Single spin measurement represents a major challenge for spin-based quantum computation. In this article we propose a new method for measuring the spin of a single electron confined in a quantum dot (QD). Our strategy is based on entangling (using unitary gates) the spin and orbital degrees of freedom. An {\em orbital qubit}, defined by a second, empty QD, is used as an ancilla and is prepared in a known initial state. Measuring the orbital qubit will reveal the state of the (unknown) initial spin qubit, hence reducing the problem to the easier task of single charge measurement. Since spin-charge conversion is done with unit probability, single-shot measurement of an electronic spin can be, in principle, achieved. We evaluate the robustness of our method against various sources of error and discuss briefly possible implementations.
We investigate two-photon entangled states using two important degrees of freedom of the electromagnetic field, namely orbital angular momentum (OAM) and spin angular momentum. For photons propagating in the same direction we apply the idea of $\textit{entanglement duality}$ and develop schemes to do $\textit{entanglement sorting}$ based either on OAM or polarization. In each case the entanglement is tested using appropriate witnesses. We finally present generalizations of these ideas to three- and four-photon entangled states.
We study the entanglement evolution of photonic orbital angular momentum qubit states with opposite azimuthal indices $l_0$, in a weakly turbulent atmosphere. Using asymptotic methods, we deduce analytical expressions for the amplitude of turbulence-induced crosstalk between the modes $l_0$ and $-l_0$. Furthermore, we analytically establish distinct, universal entanglement decay laws for Kolmogorov's turbulence model and for two approximations thereof.
We provide an analytical expression for the entanglement decay of initially maximally entangled orbital angular momentum bi-photon states, when scattered off an obstruction. We show that the decay is controlled by the diffraction-induced mutual overlap between the diffracted field modes, and quantify its dependence on the size and position of the obstruction.
Entanglement swapping generates remote quantum correlations between particles that have not interacted and is the cornerstone of long-distance quantum communication, quantum networks, and fundamental tests of quantum science. In the context of spatial modes of light, high-dimensional entanglement provides an avenue to increase the bandwidth of quantum communications and provides more stringent limits for tests of quantum foundations. Here we simultaneously swap the entanglement of multiple orbital angular momentum states of light. The system is based on a degenerate filter that cannot distingu…
To demonstrate the role played by the von Neumann entropy spectra in quantum phase transitions we investigate the one-dimensional anisotropic SU(2)$\otimes XXZ$ spin-orbital model with negative exchange parameter. In the case of classical Ising orbital interactions we discover an unexpected novel phase with Majumdar-Ghosh-like spin-singlet dimer correlations triggered by spin-orbital entanglement and having $k=π/2$ orbital correlations, while all the other phases are disentangled. For anisotropic $XXZ$ orbital interactions both spin-orbital entanglement and spin-dimer correlations extend to th…
Orbital angular momentum entanglement is one of the most intriguing topics in quantum physics. A broad range of research have been dedicated either to unravel its underlying physics or to expand the entanglement dimensions and degrees. In this paper, we present a theoretical study on the orbital angular momentum entanglement by employing the Heisenberg uncertainty principle to quantum position correlation within the azimuthal region. In this study, we decompose the pump light into a set of pump cone states characterized by their radii. The OAM entanglement can be manipulated by controlling the…
