Jae-Seung Lee, A. K. Khitrin
A scheme for direct projection of a quantum state on Bell states is described. The method is based on constructing an average Hamiltonian with Bell eigenstates and then, projecting the state on these eigenstates. The projection is performed by adding the results of a direct and time-reversed evolution. Experimental demonstration is shown for pairs of dipolar-coupled nuclear spins.
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Numerous quantum information protocols make use of maximally entangled two-particle states, or Bell states, in which information is stored in the correlations between the two particles rather than their individual properties. Retrieving information stored in this way means distinguishing between different Bell states, yet the well known no-go theorem establishes that projective linear evolution and local measurement (LELM) detection schemes can only reliably distinguish three of the four qubit Bell states. We establish maximum distinguishability of the qutrit Bell states of bosons via projecti…
Sharing Bell nonlocality of bipartite high-dimensional pure states using only projective measurements
本文证明仅使用投影测量即可共享任意高维纯两体态的Bell非定域性，推广了已有两量子比特纯态的结果，深化了对高维量子态非定域性共享的理解。
We investigate the steerability of two-qubit Bell-diagonal states under projective measurements by the steering party. In the simplest nontrivial scenario of two projective measurements, we solve this problem completely by virtue of the connection between the steering problem and the joint-measurement problem. A necessary and sufficient criterion is derived together with a simple geometrical interpretation. Our study shows that a Bell-diagonal state is steerable by two projective measurements iff it violates the Clauser-Horne-Shimony-Holt (CHSH) inequality, in sharp contrast with the strict hi…
Measuring an entangled state of two particles is crucial to many quantum communication protocols. Yet Bell state distinguishability using a finite apparatus obeying linear evolution and local measurement is theoretically limited. We extend known bounds for Bell-state distinguishability in one and two variables to the general case of entanglement in $n$ two-state variables. We show that at most $2^{n+1}-1$ classes out of $4^n$ hyper-Bell states can be distinguished with one copy of the input state. With two copies, complete distinguishability is possible. We present optimal schemes in each case…
A protocol is proposed to generate Bell states in two non-directly interacting qubits by means of repeated measurements of the state of a central ancilla connected to both qubits. An optimal measurement rate is found that minimizes the time to stably encode a Bell state in the target qubits, being of advantage in order to reduce detrimental effects from possible interactions with the environment. The quality of the entanglement is assessed in terms of the concurrence and the distance between the qubits state and the target Bell state is quantified by the fidelity.
Bell non-locality represents one of the most striking departures of quantum mechanics from classical physics. It shows that correlations between space-like separated systems allowed by quantum mechanics are stronger than those present in any classical theory. In a recent work [Sci. Adv. 7, eabc3847 (2021)], a family of Bell functionals tailored to mutually unbiased bases (MUBs) is proposed. For these functionals, the maximal quantum violation is achieved if the two measurements performed by one of the parties are constructed out of MUBs of a fixed dimension. Here, we generalize this constructi…
As a way of saving quantum resources, recycling of Bell nonlocality has been experimentally studied, but restricted to sequential unsharp measurements. However, it has been theoretically shown recently that projective measurements are sufficient for recycling nonlocality [Phys. Rev. Lett. \textbf{129}, 230402 (2022)]. Here, we go beyond unsharp measurement scenarios and experimentally demonstrate the recycling of nonlocal resources with projective measurements. By verifying the violation of Clauser-Horne-Shimony-Holt (CHSH) inequality, we find that three independent parties can recycle the Bel…
Generation of bipartite mechanical cat state by performing projective Bell state measurement
提出利用投影贝尔态测量实现纠缠交换，生成四种二分体声子贝尔猫态，并基于CHSH不等式进行贝尔不等式测试，为连续变量量子网络处理器提供新方法。
A class of self-similar sets of entangled quantum states is introduced, for which a recursive definition is provided. These sets, the "Bell gems," are defined by the subsystem exchange symmetry characteristic of the Bell states. Each Bell gem is shown to be an orthonormal basis of maximally entangled elements. A non-trivial example Bell gem is presented. Quantum circuits for producing the elements of this example from the computational basis states are provided.
Non-projective Bell state measurements
该研究定义了多于四种输出的广义非投影Bell态测量（BSM），通过最大纠缠态等角紧框架构造出五结果BSM，证明两量子比特下不存在六结果BSM，并探讨了其在局域态区分等场景的应用，丰富了量子测量与纠缠态几何研究。
Many quantum information protocols require a Bell-state measurement of entangled systems. Most optical Bell-state measurements utilize two-photon interference at a beam splitter. By creating polarization-entangled photons with spontaneous parametric down-conversion using a first-order Hermite-Gaussian pump beam, we invert the usual interference behavior and perform an incomplete Bell-state measurement in the coincidence basis. We discuss the possibility of a complete Bell-state measurement in the coincidence basis using hyperentangled states [Phys. Rev. A, \textbf{58}, R2623 (1998)].
