低纠缠的同时放大量子相干性，揭示了两种量子资源的本质差异与互补性。
We study the entanglement of families of Unruh modes in the Bell states $|Φ^\pm\rangle =1/\sqrt{2}(|00\rangle\pm|11\rangle)$ and $|Ψ^\pm\rangle=1/\sqrt{2}(|01\rangle\pm|10\rangle)$ shared by two accelerated observers and find fundamental differences in the robustness of entanglement against acceleration for these states. States $Ψ^\pm$ are entangled for all finite accelerations, whereas, due to the Unruh effect, states $Φ^\pm$ lose their entanglement for finite accelerations. This is true for Bell states of two bosonic modes, as well as for Bell states of a bosonic and a fermionic mode. Furthe…
Rapid and efficient wavefront correction for spatially entangled photons using symmetrized optimization
提出对称化遗传算法（sGA）用于空间纠缠光子的自适应波前校正，利用偶宇称分量影响双光子相关性的洞见将参数空间减半，实现比标准遗传算法更快的收敛与更高的增强效果，从而有效校正复杂介质中的纠缠退化。
In this work, there are two parties, Alice on Earth and Bob on the satellite, which initially share an entangled state, and some open problems, which emerge during quantum steering that Alice remotely steers Bob, are investigated. Our analytical results indicate that all entangled pure states and maximally entangled evolution states (EESs) are steerable, and not every entangled evolution state is steerable and some steerable states are only locally correlated. Besides, quantum steering from Alice to Bob experiences a "sudden death" with increasing decoherence strength. However, shortly after t…
Non-Maximally Entangled States for Quantum Key Distribution in Underwater Channels: BBM92 Protocol via Kraus Operators
针对水下信道吸收和散射对量子通信的影响，本文利用Kraus算符建模水下信道，研究基于非最大纠缠态的BBM92协议，推导了量子比特误码率和密钥率的解析表达式，并在不同水质条件下验证了协议性能。
Although it is widely accepted that classical information cannot travel faster than the speed of light in vacuum, the behavior of quantum correlations and quantum information propagating through actively-pumped fast-light media has not been studied in detail. To investigate this behavior, we send one half of an entangled state of light through a gain-assisted fast-light medium and detect the remaining quantum correlations. We show that the quantum correlations can be advanced by a small fraction of the correlation time while the entanglement is preserved even in the presence of noise added by …
Future quantum optical networks will require the ability to route entangled photons at high speeds, with minimal loss and added in-band noise, and---most importantly---without disturbing the photons' quantum state. Here we present an all-optical switch which fulfills these requirements and characterize its performance at the single photon level. It exhibits a 200-ps switching window, 120:1 contrast, 1.5-dB loss, and induces no measurable degradation in the switched photons' entangled-state fidelity (< 0.002). As a proof-of-principle demonstration of its capability, we use the switch to demulti…
When More Light Means Less Quantum: Modeling Bell Inequality Degradation from Accidental Counts
针对光子 Bell 测试中偶然计数随泵浦功率增强而削弱非定域性的问题，本文提出并拟合 Bell 值与泵浦强度关联的噪声模型，实现跨泵浦设置的准确预测。
We study entanglement degradation of two non-interacting qubits subject to independent baths with broadband spectra typical of solid state nanodevices. We obtain the analytic form of the concurrence in the presence of adiabatic noise for classes of entangled initial states presently achievable in experiments. We find that adiabatic (low frequency) noise affects entanglement reduction analogously to pure dephasing noise. Due to quantum (high frequency) noise, entanglement is totally lost in a state-dependent finite time. The possibility to implement on-chip both local and entangling operations …
Quantum entanglement is notorious for being a very fragile resource. Significant efforts have been put into the study of entanglement degradation in the presence of a realistic noisy environment. Here, we present a theoretical and an experimental study of the decoherence properties of entangled pairs of qubits. The entanglement dynamics of maximally entangled qubit pairs is shown to be related in a simple way to the noise representation in the Bloch sphere picture. We derive the entanglement level in the case when both qubits are transmitted through any arbitrary unital Pauli channel, and comp…
