Superactivation of bound entanglement

We show that, in a multiparty setting, two nondistillable (bound-entangled) states tensored together can make a distillable state. This is an example of true superadditivity of distillable entanglement. We also show that unlockable bound-entangled states cannot be asymptotically unentangled, providing the first proof that some states are truly bound-entangled in the sense of being both nondistillable and nonseparable asymptotically.     

Secure deterministic communication without entanglement

We propose a protocol for deterministic communication that does not make use of entanglement. It exploits nonorthogonal states in a two-way quantum channel to attain unconditional security and high efficiency of the transmission. We explicitly show the scheme is secure against a class of individual attacks regardless of the noise on the channel. Its experimental realization is feasible with current technology.     

Lower bound for electron spin entanglement from beam splitter current correlations

We determine a lower bound for the entanglement of formation of pairs of electron spins injected into a mesoscopic conductor. The bound can be expressed in terms of experimentally accessible quantities, the zero-frequency current correlators (shot noise power or cross correlators) after transmission through an electronic beam splitter and can be used to gain information about the entanglement from experiment. Spin relaxation (T1 processes) and decoherence (T2) during the ballistic coherent transmission of carriers are taken into account within Bloch theory. A variable inhomogeneous magnetic field gives rise to a useful lower bound for the entanglement of arbitrary states. The decrease in entanglement due to thermally mixed states is studied. Both the entanglement of the output of a source (entangler) and T(1,2) can be determined from current correlators.     

Decoherence in time evolution of bound entanglement

We study a dynamic process of disentanglement by considering the time evolution of bound entanglement for a quantum open system, two qutrits coupling to a common environment. Here, the initial quantum correlations of the two qutrits are characterized by the bound entanglement. Both bosonic and spin environments are considered. We found that the bound entanglement displays collapses and revivals, and it can be stable against small temperature and time change. The thermal fluctuation effects on bound entanglement are also considered.     

Experimental bound entanglement in a four-photon state

Bound entanglement is central to many exciting theoretical results in quantum information processing, but has thus far not been experimentally realized. In this work, we consider a one-parameter family of four-qubit Smolin states. We experimentally produce these states in the polarization of four optical photons produced from parametric down-conversion. Within a range of the parameter, we show that our states are entangled and undistillable, and thus bound entangled. Using these bound-entangled states we demonstrate entanglement unlocking.     

Rotationally invariant bipartite states and bound entanglement

We consider rotationally invariant states in CN₁⊗CN₂${\mathbb{C}}^{N_1}\otimes {\mathbb{C}}^{N_2}$ Hilbert space with even N₁?4$N_1?4$ and arbitrary N₂?N₁$N_2?N_1$, and show that in such case there always exist states which are inseparable and remain positive after partial transposition, and thus the PPT criterion does not suffice to prove separability in such systems. We demonstrate it applying a map developed recently by Breuer [H.-P. Breuer, Phys. Rev. Lett. 97 (2006) 080501] to states that remain invariant after partial time reversal.     

Multiparty semiquantum key agreement without entanglement

The key agreement protocols allow two or more users to negotiate a shared key for establishing a secure communication channel without a third trusted party in such a way that the shared key is determined by all authorized players rather than any subset of them. We propose the first real multiparty semiquantum key agreement(SQKA) protocols based on single-photons. Our protocols include only one quantum player, while the others are classical players who only need to measure and prepare states in the classical basis. We first present a symmetric three-party SQKA protocol,where two classical players can fairly negotiate a key with a quantum player by using single-photons as message carriers. Then we present an asymmetric SQKA protocol where a relatively low percentage of quantum states are used for eavesdropping detection. And we further extend them to an asymmetric multiparty SQKA protocol. Our SQKA protocols require fewer quantum resources than the previous SQKA protocols for classical players, especially without requirement of entanglement,which makes them easier to implement using current technologies. Our protocols are secure against external eavesdroppers and are fair against a minority of internal dishonest players.     

The geometry of bipartite qutrits including bound entanglement

We investigate the state space of bipartite qutrits, in particular we construct an analog to the “magic” tetrahedron for bipartite qubits—a magic simplex W. Due to the high symmetry it is enough to consider certain typical slices through W and optimal entanglement witnesses can be calculated. A whole region of bound entangled states is found.     

Spin-orbit hybrid entanglement quantum key distribution scheme

We propose a novel quantum key distribution scheme by using the SAM-OAM hybrid entangled state as the physical resource.To obtain this state,the polarization entangled photon pairs are created by the spontaneous parametric down conversion process,and then,the q-plate acts as a SAM-to-OAM transverter to transform the polarization entangled pairs into the hybrid entangled pattern,which opens the possibility to exploit the features of the higher-dimensional space of OAM state to encode information.In the manipulation and encoding process,Alice performs the SAM measurement by modulating the polarization stateπ lθx on one photon,whereas Bob modulates the OAM sector state lx' on the other photon to encode his key elements using the designed holograms which is implemented by the computer-controlled SLM.With coincidence measurement,Alice could extract the key information.It is showed that N-based keys can be encoded with each pair of entangled photon,and this scheme is robust against Eve’s individual attack.Also,the MUBs are not used.Alice and Bob do not need the classical communication for the key recovery.     

