جستجوی مقالات مرتبط با کلیدواژه « entanglement » در نشریات گروه « فیزیک »

Recently, coherent perfect absorbers have received much attention from researchers in the field of classical optics. Such absorbers, known as a time-reversal of lasers, provide perfect absorption of the incident light. Considering the very attractive features of these materials, in this paper, we investigate the quantum optics of these materials. To do this, we consider two coherent perfect absorber structures and assume two identical two-level atoms; one of which is prepared in its ground state and the other in the excited state, are placed on both sides of the coherent perfect absorber slabs. We compute the spontaneous emission rate, Lamb shift, and collective decay of atoms near two slabs, and then investigate the entanglement dynamics of the atomic system using the concurrence. We find that these parameters exhibit a damped oscillatory behavior with increasing the distance between the atoms and the slabs. In contrast to the classical regime, we also observe that these structures are not the perfect absorbers in the quantum regime.

This paper describes the interaction of three two-level atoms with a single-mode quantized field in the intensity-dependent coupling regime. Under a choice for the initial conditions for subsystems, where the atoms are prepared in the excited state and the cavity field is in the standard coherent state, the explicit form of the state vector of the whole system are obtained. To achieve this goal, the Laplace transform technique can be applied. By considering the intensity-dependent and constant coupling regimes, some of the most important physical properties of the system such as quantum entanglement between the atomic subsystem and the radiation field subsystem, atomic population inversion, quantum statistics of photons, and quadrature squeezing are numerically investigated. The numerical results show that the presence of nonlinear function can affect in the depth and the domain of the nonclassicality of the system. Also, by choosing different nonlinearity functions corresponding to any nonlinear oscillator with arbitrary nonlinear function, or corresponding to any solvable quantum system with a known discrete spectrum, the presented formalism would clearly be distinguished.

In this paper, we use a direct detection in an optical parametric amplifier (OPA) receiver in a lossy and noisy environment and obtain the quantum enhancement factor (QEF) and error probability, which improve the quantum illumination (QI) performance. QI uses the entanglement between the signal and the idler to detect the target. Different parameters of the assumed scheme help us to improve the target detection in the current configuration. Thus, we show that the channel transmissivity function from the transmitter output to the receiver input plays an significant role in improving the QI performance. In addition, we state that the QEF increases with the increase of detector quantum efficiency and transmissivity. One of the most important applications of this work is the manufacturing of QI with optimal performance, which is very beneficial in the country's defense and research industries.

In this paper, we model an entangled system of two identical superconducting qubits in which Josephson junctions are coupled by a fixed capacitor. This coupling of the capacitor with Josephson junctions is added due to increase the coherence and neutralize the effects of decoherence in the system. To better understand the state of the system, using the theory of quantum computing, the qualitative behaviors of entanglement, coherence, and comparison between them are numerically investigated. It was observed that there is a limit for increasing the mutual coupling energy between two qubits, and beyond that, it will weaken the system performance. It was also seen that the behaviors of the entanglement and the coherence are almost similar and they can be maintained at high temperatures under some conditions. A remarkable point was is that, as the Josephson energy of the qubits increases, the coherence and entanglement increase even at high temperatures. Also, by increasing the mutual coupling energy between qubits, coherence and entanglement are maintained at high temperatures, which can be considered in the construction of entanglement sources. In addition, the difference between an identical and a dissimilar two-qubit system is also stated.

In this article, we first introduce a system consisting of two dissipative cavities, in each of which there is a two-level atom. Each of the atoms is under the influence of a classical laser field, which causes the displacement of their energy levels. The correlation between two cavities is given by the field-field interaction term. We use the Gardiner-Collet model to describe the dissipation in this research. After introducing the Hamiltonian of the system, we simplify the Hamiltonian by using the Fano technique and introducing two sets of new operators. Also, with introducing the atomic bare bases, the Hamiltonian of the system will become a solvable form. Then, by using the technique of Laplace transforms, we solve the time-dependent Schrödinger equation and find the explicit form of the system's wave function at every moment of time. Having the wave function of the atom-atom system and using the concurrence criterion, we investigate the entanglement dynamics of the system in two strong and weak interaction regimes corresponding to the non-Markovian and Markovian regimes. The results show that there will be no stable state of entanglement in this system. We will also show that the classical laser field will play a constructive role in maintaining the initial entanglement as well as the generated entanglement.

The quantum synchronization and correlation have been studied between two coupled dissipative van der pol oscillators in a quantum system. Using the master equation approach, this study calculated mutual information, entanglement and synchronization error and investigated the effect of dissipative coupled on quantum synchronization and correlations of two coupled dissipative van der pol oscillators, considering the effect of temperature change in the squeeze states,. The results show that increasing the temperature in the squeeze states has a negative effect on the quantum synchronization and quantum correlation, while mutual information, which is the sum of classical and quantum correlations, indicates a decrease.