To deploy and operate a quantum network which utilizes existing telecommunications infrastructure, it is necessary to be able to route entangled photons at high speeds, with minimal loss and signal-band noise, and---most importantly---without disturbing the photons' quantum state. Here we present a switch which fulfills these requirements and characterize its performance at the single photon level; it exhibits a 200-ps switching window, a 120:1 contrast ratio, 1.5 dB loss, and induces no measurable degradation in the switched photons' entangled-state fidelity (< 0.002). Furthermore, because th…
The inevitable dissipative interaction of an entangled quantum system with its environment causes degradation in quantum correlations present in the system. This can lead to a finite-time disappearance of entanglement, which is known as entanglement sudden death (ESD). Here, we consider an initially entangled qubit-qutrit system and a dissipative noise which leads to ESD, and propose a set of local unitary operations, which when applied on the qubit, qutrit, or both subsystems during the decoherence process, cause ESD to be hastened, delayed, or avoided altogether, depending on its time of app…
We investigate the dynamics of two qubits state through the Bloch channel. It is found that the degradation and sudden-death of the entanglement depend on the channel's parameters and the structure of the input state. Starting from partially entangled states as input state, the output states are more robust compared with those obtained from initial maximally entangled states. Also the survivability of entanglement increased as the absolute equilibrium values of the channel increased or the ratio between the longitudinal and transverse relaxation times gets smaller. Ability of using the output …
Extraction of entanglement from quantum fields with entangled particle detectors
本文研究两个初始纠缠的 Unruh-DeWitt 粒子探测器与量子标量场作用后的纠缠变化，发现初始弱关联的探测器可提取更多纠缠，而充分纠缠的探测器仅发生退化，并证明引力冲击波时空中能在更大间距下增强弱纠缠探测器的初始纠缠。
We study the role of driving in an initial maximally entangled state evolving under the presence of a structured environment in a weak and strong regime. We focus on the enhancement and degradation of maximal Concurrence when the system is driven on and out of resonance for a general evolution, as well as the effect of adding a transverse coupling among the particles of the model. We further investigate the role of driving in the acquisition of a geometric phase for the maximally entangled state. As the model studied herein can be used to model experimental situations such as hybrid quantum cl…
Advantages and disadvantages of maximally entangled states in dilaton black hole background
研究发现，在稀顿黑洞背景中，非最大纠缠态可能在引力效应下提供操作优势，而量子相干性则表现出更强的抗退化能力。
We present the experimental realization of a theoretical effect discovered by Olivares and Paris, in which a pair of entangled optical beams undergoing independent losses can see nonlocal correlations restored by the use of a nonlocal resource correlating the losses. Twin optical beams created in an entangled Einstein-Podolsky-Rosen (EPR) state by an optical parametric oscillator above threshold were subjected to 50% loss from beamsplitters in their paths. The resulting severe degradation of the signature quantum correlations observed between the two beams was then suppressed when another, ind…
Beyond the four-level model: Dark and hot states in quantum dots degrade photonic entanglement
本研究测量GaAs量子点在最高65 K下双激子—激子级联光子对的偏振纠缠，结合附加热态、暗态及声子辅助跃迁解释纠缠退化，为高温纠缠光子源优化提供依据。
We analyze the entanglement degradation provoked by the Hawking effect in a bipartite system Alice-Rob when Rob is in the proximities of a Schwarzschild black hole while Alice is free falling into it. We will obtain the limit in which the tools imported from the Unruh entanglement degradation phenomenon can be used properly, keeping control on the approximation. As a result, we will be able to determine the degree of entanglement as a function of the distance of Rob to the event horizon, the mass of the black hole, and the frequency of Rob's entangled modes. By means of this analysis we will s…