Measurement incompatibility at all remote parties do not always permit Bell nonlocality
本文研究双输入双输出场景下贝尔非局域性与测量不相容性的关系，发现双方采用不相容投影测量总能激活非局域性，但存在一类不相容POVM无法激活，并给出实现非局域关联的充分判据。
We provide an interesting two-party parity oblivious communication game whose success probability is solely determined by the Bell expression. The parity-oblivious condition in an operational quantum theory implies the preparation non-contextuality in an ontological model of it. We find that the aforementioned Bell expression has two upper bounds in an ontological model; the usual local bound and a non-trivial preparation non-contextual bound arising from the non-trivial parity-oblivious condition, which is smaller that the local bound. We first demonstrate the communication game when both Ali…
Special features of the Weyl-Heisenberg Bell basis imply unusual entanglement structure of Bell-diagonal states
本文研究Weyl–Heisenberg标准Bell基的群结构，揭示其导致Pauli信道与twirl信道等价，并使Bell对角态具有最高的PPT及PPT纠缠态占比。
The Bell basis, a set of maximally entangled biphoton state, is a critical prerequisite towards quantum information processing, and many quantum applications have highlighted the requirement for the manipulation of high-dimensional Bell basis. While the Bell states can be created by using ingenious single-photon quantum gates, its implementation complexity in higher dimensions is significantly increased. Here we present an elaborate approach to show that the adaptive pump modulation enable the efficient preparation of Bell basis in arbitrary-dimensional Hilbert space. A complete set of four-di…
Bell-Inequality Violation for Continuous, Non-Projective Measurements
本研究提出了一种基于连续非投影测量的贝尔不等式违反方法，通过相位敏感投影和粗粒化构建有效二元可观测量，展示了在连续测量平台中验证量子非局域性的可行性。
We report the experimental generation of all four frequency-bin Bell states in a single versatile setup via successive pumping of spontaneous parametric downconversion with single and dual spectral lines. Our scheme utilizes intensity modulation to control the pump configuration and offers turn-key generation of any desired Bell state using only off-the-shelf telecommunication equipment. We employ Bayesian inference to reconstruct the density matrices of the generated Bell states, finding fidelities $\geq$ 97% for all cases. Additionally, we demonstrate the sensitivity of the frequency-bin Bel…
We demonstrate that multipartite Bell-inequality violations can be fully destroyed in finite time in three-qubit systems subject only to the mechanism of local external asymptotic dephasing noise. This broadens the study of local-noise-induced sudden death of nonlocal behavior, extending it beyond the realm of bipartite systems, to which it had previously been restricted.
Disjoint Bell measurements enable near-projective GHZ certification
提出BM-Cert协议，仅用不相交两量子比特贝尔基测量实现n量子比特GHZ态的单拷贝验证，渐近达到理想投影认证的谱间隙1-O(1/n)。
The Bell basis is a distinctive set of maximally entangled two-particle quantum states that forms the foundation for many quantum protocols such as teleportation, dense coding and entanglement swapping. While the generation, manipulation, and measurement of two-level quantum states is well understood, the same is not true in higher dimensions. Here we present the experimental generation of a complete set of Bell states in a four-dimensional Hilbert space, comprising of 16 orthogonal entangled Bell-like states encoded in the orbital angular momentum of photons. The states are created by the app…
In this letter, we propose and experimentally test a quantumness criterion for single systems. The criterion is based on the violation of an already reported classical inequality. This inequality is expressed in terms of joint probabilities that we identify as realizations of preparation-and-measurement schemes. Our analysis points out that superposition is the key notion to grasp the quantum nature of physical systems. We verify our criterion in an all-optical experimental setup with heralded single photons generated via spontaneous parametric down conversion and use exclusively projective pr…
We show that a reduced form of the structural requirements for deterministic hidden variables used in Bell-Kochen-Specker theorems is already sufficient for the no-go results. Those requirements are captured by the following principle: an observable takes a spectral value x if and only if the spectral projector associated with x takes the value 1. We show that the only if part of this condition suffices. The proof identifies an important structural feature behind the no-go results; namely, if at least one projector is assigned the value 1 in any resolution of the identity, then at most one is.
