International Journal of Quantum Information

Heat capacity of a mixed-three-spin (1/2,1,1/2) antiferromagnetic XXX Heisenberg chain is precisely investigated by use of the partition function of the system for which, spins (1,1/2) have coupling constant $J_1$ and spins (1/2,1/2) have coupling constant $J_2$. We verify tripartite entanglement for the model by means of the convex roof extended negativity (CREN) and concurrence as functions of temperature T, homogeneous magnetic field B and the coupling constants $J_1$ and $J_2$. As shown in our previous work, [H. A. Zad, Chin. Phys. B 25 (2016) 030303.] the temperature, the magnetic field and the coupling constants dependences of the heat capacity for such spin system have different behaviors for the entangled and separable states, hence, we did some useful comparisons between this quantity and negativities of its organized bipartite (sub)systems at entangled and separable states. Here, we compare the heat capacity of the mixed-three-spin (1/2,1,1/2) system with the CREN and the tripartite concurrence (as measures of the tripartite entanglement) at low temperature. Ground state phase transitions, and also, transition from ground state to some excited states are explained in detail for this system at zero temperature. Finally, we investigate the heat capacity behavior around those critical points in which these quantum phase transitions occur.

This paper presents a steganographical algorithm based on least significant bit (LSB) from the most significant bit information (MSBI) and the equivalence of a bit pixel image to a quantum pixel image, which permits to make the information communicate secretly onto quantum pixel images for its secure transmission through insecure channels. This algorithm offers higher security since it exploits the Shannon entropy for an image.

We propose a scheme to generate the case of macroscopic entanglement in the optomechanical system, which consist of Fabry–Perot cavity and a mechanical oscillator by applying a homodyne-mediated quantum feedback. We explore the effect of feedback on the entanglement in vacuum and coherent state, respectively. The results show that the introduction of quantum feedback can increase the entanglement effectively between the cavity mode and the oscillator mode.

We firstly propose a simultaneous dense coding protocol with two-photon four-qubit cluster states in which two receivers can simultaneously get their respective classical information sent by a sender. Because each photon has two degrees of freedom, the protocol will achieve a high transmittance. The security of the simultaneous dense coding protocol has also been analyzed. Secondly, we investigate how to simultaneously teleport two different quantum states with polarization and path degree of freedom using cluster states to two receivers, respectively, and discuss its security. The preparation and transmission of two-photon four-qubit cluster states is less difficult than that of four-photon entangled states, and it has been experimentally generated with nearly perfect fidelity and high generation rate. Thus, our protocols are feasible with current quantum techniques.

More general probability sum-rules for describing interference found in quantum mechanics (QM) were formulated by Sorkin in a hierarchy of such rules. The additivity of classical measure theory corresponds to the second sum-rule. QM violates this rule, but satisfies the third and higher sum-rules. This evokes the question of whether there are physical principles that forbid their violation. We show that in a theory that is indistinguishable from quantum mechanics in first and second-order interferences, the violation of higher sum-rules allows for superluminal signaling, essentially because probability measures can be contextual in such theories.

Wireless sensor networks (WSNs) can take the advantages by utilizing the security schemes based on the concepts of quantum computation and cryptography. However, quantum wireless sensor networks (QWSNs) are shown to have many practical constraints. One of the constraints is the number of entangled qubits which is very high in the quantum security scheme proposed by [Nagy et al., Nat. Comput. 9 (2010) 819]. In this work, we propose a modification of the security scheme introduced by Nagy et al. and hence the reduction in the number of entangled qubits is shown. Further, the modified scheme can overcome some of the constraints in the QWSNs.

Nitrogen-vacancy (NV) centers implanted beneath the diamond surface have been demonstrated to be effective in the processing of controlling and reading-out. In this paper, NV center entangled with the fluorine nuclei collective ensemble is simplified to Jaynes–Cummings (JC) model. Based on this system, we discussed the implementation of quantum state storage and single-qubit quantum gate.

We study qutrit teleportation and its fidelity in the presence and absence of intrinsic decoherence through a qutrit channel. The channel consists of a Heisenberg chain with $xyz$ interaction model and the intrinsic decoherence is implemented through the Milburn model. It is shown that while the fidelity diminishes due to intrinsic decoherence, it may be enhanced if the channel is initially in an entangled state. It is also observed that, for stronger intrinsic decoherence, the initial entanglement of the channel is more effective in enhancing of fidelity.

Quantum computation strongly relies on the realization, manipulation, and control of qubits. A central method for realizing qubits is by creating a double-well potential system with a significant gap between the first two eigenvalues and the rest. In this work, we first revisit the theoretical grounds underlying the double-well qubit dynamics, then proceed to suggest novel extensions of these principles to a triple-well qutrit with periodic boundary conditions, followed by a general $d$-well analysis of qudits. These analyses are based on representations of the special unitary groups SU$(d)$ which expose the systems’ symmetry and employ them for performing computations. We conclude with a few notes on coherence and scalability of $d$-well systems.

Hilbert–Schmidt (HS) decompositions are employed for analyzing systems of $n$-qubit, and a qubit with a qudit. Negative eigenvalues, obtained by partial-transpose (PT) plus local unitary (PTU) transformations for one qubit from the whole system, are used for indicating entanglement/separability. A sufficient criterion for full separability of the $n$-qubit and qubit–qudit systems is given. We use the singular value decomposition (SVD) for improving the criterion for full separability.

General properties of entanglement and separability are analyzed for a system of a qubit and a qudit and $n$-qubit systems, with emphasis on maximally disordered subsystems (MDS) (i.e. density matrices for which tracing over any subsystem gives the unit density matrix). A sufficient condition that $\rho$ (MDS) is not separable is that it has an eigenvalue larger than $1∕d$ for a qubit and a qudit, and larger than $1∕2^{n-1}$ for $n$-qubit system. The PTU transformation does not change the eigenvalues of the $n$-qubit MDS density matrices for odd $n$. Thus, the Peres–Horodecki (PH) criterion does not give any information about entanglement of these density matrices. The PH criterion may be useful for indicating inseparability for even $n$.

The changes of the entanglement and separability properties of the GHZ state, the Braid entangled state and the $W$ state by mixing them with white noise are analyzed by the use of the present methods. The entanglement and separability properties of the GHZ-diagonal density matrices, composed of mixture of 8GHZ density matrices with probabilities $p_i(i=1,2,\dots ,8)$, is analyzed as function of these probabilities. In some cases, we show that the PH criterion is both sufficient and necessary.