The unavoidable and irreversible interaction between an entangled quantum system and its environment causes decoherence of the individual qubits as well as degradation of the entanglement between them. Entanglement sudden death (ESD) is the phenomenon wherein disentanglement happens in finite time even when individual qubits decohere only asymptotically in time due to noise. Prolonging the entanglement is essential for the practical realization of entanglement-based quantum information and computation protocols. For this purpose, the local NOT operation in the computational basis on one or bot…
We analyse the quantum mechanical limits to the plasmon-assisted entanglement transfer observed by E. Altewischer, M.P. van Exter, and J.P. Woerdman [Nature, 418, 304 (2002)]. The maximal violation S of Bell's inequality at the photodetectors behind two linear media (such as the perforated metal films in the experiment) can be described by two ratio's tau_1, tau_2 of polarization-dependent transmission probabilities. A fully entangled incident state is transferred without degradation for tau_1=tau_2, but a relatively large mismatch of tau_1 and tau_2 can be tolerated with a small reduction of …
Non-local temporal interferometry for highly resilient free-space quantum communication
本文通过部署相位稳定的非局域Franson干涉仪，在10.2公里自由空间分发多维能量-时间纠缠光子，实现维度自适应QKD协议，获得正渐近密钥率，展示了高维纠缠在量子通信中的潜力。
Characterizing Noise Effects on Multipartite Entanglement via Phase-Space Visualization
本文通过相空间可视化研究噪声对多体纠缠态的影响，利用自旋Wigner函数分析纠缠结构在噪声下的响应，为量子计算中的抗噪声协议设计提供理论支持。
Enhanced quantum emission from a topological Floquet resonance
本研究利用非磁性可调谐反常Floquet绝缘体，通过自发四波混频增强纠缠光子对产生，验证了非经典性与时间-能量纠缠，为鲁棒量子源提供新途径。
Satellite-Aided Entanglement Distribution for Optimized Quantum Networks
针对量子互联网中纠缠预分发效率问题，提出卫星辅助纠缠分发（SED）算法，通过卫星缩短拓扑减少量子比特使用，数值结果显示SED显著提升小型网络性能，而纠缠共享约束对大型网络关键。
We investigate the degradation of quantum entanglement in the Schwarzschild-de Sitter black hole spacetime, by studying the mutual information and the logarithmic negativity for maximally entangled, bipartite states for massless minimal scalar fields. This spacetime is endowed with a black hole as well as a cosmological event horizon, giving rise to particle creation at two different temperatures. We consider two independent descriptions of thermodynamics and particle creation in this background. The first involves thermal equilibrium of an observer with either of the horizons. We show that as…
Entanglement degradation of static black holes in effective quantum gravity
本文研究了有效量子引力中第三类黑洞对标量和狄拉克场量子纠缠退化的机制，揭示了量子参数对引力压制量子相关性的保护作用。
We introduce an arbitrary number of accessible modes when analyzing bipartite entanglement degradation due to Unruh effect between two partners Alice and Rob. Under the single mode approximation (SMA) a fermion field only had a few accessible levels due to Pauli exclusion principle, conversely to bosonic fields which had an infinite number of excitable levels. This was argued to justify entanglement survival in the fermionic case in the SMA infinite acceleration limit. Here we relax SMA. Hence, an infinite number of modes are excited as the observer Rob accelerates, even for a fermion field. W…
Distributed Entanglement Distribution Using Multiple Entanglement Sources in WDM-based Quantum Optical Networks
研究基于WDM的量子光网络中多纠缠源的分布式纠缠分发，采用多跳无中继网状网络架构，利用单光子和双光子纠缠分发的混合方法，实验表征自制EPPS并模拟，发现该方法在EPPS放置上更灵活，能更好利用纠缠资源并接受更高需求。
Quantum illumination via frequency-mode-based correlation-to-displacement conversion
针对量子照明中关联到位移转换因模式数量庞大而难以实现的问题，提出频率模式纠缠源、跨模式外差探测与腔增强量子脉冲门的集成框架。
Quantum properties of fermionic fields in multi-event horizon spacetime
本文研究多事件视界史瓦西-德西特时空中无质量狄拉克场的量子纠缠与互信息，发现Nariai极限下物理可及相关性最大、不可及相关性为零，且初始态保持纠缠可用于量子信息处理，与标量场情形不同。
Improving Entanglement Resilience in Quantum Memories with Error-Detection-Based Distillation
针对量子存储器纠缠退化问题，提出基于[[4,2,2]]量子误差检测码的纠缠蒸馏协议，结合存储阶段恢复策略，可延长存储寿命，为相关场景提供定量评估框架。
We investigate the Hawking effect on entangled fields. By considering a scalar field which is in a two-mode squeezed state from the point of view of freely falling (Kruskal) observers crossing the horizon of a Schwarzschild black hole, we study the degradation of quantum and classical correlations in the state from the perspective of Schwarzschild observers confined outside the horizon. Due to monogamy constraints on the entanglement distribution, we show that the lost bipartite entanglement is recovered as multipartite entanglement among modes inside and outside the horizon. In the limit of a…
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