One of the most critical aspects of studying entanglement states is quantifying the degree of entanglement. One of the entanglement measures for pure states is Van- Neumann entropy, which is why Van-Neumann entropy is also called entanglement entropy. In this work, to calculate the entanglement of a three-particle atom called the Moshinsky atom; first, the reduced density matrix was calculated and then using the coordinates of the three particles in the lithium atom, the entropy entanglement was calculated. We obtained the entropy of entanglement in terms of the frequency parameter related to the external coordinate field.

In this work, for the first time, the qualitative behaviors of squeezing and entanglement in quantum two-mode squeezed radars (QTMS) when the target is present and the signal is transmitted to the target are calculated and their qualitative behaviors are evaluated. The squeezing parameter is a tool in theory similar to the laboratory's signal power. Therefore, the correlation between signal-idler increases or decreases with signal power changes. This increase or decrease of correlation (especially entanglement) leads to improvement or weakening of the performance of quantum radars. In this work, it can be seen that with the increase of the squeezing parameter even at room temperature (300 K), the behavioral quality of the squeezing increases, and hence, the correlation between the signal and the idler also increases. We also examine the entanglement, where we see that there is a maximum limit to increase in signal power that cannot be exceeded, however, this limit can be violated by choosing a suitable receiver, hence at high powers maintained entanglement. Therefore, by controlling the squeezing parameter and choosing a suitable receiver, which leads to the improvement of squeezing and entanglement behaviors, the performance of a QTMS radar can be optimized at room temperature.

In this article, we investigate photonic squeezed states with a special initial state. For this purpose, first, we consider a special initial state of two modes, then we squeezed one or two modes. In the following, we describe a method based on the Wigner function for the entanglement of the system, first we write the state of the system in the phase space using the Wigner function, and then apply the squeezing operator to the Wigner function of a system, next the state of the system transferred from the phase space to the Hilbert space. I do. In the end, we obtain the degree of entanglement for N=1,2 states by using concurrence. In this article, you can see the effect of the initial state and the number of modes used on the degree of entanglement of the system.

This paper uses the Jaynes-Cummings model to investigate the entanglement of a two-qutrit state in a cavity. The entanglement is analyzed as a function of decoherence rate, coupling constant, and frequency of atomic transition. We note that this entanglement is decreased passing time and the negativity is an increasing function of the frequency of atomic transition. The negativity at first is an increasing function of the coupling constant, then for higher values of the coupling constant, the negativity decreases with the increase of the coupling constant and over time it tends to zero. We also investigate the influence of intrinsic decoherence on quantum teleportation via this two-qutrit state. We plot the fidelity as a function of the decoherence rate, the coupling constant, and the atomic transition frequency. The results show that the fidelity decreases with an increasing decoherence rate. Moreover, the frequency of fidelity oscillations is an increasing function of the atomic transition frequency. The fidelity is relatively independent of the coupling constant, especially at a higher value of the coupling constant.

One of the most powerful and popular quantum entanglement-based computation methods for the simulation of one-dimensional and quasi-two-dimensional strongly correlated materials is the Density Matrix Renormalization Group (DMRG) technique. A lower quantum entanglement between the system’s degrees of freedom will result in a higher accuracy of the method. However, quantum entanglement between two complementary subsystems of a closed system depends on how the degrees of freedom have been distributed between them, and therefore, it is not in general invariant under arbitrary unitary transformations. Entanglement is invariant only under those unitary transformations which do not mix the degrees of freedom associated with the two subsystems. Consequently, a fundamental question is that if it is possible to construct, algorithmically, unitary transformations which minimize entanglement between degrees of freedom of the system and therefore maximize the DMRG technique? A general solution to this problem is highly nontrivial in general. In this paper, we study interacting fermionic models and by considering single-particle unitary transformations only. We define a novel and efficient method to find the optimal single-particle unitary transformations. In this framework, we show that using our algorithm, the accuracy of the DMRG method at a given bond dimension is increased and the ground-state energy is decreased and approaches to its exact value. Our method paves the way toward finding more general (many-particle) optimized unitary transformations and helps us to better understand the behavior of fermions in strongly correlated materials.