Activating distillation with an infinitesimal amount of bound entanglement

We show that bipartite quantum states of any dimension, which do not have a positive partial transpose (NPPT), become 1-distillable when one adds an infinitesimal amount of bound entanglement. To this end we investigate the activation properties of a new class of symmetric bound entangled states of full rank. It is shown that in this set there exist universal activator states capable of activating the distillation of any NPPT state. The result shows that even a small amount of bound entanglement can be useful for quantum information purposes.     

Thermal bound entanglement in macroscopic systems and area law

Does bound entanglement naturally appear in quantum many-body systems? We address this question by showing the existence of bound-entangled thermal states for harmonic oscillator systems consisting of an arbitrary number of particles. By explicit calculations of the negativity for different partitions, we find a range of temperatures for which no entanglement can be distilled by means of local operations, despite the system being globally entangled. We offer an interpretation of this result in terms of entanglement-area laws, typical of these systems. Finally, we discuss generalizations of this result to other systems, including spin chains.     

Nonadditivity of bipartite distillable entanglement follows from a conjecture on bound entangled Werner states

Assuming the validity of a conjecture given by DiVincenzo et al. [Phys. Rev. A 61, 062312 (2000)] and by Dür et al. [Phys. Rev. A 61, 062313 (2000)], we show that the distillable entanglement for two bipartite states, each of which individually has zero distillable entanglement, can be nonzero. We show that this also implies that the distillable entanglement is not a convex function. Our example consists of the tensor product of a bound entangled state based on an unextendible product basis with an entangled Werner state which lies in the class of conjectured undistillable states.     

Path integral calculation of entanglement probabilities for bound macromolecules

The path integral for an entangled macromolecule chain in two dimensions is calculated. The entanglement probabilities for a bound macromolecule so reached coincide with Wiegel's results obtained by solving a differential equation.     

Equivalence between two-qubit entanglement and secure key distribution

We study the problem of secret key distillation from bipartite states in the scenario where Alice and Bob can perform measurements only at the single-copy level and classically process the obtained outcomes. Even with these limitations, secret bits can be asymptotically distilled by the honest parties from any two-qubit entangled state, under any individual attack. Our results point out a complete equivalence between two-qubit entanglement and secure key distribution: a key can be established through a one-qubit channel if and only if it allows one to distribute entanglement. These results can be generalized to a higher dimension for all those states that are one-copy distillable.     

High-efficient quantum key distribution based on hybrid entanglement

A novel quantum key distribution scheme based on the path-spin hybrid entanglement is proposed and analyzed. In this proposed scheme, the entanglement between the path and the spin degrees of freedom is confined locally with the single particle and transmitted in one-way direction. Two split pulses of a single spin-1/2 particle are not simultaneously transmitted through the public quantum channels for the security goal. The scheme is robust against any individual attack even in noisy environments. Moreover, it also has high-efficiency since one single particle can be used to generate one bit key on average.     

Optimal spin squeezing inequalities detect bound entanglement in spin models

We determine the complete set of generalized spin squeezing inequalities. These are entanglement criteria that can be used for the experimental detection of entanglement in a system of spin-1/2 particles in which the spins cannot be individually addressed. They can also be used to show the presence of bound entanglement in the thermal states of several spin models.     

The bound of entanglement of superpositions with more than two components

A bipartite quantum state (for two systems in any dimensions) can be decomposed as a superposition of many components. For a superposition of more than two components we prove that there is a bound of the entanglement of the superposition state which can be expressed according to entanglements of its component states. Especially, if the component states are mutually bi-orthogonal, the entanglement of the superposition state can be exactly given in terms of the entanglements of the states being superposed.     

Secure quantum key distribution with an uncharacterized source

We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol for an arbitrary source whose averaged states are basis independent, a condition that is automatically satisfied if the source is suitably designed. The proof is based on the observation that, to an adversary, the key extraction process is equivalent to a measurement in the sigma(x) basis performed on a pure sigma(z)-basis eigenstate. The dependence of the achievable key length on the bit error rate is the same as that established by Shor and Preskill [Phys. Rev. Lett. 85, 441 (2000)]] for a perfect source, indicating that the defects in the source are efficiently detected by the protocol.     

基于纠缠交换的多方多级量子密钥分配协议

提出了一种基于纠缠交换的多方多级量子密钥分配协议.构造了一种两方三级系统的完备正交归一化基，利用该正交归一化基和纠缠交换可以实现两方量子密钥分配.同时，三级可以推广到多级以及两方推广到多方，即可以实现基于纠缠交换的多方多级量子密钥分配.这样，利用纠缠交换和多级密钥分配可以极大地提高检测窃听的效率、密钥生成率以及信息容量. 关键词： 量子密钥分配 纠缠交换 多方 多级     

Secure quantum key distribution using continuous variables of single photons

We analyze the distribution of secure keys using quantum cryptography based on the continuous variable degree of freedom of entangled photon pairs. We derive the information capacity of a scheme based on the spatial entanglement of photons from a realistic source, and show that the standard measures of security known for quadrature-based continuous variable quantum cryptography (CV-QKD) are inadequate. A specific simple eavesdropping attack is analyzed to illuminate how secret information may be distilled well beyond the bounds of the usual CV-QKD measures.