Full Bell-basis measurement of an atom-photon 2-qubit state and its application for quantum networks
本研究提出并实现了一种利用单离子量子存储器和heralded absorption区分所有四个Bell态的协议，通过光子qubit保态映射和原子到光子量子态隐形传态演示了完整Bell态投影，突破了线性光学50%的测量效率限制。
Algebraic and Topological Study of Bell States and Quantum Teleportation
本文研究广义贝尔态的代数性质和量子传送的拓扑特征，通过代数和拓扑方法揭示量子信息协议的本质。
We propose "super one-way deficit" by weak measurement as the generalization of one-way deficit defined for standard projective measurement. The super one-way deficit for Werner state is obtained analytically. We find that super one-way deficit is smaller than the standard one-way deficit, which contrasts with a straightforward expectation based on the known fact that super-quantum discord by weak measurement is always larger than the quantum discord defined by projective measurement. On the other hand, by tuning the weak measurement continuously to the projective measurement, both super one-w…
A constraint on two complementary knowledge excesses by maximal violation of Bell inequalities for a single copy of any mixed state of two qubits $S,M$ is analyzed. The complementary knowledge excesses ${\bf ΔK}(Π_{M}\to Π_{S})$ and ${\bf ΔK}(Π'_{M}\to Π'_{S})$ quantify an enhancement of ability to predict results of the complementary projective measurements $Π_{S},Π'_{S}$ on the qubit $S$ from the projective measurements $Π_{M},Π'_{M}$ performed on the qubit $M$. For any state $ρ_{SM}$ and for arbitrary $Π_{S},Π'_{S}$ and $Π_{M},Π'_{M}$, the knowledge excesses satisfy the following inequality…
Experimental Quantum Bernoulli Factories via Bell-Basis Measurements
该研究通过贝尔基测量在IBM超导硬件上实现了量子伯努利工厂，展示了量子资源在随机性处理中的优势，包括实现经典无法构造的伯努利倍增函数和公平硬币。
Bell State Analysis Provides an Optimal Basis Saturating the Quantum Cramer-Rao in Rotation Sensing
针对二阶反相干态在旋转传感中参数提取困难的问题，提出利用附加路径自由度的成对贝尔态分析方法，实现了N=4和N=6二阶反相干态的旋转角度提取。
Multipartite entanglement measures via Bell basis measurements
本文提出利用贝尔基测量数据估计多体纠缠度量的新方案，重点扩展Concentratable Entanglements至混合态并构造可估计下界，同时降低实验要求并简化重复次数分析，最终在里德伯原子量子计算机模拟中验证了可行性。
A bipartite Bell inequality is derived which is maximally violated on the two-qubit state space if measurements describable by positive operator valued measure (POVM) elements are allowed rather than restricting the possible measurements to projective ones. In particular, the presented Bell inequality requires POVMs in order to be maximally violated by a maximally entangled two-qubit state. This answers a question raised by N. Gisin.
What classical resources are required to simulate quantum correlations? For the simplest and most important case of local projective measurements on an entangled Bell pair state, we show that exact simulation is possible using local hidden variables augmented by just one bit of classical communication. Certain quantum teleportation experiments, which teleport a single qubit, therefore admit a local hidden variables model.
Sharing Asymmetric Einstein-Podolsky-Rosen Steering with Projective Measurements
提出基于投影测量的 EPR 导引共享协议，理论与实验证明无共享随机性时任意多方可共享单向导引，仅靠局部随机性还可共享不对称双向导引。
In several recent papers on entanglement in relativistic quantum systems and relativistic Bell's inequalities, relativistic Bell-type two-particle states have been constructed in analogy to non-relativistic states. These constructions do not have the form suggested by relativistic invariance of the dynamics. Two relativistic formulations of Bell-type states are shown for massive particles, one using the standard Wigner spin basis and one using the helicity basis. The construction hinges on the use of Clebsch-Gordan coefficients of the Poincaré group to reduce the direct product of two unitary …
We study the mathematical structures and relations among some quantities in the theory of quantum entanglement, such as separability, weak Schmidt decompositions, Hadamard matrices etc.. We provide an operational method to identify the Schmidt-correlated states by using weak Schmidt decomposition. We show that a mixed state is Schmidt-correlated if and only if its spectral decomposition consists of a set of pure eigenstates which can be simultaneously diagonalized in weak Schmidt decomposition, i.e. allowing for complex-valued diagonal entries. For such states, the separability is reduced to t…
Exchange-Symmetrized Qudit Bell Bases and Bell-State Distinguishability
本文针对偶数维量子比特对引入交换对称化的广义Bell基，证明奇数维不存在完整基，并表明在LELM限制下可无歧义区分$2d-1$个Bell态，达到上界，扩展了量子通信应用。
Localizable Bipartite Rank-One Ideal Measurements Have a Block-Replicated Nice-Bell Structure
本文证明了无需通信即可局域化的二分秩一理想测量等价于在等维块上复制单个尼斯-贝尔基，证实了Akibue和Miyazaki的猜想1。
Generation of the Complete Bell Basis via Hong-Ou-Mandel Interference
本文利用光学矢量模式的 Hong-Ou-Mandel 干涉，同时生成了所有四种 Bell 态，为高维量子信息处理和先进光子技术提供了新途径。
Exploring quantum properties of bipartite mixed states under coherent and incoherent basis
本文探究了两体混合态在相干和非相干基下的量子性质，证明相干性只能在可分基中测量，并发现非定域性并非量子隐形传态的必要条件。
A new interpretation offers a consistent conceptual basis for nonrelativistic quantum mechanics. The violation of Bell's inequality is explained by maintaining realism, inductive inference and Einstein separability.
