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Quantum secure direct communication (QSDC) can transmit secret messages without keys, making it an important branch of quantum communication. We present a hybrid entanglement-based quantum secure direct communication (HE-QSDC) protocol with simple linear optical elements, combining the benefits of both continuous variables (CV) and discrete variables (DV) encoding. We analyze the security and find that the QSDC protocol has a positive security capacity when the bit error rate is less than 0.073. Compared with previous DV QSDC protocols, our protocol has higher communication efficiency due to performing nearly deterministic Bell-state measurement. On the other hand, compared with CV QSDC protocol, this protocol has higher fidelity with large α. Based on these advantages, our protocol may provide an alternative approach to realize secure communication. [ABSTRACT FROM AUTHOR]

Entanglement plays a fundamental role in the field of quantum computing and quantum communication. In real conditions, a physical system is never isolated and inevitably interacts with its surroundings. Temperature is one of these effects that mainly reduces entanglement. In real conditions, the system is at a non-zero temperature, which will lead to a mixed state. Therefore, in the present work, the combined thermal entanglement of electron spin-oscillation in an anisotropic two-dimensional quantum dot is investigated using the negativity criterion. The results show that the combined entanglements are strongly dependent on the changes of the controllable parameters, such as the Rashba parameter and the magnetic field. The thermal entanglement between the spin and the assembly of oscillators is zero at absolute zero temperature and reaches a maximum with increasing temperature and then tends to zero asymptotically. This temperature, at which the amount of entanglement reaches its maximum value, can be controlled by changing the magnetic field and the coupling of the Rashba parameter. These two factors also control the rate of reaching the asymptotic state. These results provide a way to control the degree of entanglement between electron degrees of freedom, which is a fundamental requirement of quantum information processing.

Hybrid entangled states (HES) have attracted significant attention and been utilized in various quantum information processing applications. However, like many other entangled states, maximally entangled HES may degrade to mixed states due to environmental noise and operational imperfections. In this paper, a hybrid entanglement purification protocol (HEPP) for the HES, which consists of photon‐number state and coherent state is proposed. This HEPP is designed to effectively purify a bit‐flip error occurring in any qubit of the HES. Furthermore, HEPP is extended to a general condition, say, the multi‐party scenario, and integrates the generation of HES into the HEPP. Moreover, if different initial mixed states are chosen, then the residual entanglement can be utilized to distill high‐quality entanglement. The HEPP has important applications in the future quantum information processing field. [ABSTRACT FROM AUTHOR]

Hybrid entangled states exhibit non-local correlations between photons with independent degrees of freedom and are currently gaining much interest. In particular, hybrid entanglement between polarisation and spatial modes of two photons are promising candidates for future heterogeneous quantum channels, but their versatility is limited by current generation methods that rely on static elements. Here, we present a technique that exploits polarisation and spatial mode dependent phase modulation in an all-digital approach using spatial light modulators. We show that we can tailor hybrid entangled states using spatial modes with Cylindrical and Cartesian symmetry, making our approach flexible, dynamic, and adaptable. [ABSTRACT FROM AUTHOR]

The development of intelligent Internet of vehicles (IoV) is based on communication technology, especially the quantum communication of hybrid entangled states based on orbital angular momentum (OAM). The quantum key distribution (QKD) method is demonstrated using hybrid entangled states with multi-degree of freedom and high purity based on OAM. First, the polarization entangled photon pairs are produced by two type I BBO crystals by means of spontaneous parametric down-conversion (SPDC). The polarization-OAM hybrid entangled states are created by q-plate (QP) to realize QKD. It is found that the polarization entangled photon pairs have the characteristics of OAM. A single mode fiber (SMF) is proposed to filter the topological charge non-zero photons and purify the polarization entangled photon pairs. Then the purification setup with M-Z interferometer and beam rotator (BR) is presented which can purify the polarization-OAM hybrid entangled states. The high purity polarization-OAM hybrid entangled states are implemented, and the QP conversion rate of 3 secondary feedbacks is 99.84% theoretically. Analysis indicates that precise purity polarization-OAM hybrid entangled state can increase the fidelity of information and improve the anti-interference ability in this program without mutually unbiased bases (MUBs). The photon utilization rate, energy-efficient and low-latency communication can be greatly improved by using the OAM carrying information.