We investigate the phase diagram and the spin-orbital entanglement of a one-dimensional SU(2)$\otimes XXZ$ model with SU(2) spin exchange and anisotropic $XXZ$ orbital exchange interactions and negative exchange coupling. As a unique feature, the spin-orbital entanglement entropy in the entangled ground states increases here linearly with system size. In the case of Ising orbital interactions we identify an emergent phase with long-range spin-singlet dimer correlations triggered by a quadrupling of correlations in the orbital sector. The peculiar translational invariant spin-singlet dimer phas…
Entanglement of orbital angular momentum states between an ensemble of cold atoms and a photon
近年来，利用集体增强技术成功地实现了原子系综与单光子的纠缠[D. N. Matsukevich，\textit{et al.}，物理学评论快报。\textbf{95}，040405（2005）.]，其中原子内态和光子偏振态以非局域方式关联。在这里，我们通过实验阐明了在原子系综和光子系统中，也存在着与空间自由度有关的纠缠。讨论了远程原子系综之间高维纠缠的产生及其在凝聚态物理中的应用。
With progress towards more compact quantum computing architectures, fundamental questions regarding the entanglement of indistinguishable particles need to be addressed. In a solid state device, this quest is naturally connected to the quantum correlations of electrons. Here, we investigate the entanglement between electrons, focusing on the entanglement of modes, the entanglement of particles and the effect of particle-number superselection rules. We elucidate the formation of mode and particle entanglement in strongly correlated materials and show that both represent important resources in q…
The Einstein Podolsky Rosen (EPR) entangled quantum state is of special importance not only for fundamental research in quantum mechanics, but also for information processing in the field of quantum information. Previous EPR entangled state demonstrations were constructed with photons of equal phase wave fronts. More complex scenarios with structured wave fronts have not been investigated. Here, we report the first experimental demonstration of EPR entanglement for photon pairs carrying orbital angular momentum (OAM) information, resulting in an OAM embedded EPR entangled state. We measured th…
We demonstrate entanglement between the orbital angular momentum (OAM) of two photoelectrons ionized via the strongly correlated process of non-sequential double ionization (NSDI). Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. We explore detection by an entanglement witness, decomposable into local measurements, which strongly reduces the difficulty of experimental implementation. We compute the logarithmic negativity measure, which is directly applicable to mixed states, to demonstrate that the en…
Fingerprints of spin-orbital entanglement in transition metal oxides
介绍了过渡金属氧化物中超交换键的自旋轨道纠缠的概念，并举例说明。结果表明，自旋-轨道纠缠在超交换模型不稳定的长程（自旋和轨道）秩序，并可能导致一个无序的自旋液体状态或新的阶段，在低温下所产生的强烈受挫的相互作用。这种新颖的基态不能在常规使用的平均场论中描述，该平均场论将自旋和轨道自由度分开。即使在基态解纠缠的情况下，自旋-轨道纠缠也发生在激发态中，并且可能对有限温度下物理性质的正确描述至关重要。作为这种行为的一个重要例子，我们提出了$R$ VO $_3$钙钛矿中的自旋轨道纠缠，其中$R$ =La，Pr，.，Yb，Lu，其中这些化合物的这种有限温度特性只能使用纠缠态来理解：（$i$）光谱权重的热演化，（$ii$）在$R$ VO $_3$钙钛矿的相图中，轨道和磁序开始的转变温度对离子半径的依赖性，和（$iii$）二聚化在磁振子谱中观察到的$C$型反铁磁相的YVO $_3$。最后，它表明，联合自旋-轨道激发在一个有序的相位与共存的反铁磁和交替的轨道顺序引入拓扑约束的空穴传播，从而从根本上改变输运性质的掺杂莫特绝缘体中的空穴运动意味着同时自旋和轨道激发。
We review the consequences of intrinsic frustration of the orbital superexchange and of spin-orbital entanglement. While Heisenberg perturbing interactions remove frustration in the compass model, the lowest columnar excitations are robust in the nanoscopic compass clusters and might be used for quantum computations. Entangled spin-orbital states determine the ground states in some cases, while in others concern excited states and lead to measurable consequences, as in the $R$VO$_3$ perovskites. On-site entanglement for strong spin-orbit coupling generates the frustrated Kitaev-Heisenberg mode…
We propose a protocol to certify the presence of entanglement in artificial on-surface atomic and molecular spin arrays using electron spin resonance carried by scanning tunnel microscopes (ESR-STM). We first generalize the theorem that relates global spin susceptibility as an entanglement witness to the case of anisotropic Zeeman interactions, relevant for surfaces. We then propose a method to measure the spin susceptibilities of surface-spin arrays combining ESR-STM with atomic manipulation. Our calculations show that entanglement can be certified in antiferromagnetically coupled spin dimers…