A suitable generalized measurement described by a 4-element positive operator-valued measure (POVM) on each particle of a two-qubit system in the singlet state is, from the point of view of Einstein, Podolsky, and Rosen's (EPR's) criterion of elements of reality, equivalent to a random selection between two alternative projective measurements. It is shown that an EPR-experiment with a fixed POVM on each particle provides a violation of Bell's inequality without requiring local observers to choose between the alternatives. This approach could be useful for designing a loophole-free test of Bell…
Clauser-Horne-Shimony-Holt Bell-inequality Violability with the Full Poincaré-Bloch Sphere
本文研究了使用相同或不同线性和椭圆偏振基态对光子进行Bell和非Bell状态时，CHSH不等式违反的实验验证，发现Bell态在相同基态下违反不等式，而非Bell态则相反。
In this paper, we define a cross product operator and construct the cross Bell basis, by use this basis and Bell measurements we give a simple scheme of the teleportation of arbitrary multipartite qubit entanglement.
A new interpretation offers a consistent conceptual basis for nonrelativistic quantum mechanics. The Einstein-Podolsky-Rosen (EPR) paradox is solved and the violation of Bell's inequality is explained by maintaining realism, inductive inference and Einstein separability.
Bipartite quantum entangled systems can exhibit measurement correlations that violate Bell inequalities, revealing the profoundly counter-intuitive nature of the physical universe. These correlations reflect the impossibility of constructing a joint probability distribution for all values of all the different properties observed in Bell inequality tests. Physically, the impossibility of measuring such a distribution experimentally, as a set of relative frequencies, is due to the quantum back-action of projective measurements. Weakly coupling to a quantum probe, however, produces minimal back-a…
Quantum Computing has been evolving in the last years. Although nowadays quantum algorithms performance has shown superior to their classical counterparts, quantum decoherence and additional auxiliary qubits needed for error tolerance routines have been huge barriers for quantum algorithms efficient use. These restrictions lead us to search for ways to minimize algorithms costs, i.e the number of quantum logical gates and the depth of the circuit. For this, quantum circuit synthesis and quantum circuit optimization techniques are explored. We studied the viability of using Projective Simulatio…
We show that measuring pairs of qubits in the Bell basis can be used to obtain a simple quantum algorithm for efficiently identifying an unknown stabilizer state of n qubits. The algorithm uses O(n) copies of the input state and fails with exponentially small probability.