Heterogeneousquantum networks consisting of mixed‐technologies—continuous variable (CV) and discrete variable (DV)—will become ubiquitous as global quantum communication matures. Hybrid quantum‐entanglement between CV and DV modes will be a critical resource in such networks. A leading candidate for such hybrid quantum entanglement is that between Schrödinger‐cat states and photon‐number states. In this work, we explore, for the first time, the use of two‐mode squeezed vacuum (TMSV) states, distributed from satellites, as a teleportation resource for the redistribution of our candidate hybrid entanglement prestored within terrestrial quantum networks. We determine the loss conditions under which teleportation via the TMSV resource outperforms direct‐satellite distribution of the hybrid entanglement, in addition to quantifying the advantage of teleporting the DV mode relative to the CV mode. Our detailed calculations show that under the loss conditions anticipated from low‐Earth‐orbit, DV teleportation via the TMSV resource will always provide for significantly improved outcomes, relative to other means for distributing hybrid entanglement within heterogeneous quantum networks. [ABSTRACT FROM AUTHOR]

As heterogeneous quantum systems become a high-impact topic in the state of the art through hybrid entanglement, none of the many competing measures of multipartite entanglement has yet been shown to provide a complete characterization. In this paper, for the first time, we connect two of the most popular research directions on entanglement of heterogeneous systems: correlation tensors on the one hand, and concurrence and tangle on the other. We find the first ever separability criteria for heterogeneous quantum systems under any arbitrary partition in terms of multipartite concurrence and tangle, respectively, generalizing and refining previous work on m-separability of homogeneous systems, and we derive the first relations between the Frobenius norms of correlation tensors, concurrence and tangle, generalizing previous work for homogeneous systems. [ABSTRACT FROM AUTHOR]

By coherently combining advantages while largely avoiding limitations of two mainstream platforms, optical hybrid entanglement involving both discrete and continuous variables has recently garnered widespread attention and emerged as a promising idea for building heterogenous quantum networks. In contrast to previous results, here we propose a new scheme to remotely generate hybrid entanglement between discrete polarization and continuous quadrature optical qubits heralded by two-photon Bell-state measurement. As a novel nonclassical light resource, we further use it to discuss two examples of ways—entanglement swapping and quantum teleportation—in which quantum information processing and communications could make use of this hybrid technique.

We propose the use of hybrid entanglement in an entanglement swapping protocol, as means of distributing a Bell state with high fidelity to two parties. The hybrid entanglement used in this work is described as a discrete variable (Fock state) and a continuous variable (cat state super- position) entangled state. We model equal and unequal levels of photonic loss between the two propagating continuous variable modes, before detecting these states via a projective vacuum-one-photon measurement, and the other mode via balanced homodyne detection. We investigate homodyne measurement imperfections, and the associated success probability of the measurement schemes chosen in this protocol. We show that our entanglement swapping scheme is resilient to low levels of photonic losses, as well as low levels of averaged unequal losses between the two propagating modes, and show an improvement in this loss resilience over other hybrid entanglement schemes using coherent state superpositions as the propagating modes. Finally, we conclude that our protocol is suitable for potential quantum networking applications which require two nodes to share entanglement separated over a distance of 5--10 km , when used with a suitable entanglement purification scheme. [ABSTRACT FROM AUTHOR]

Motivated by the block encryption and quantum Fourier transfer, a quantum blind signature scheme is proposed. Alice encodes the original message into three pairs of hybrid entangled states, which are distributed among three parties of communication. The measurement result of entangled pairs serves as the blind message of Alice and signature of Charlie, the verifier Bob recovers the message and verifies the signature. The information is delivered after being encoded into multi-dimensional quantum states and encrypted by quantum Fourier transforms. The analysis shows that the scheme is non-forgery, non-repudiation and blindness. In addition, the proposed scheme is more efficient against the universal and selective forgery attacks. [ABSTRACT FROM AUTHOR]