Here we present the quantum storage of three-dimensional orbital-angular-momentum photonic entanglement in a rare-earth-ion-doped crystal. The properties of the entanglement and the storage process are confirmed by the violation of the Bell-type inequality generalized to three dimensions after storage ($S=2.152\pm0.033$). The fidelity of the memory process is $0.993\pm0.002$, as determined through complete quantum process tomography in three dimensions. An assessment of the visibility of the stored weak coherent pulses in higher-dimensional spaces, demonstrates that the memory is highly reliab…
A recent development in quantum chemistry has established the quantum mutual information between orbitals as a major descriptor of electronic structure. This has already facilitated remarkable improvements of numerical methods and may lead to a more comprehensive foundation for chemical bonding theory. Building on this promising development, our work provides a refined discussion of quantum information theoretical concepts by introducing the physical correlation and its separation into classical and quantum parts as distinctive quantifiers of electronic structure. In particular, we succeed in …
Quantum entanglement shared between more than two particles is essential to foundational questions in quantum mechanics, and upcoming quantum information technologies. So far, up to 14 two-dimensional qubits have been entangled, and an open question remains if one can also demonstrate entanglement of higher-dimensional discrete properties of more than two particles. A promising route is the use of the photon orbital angular momentum (OAM), which enables implementation of novel quantum information protocols, and the study of fundamentally new quantum states. To date, only two of such multidimen…
Orbital angular momentum (OAM) multiplexing provides an efficient method to improve data-carrying capacity in various quantum communication protocols. It is a precondition to distribute OAM multiplexed quantum resources in quantum channels for implementing quantum communication. However, quantum steering of OAM multiplexed optical fields and the effect of channel noise on OAM multiplexed quantum resources remain unclear. Here, we generate OAM multiplexed continuous-variable (CV) entangled states and distribute them in lossy or noisy channels. We show that the decoherence property of entangleme…
High-dimensional entanglement has demonstrated potential for increasing channel capacity and resistance to noise in quantum information processing. However, its distribution is a challenging task, imposing a severe restriction on its application. Here we report the first distribution of three-dimensional orbital angular momentum (OAM) entanglement via a 1-km-long optical fibre. Using an actively-stabilizing phase pre-compensation technique, we successfully transport one photon of a three-dimensional OAM entangled photon pair through the fibre. The distributed OAM entangled state still shows a …
Spin-Orbital Entanglement Emerging from Frustration in the Kugel-Khomskii Model
利用平均场集团方法研究了双层正方晶格Kugel-Khomskii（$d^9$）模型的零温相图，其中包含纠缠相和单重相.该图包括在氟化物K $_3$ Cu $_2$ F $_7$中观察到的层间单线态相和奇异纠缠自旋轨道相。对于单层的情况下，实现在K $_2$ CuF $_4$，我们在有限的温度下进行类似的计算，并表明，交替轨道的铁磁体衰减首先纠缠均匀的铁磁体，然后顺磁相。发表于：J. Phys.：系列391，012085（2012）。
Entanglement is a vital resource for realizing many tasks such as teleportation, secure key distribution, metrology and quantum computations. To effectively build entanglement between different quantum systems and share information between them, a frequency transducer to convert between quantum states of different wavelengths while retaining its quantum features is indispensable. Information encoded in the photons orbital angular momentum OAM degrees of freedom is preferred in harnessing the information carrying capacity of a single photon because of its unlimited dimensions. A quantum transdu…
One of the most critical tasks at the very beginning of a quantum chemical investigation is the choice of either a multi- or single-configurational method. Naturally, many proposals exist to define a suitable diagnostic of the multi-configurational character for various types of wave functions in order to assist this crucial decision. Here, we present a new orbital-entanglement based multi-configurational diagnostic termed $Z_{s(1)}$. The correspondence of orbital entanglement and static (or nondynamic) electron correlation permits the definition of such a diagnostic. We chose our diagnostic t…