Bell derived the given inequalities on the basis of one rather forceful assumption that was supposed to hold in the hidden variable theory. However, this assumption has been so strong that it has corresponded only to the classical physics; any dependence on spin orientations of individual objects (i.e., on quantum characteristics) having been practically excluded. And consequently, the hidden-variable theory has not been refused by corresponding experimental data. On the other side, Einstein's criticism of Copenhagen quantum mechanics based on ontological approach refusing nonlocality has been…
We address the possibility of using even/odd states of orbital angular momentum (OAM) of photons for the quantum information tasks. Single photon qubit states and two photon entangled states in even/odd basis of OAM are considered. We present a method for the tomography and general projective measurement in even/odd basis. With the general projective measurement, we show the Bell violation and quantum cryptography with Bell's inequality. We also describe hyper and hybrid entanglement of even/odd OAM states along with polarization, which can be applied in the implementation of quantum protocols…
Einstein-Podolsky-Rosen steering is a form of inseparability in quantum theory commonly acknowledged to be intermediate between entanglement and Bell nonlocality. However, this statement has so far only been proven for a restricted class of measurements, namely projective measurements. Here we prove that entanglement, one-way steering, two-way steering and nonlocality are genuinely different considering general measurements, i.e. single round positive-operator-valued-measures. Finally, we show that the use of sequences of measurements is relevant for steering tests, as they can be used to reve…
Both the quantum mechanical and classical Bells experiment are within the focus of this paper. The fact that one measures different probabilities in both experiments is traced back to the superposition of two orthogonal but nonentangled substates in the quantum mechanical case. This superposition results in an interference term that can be splitted into two additional states representing a sink and a source of probabilities in the classical event space related to Bells experiment. As a consequence, a statistical operator can be related to the quantum mechanical Bells experiment that contains a…
We introduce two multiple qubit controlled-unitary gates with different working principles. We employ these gates and existing quantum gates to propose simple and efficient algorithms that generate multi-term orthonormal entangled Bell-like bases. All algorithms thus far known for constructing entangled bases turn out to be special cases of our method. The Bell-like states in any basis is superposition of $2^m$-terms with equal probabilities (that is, their amplitudes being $\pm 1/\sqrt{2^m}$). The orthogonality of the basis does not permit arbitrary amplitudes. The quantum correlations of the…
Beating one bit of communication with and without quantum pseudo-telepathy
本研究提出类Bell场景，证明高维态投影测量的两方关联无法用单比特通信模拟，并发现Magic square伪心灵感应游戏等两份拷贝需16x16态打破单比特界限，有望在光学平台实现。
The distinctive symmetry of Bell states
本文从一般密度矩阵出发，通过逐步施加物理约束与对称条件来唯一确定Bell态，提出了所需的附加“原子对称性”，并揭示其满足程度与Concurrence偏离最大值呈线性关系。
We investigate the complexity cost of demonstrating the key types of nonclassical correlations --- Bell inequality violation, EPR-steering, and entanglement --- with independent agents, theoretically and in a photonic experiment. We show that the complexity cost exhibits a hierarchy among these three tasks, mirroring the recently-discovered hierarchy for how robust they are to noise. For Bell inequality violations, the simplest test is the well-known CHSH test, but for EPR-steering and entanglement the tests that involve the fewest number of detection patterns require non-projective measuremen…
Doubts regarding the completeness of quantum mechanics as raised by Einstein, Podolsky and Rosen(EPR) have predominantly been resolved by resorting to a measurement of correlations between entangled photons which clearly demonstrate violation of Bell's inequality. This article is an attempt to reconcile incompatibility of hidden variable theories with reality by demonstrating experimentally a violation of Bell's inequality in locally correlated systems whose two degrees of freedom, the spin and orbital angular momentum, are maximally correlated. To this end we propose and demonstrate a linear,…
Local-projective-measurement-enhanced quantum battery capacity
本文研究二分量子系统中局域投影测量对量子电池容量的增强作用，并以贝尔对角态和X型态为例证明该方法可提升整体或子系统容量。
We report on an optical setup generating more than one bit of randomness from one entangled bit (i.e. a maximally entangled state of two-qubits). The amount of randomness is certified through the observation of Bell non-local correlations. To attain this result we implemented a high-purity entanglement source and a non-projective three-outcome measurement. Our implementation achieves a gain of 27$\%$ of randomness as compared with the standard methods using projective measurements. Additionally we estimate the amount of randomness certified in a one-sided device independent scenario, through t…
We propose a feasible approach to test Bell's inequality with the experimentally-demonstrated circuit QED system, consisting of two well-separated superconducting charge qubits (SCQs) dispersively coupled to a common one-dimensional transmission line resonator (TLR). Our proposal is based on the joint spectral measurements of the two SCQs, i.e., their quantum states in the computational basis $\{|kl>,\,k,l=0,1\}$ can be measured by detecting the transmission spectra of the driven TLR: each peak marks one of the computational basis and its relative height corresponds to the probability superpos…
Unbounded Sharing of Nonlocality Using Projective Measurements
本研究提出局部随机性辅助的投影测量协议，突破投影测量破坏纠缠的认知，实现任意数量顺序观察者与单一爱丽丝共享非局域性，揭示双方不相容性相互作用，为量子非局域性研究及信息处理提供新范式。
Dynamically preparing robust Bell states by time-boundary engineering
提出一种基于时间边界工程和动量空间投影测量的动力学框架，在Su-Schrieffer-Heeger系统中制备对环境和参数涨落具有鲁棒性的贝尔态，实现近乎完美的量子保真度。
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