We propose and develop the theory of quantum teleportation of an unknown qubit based on the interaction mechanism between discrete-variable (DV) and continuous-variable (CV) states on highly transmissive beam splitter (HTBS). This DV-CV interaction mechanism is based on the simultaneous displacement of the DV state on equal in absolute value, but opposite in sign displacement amplitudes by coherent components of the hybrid in such a way that all the information about the displacement amplitudes is lost with subsequent registration of photons in the auxiliary modes. The relative phase of the displaced unknown qubit in the measurement number state basis can vary on opposite, depending on the parity of the basis states in the case of the negative amplitude of displacement that is akin to action of nonlinear effect on the teleported qubit. All measurement outcomes of the quantum teleportation are distinguishable, but the teleported state at Bob’s disposal may acquire a predetermined amplitude-distorting factor. Two methods of getting rid of the factors are considered. The quantum teleportation is considered in various interpretations. A method for increasing the efficiency of quantum teleportation of an unknown qubit is proposed.

We consider novel method for implementation of hybrid entanglement between the states in signal/idler and pumping modes or between the states from different vector spaces which are characterized by different displacement amplitudes on output from spontaneous parametric down converter (SPDC). The entanglement is caused by the depletion of the pump wave. In general, the entangled state is defined by two parameters: squeezing and coupling ones. The generated entanglement is mainly defined by the coupling parameter. We characterize it and show the limits of applicability of the depleted pump regime. The hybrid entanglement is tested by conditional generation of maximally entangled states of two qubits and qutrits. Possibilities to enhance the hybrid entanglement and, as consequence, success probabilities of the conditional generation are considered. The optical scheme for test is realizable in practice and can do without photon number resolving detection (PNRD). [ABSTRACT FROM AUTHOR]

The realization of hybrid entanglement between a microscopic (quantum) and a macroscopic (classical) system, in analogy to the situation of the famous Schrödinger's cat paradox, is an important milestone, both from the fundamental perspective and for possible applications in the processing of quantum information. The most straightforward optical implementation of this condition is that of the entanglement between a single-photon and a coherent state. In this work, we describe the first step towards the generation of this type of hybrid entanglement from the experimental perspective. [ABSTRACT FROM AUTHOR]

In the present article we report the dynamics of electronic spin-subbands, as well as subband-subband, hybrid entanglements in a two-dimensional anisotropic quantum dot. The dot is under the influence of Rashba effect and an external magnetic field. To study the hybrid entanglements, we partition the system into two categories in which either spatial degrees of freedom, subbands, entangle with the spin or the subbands become entangled amongst themselves. For the first case we calculate the von Neumann entropy, while for the latter the negativity is calculated. Our calculations show that for both cases information is periodically distributed between the corresponding subspaces. Effects of Rashba parameter and magnetic field on the characteristics of such oscillatory behavior are also discussed. For spin-subband entanglement the oscillations include dips, surrounded by plateaus of maximal entanglement. The subband-subband entanglement shows vanishingly small plateaus. The duration of plateaus is controlled by Rashba coupling and the external field. [ABSTRACT FROM AUTHOR]

We present two hybrid entanglement concentration protocols based on quantum dots (QDs) and optical microcavity coupled systems. The system is theoretically analyzed and used for photon and electron hybrid entanglement generation. Also, the proposed system can be further used for parity check that allows a quantum nondemolition measurement on the spin parity. By performing parity check process on electron spins, the entangled state can be concentrated into maximally entangled state efficiently. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

By coherently combining advantages while largely avoiding limitations of two mainstream platforms, optical hybrid entanglement involving both discrete and continuous variables has recently garnered widespread attention and emerged as a promising idea for building heterogenous quantum networks. In contrast to previous results, here we propose a new scheme to remotely generate hybrid entanglement between discrete polarization and continuous quadrature optical qubits heralded by two-photon Bell-state measurement. As a novel nonclassical light resource, we further use it to discuss two examples of ways—entanglement swapping and quantum teleportation—in which quantum information processing and communications could make use of this hybrid technique. [ABSTRACT FROM AUTHOR]

• Discretization of hybrid entanglement allows quantification with discrete measures. • The quantum state of the Stern-Gerlach Experiment (SGE) possesses a simple discrete form. • Concurrence and entanglement entropy quantify the creation of entanglement in the SGE. • The partial trace allows the quantification with the entanglement entropy. • The partial trace and discretization methods give identical results in the SGE. Hybrid entanglement occurs between discrete and continuous variables, resulting in a considerable difficulty for its quantification. However, some advancements have been made, one of them is the discretization of the continuous variable as a tool to quantify it. Here, in this paper, the applicability of discrete entanglement measures to the final quantum state of the Stern-Gerlach Experiment is discussed in order to quantify its hybrid entanglement. Additionally, the possibility of using an equivalent discrete state is compared to the employment of the partial trace in order to consider the discrete component only, we show that the entanglement entropy distinctively represents the discrete part. Hence, the known entanglement capabilities of the SGE allow us to support the use of both techniques. Therefore, these measures give a detailed description of the entanglement creation in this famous experiment. [ABSTRACT FROM AUTHOR]

We design a scheme to generate a new type of hybrid entanglement between a macroscopic polarized coherent state and a single polarized photon. Concretely, a polarized Schrodinger cat state, an entangled polarized photon pair, and a polarized coherent state, which have been prepared off-line, are supplied as inputs to our scheme. Then, using conventional optical devices such as beam splitters, polarizing beam splitters, half-wave plates, and photon-number-resolving detectors, all the necessary operations can be effectively performed in-line. Conditioned on photon detection results, creation of the desired hybrid entanglement is heralded. The total success probability of our scheme is twice as high as that in existing schemes for unpolarized coherent states. Our scheme is not only feasible but also economical with regard to resource consumption because no active displacement operations must be utilized in-line. We also consider practical factors affecting the scheme performance, including decoherence of the input states, realistic resources, and device imperfections. Detailed analyses show that our scheme is robust against small imperfection effects and could be implemented with current technology. The proposed hybrid entanglement would serve as a vital resource for processing information encoded in superpositions of differently polarized coherent states which represent a new format of qubit. [ABSTRACT FROM AUTHOR]

As a multi-particle entangled state, W state is more robust than GHZ state, because even if one of the entangled particles is traced out or projected to a specific state, the remaining particles of W state are still entangled. In this paper, we propose a novel scheme to generate hybrid W state entanglement where three photons entangled in spin angular momentum (SAM), path (line momentum), and orbital angular momentum (OAM). Based on the hybrid entanglement W state, we present a probabilistic open-destination teleportation scheme, in which Alice can remotely control the OAM generation of Bob's photon. Different from the original teleportation, with certain probability, the specified state can be transmit to any photon of the multi-particle entanglement, and its information can be obtained from any individual DOF of OAM or linear momentum, depending on postprocessing. At last, we evaluate the performance of this scheme using quantum state tomography, and we find Bob's OAM state not only electrically tuned by Alice, but also depend on the linear momentum (path) of the W state and the polarization of Carol's photon. [ABSTRACT FROM AUTHOR]

We propose and develop the theory of quantum teleportation of an unknown qubit based on the interaction mechanism between discrete-variable (DV) and continuous-variable (CV) states on highly transmissive beam splitter (HTBS). This DV-CV interaction mechanism is based on the simultaneous displacement of the DV state on equal in absolute value, but opposite in sign displacement amplitudes by coherent components of the hybrid in such a way that all the information about the displacement amplitudes is lost with subsequent registration of photons in the auxiliary modes. The relative phase of the displaced unknown qubit in the measurement number state basis can vary on opposite, depending on the parity of the basis states in the case of the negative amplitude of displacement that is akin to action of nonlinear effect on the teleported qubit. All measurement outcomes of the quantum teleportation are distinguishable, but the teleported state at Bob's disposal may acquire a predetermined amplitude-distorting factor. Two methods of getting rid of the factors are considered. The quantum teleportation is considered in various interpretations. A method for increasing the efficiency of quantum teleportation of an unknown qubit is proposed.