‎We present a theoretical interacting one-dimensional Bose-Einstein condensate (BEC) inside an optical cavity which is driven through of the fixed end mirrors. Under the Bogoliubov approximation and when the number of photons inside the cavity is not too large, the atomic field operator can be considered as a single-mode quantum field which is coupled to the radiation pressure of the intracavity field. In this way, the system behaves like an optomechanical system with an extra nonlinear term corresponding to the atom-atom interaction.We show that one of the best ways of tracing the effect of atomic interaction is to study the noise power spectrum of the field of the cavity. For this purpose, we study the light intensity spectrum of the cavity as well as the entanglement between the optical cavity and the BEC. We show how the pattern of the power spectrum of the cavity changes due to the nonlinear effect of atomic collisions. Furthermore, it is shown that due to the s-wave scattering frequency of the atom-atom interaction, one can measure the strength of interatomic interaction. Besides, we show how the atomic collisions affect the entanglement between subsystems and cooling behavior of the BEC atoms.

In this study, spin squeezing was calculated for two common square systems in terms of the system variables using the Kitagawa parameter, and entanglement of these systems was compared based on the Meyer–Wallach and LE measures. Calculations showed a certain relationship between the Kitagawa parameter and the Meyer–Wallach measure in the two square systems. The correlation between squeezing and the Meyer–Wallach measure remains unchanged in cases where squeezing is dependent on the interaction between the sites. No correlation was found between entanglement in terms of the LE measure and squeezing in the studied systems. However, in one of the systems, a weak correlation between them was observed.

In this paper, we considered the two-qubit Heisenberg XXZ model with Dzyaloshinski-Moriya (DM) interaction under the Calogero-Moser model type I, where each spin is affected by the homogeneous and inhomogeneous magnetic field. Assuming that the initial state of the system is , we examined the effects of the relative distance between spins, intrinsic decoherence, magnetic field, DM interaction and the parameters of the initial state of the system on the dynamics of quantum entanglement. The results showed that by increasing the relative distance between the spins, the entanglement decreased. In addition, by applying the inhomogeneous magnetic field in the presence of DM interaction, the effect of intrinsic decoherence on entanglement dynamics could be observed. We also used the two-qubit Heisenberg XXZ model density matrix in the presence of intrinsic decoherence as a quantum channel to teleport the quantum states from the sender to the receiver. Using the fidelity measure, we studied the effect of the different parameters on the quality of the teleported quantum state. We observed that by applying the inhomogeneous magnetic field and in the presence of the DM interaction, the average fidelity is larger than the minimum of the average fidelity i.e.,

Quantum multi-hop teleportation is important in the field of quantum communication. In this paper, we proposed a Multi-hop quantum teleportation scheme of the 6-qubit entangled state. In the proposed scheme,  the sender teleports the desired information by transmitting a 6-qubit entangled quantum state to the receiver through the N intermediate nodes between the source node and destination node. Therefore,  the sender and receiver are not in direct connection with each other, and the information is teleported in Multi-hop between the N neighbor nodes. Also, in this scheme, the 7-qubit entangled states as quantum channels are shared between neighbor nodes.  Always, teleportation processes over long distances are problematic because of the existence of the environmental noise which is almost inescapable and adversely affects the entangled quantum channel. This scheme explains the quantum teleportation between N nodes in N-1 steps and it is suitable for teleportation processes over long distances. Also, we calculate the efficiency of this scheme and indicate the advantages of this protocol.

In this study, we introduce a new class of two-mode qutrit-like entangled states based on the `Near' coherent states. They link to a specific class of non-classical states, namely, photon added coherent states, which makes them capable candidates in quantum information processes. Based on these states, various superpositions such as the two-qubit entangled states, have been introduced and studied by various authorities, which is evidence for their valuable quantum properties. Therefore, based on the above-mentioned relationship and the emergence of controllable non-classical properties of these states, in this work, we seek to analyze the effect of adding photons to two-qutrit entangled states. For this purpose, we present a general analysis of non-classical properties such as the photon statistics and entanglement with emphases on the control role of the shift parameter of these states. We apply the generalized I-concurrence measure to quantify the entanglement and the condition in which quantum entanglement can be enhanced and maximized. Comparing with some cases already discussed in the literature, we can see that the photon addition, which is equivalent to selecting a specific shift parameter, plays an important role in non-classical effects, and this operation can be applied to enhance and preserve the entanglement.

AbstractThe violation of Bell’s inequality in quantum mechanics implies that there exist nonlocality and entanglement. When the density matrix of a composite system cannot be written as a convex combination of the product of the density matrices of its subsystems, we say there exists entanglement. For pure states, the existence of entanglement always leads to the violation of Bell’s inequality. However, in the case of the mixed states, there may be entanglement, but Bell's inequality is not violated and in other words, the nonlocality is not manifested. In addition to Bell's inequality, quantum teleportation is also a manifestation of nonlocality. Quantum teleportation using entangled states is more successful than quantum teleportation with separable states. Therefore, the corresponding fidelity of teleported state with the initial state (in short, the fidelity) of the former is always greater than the fidelity of the latter. In this paper, for Werner's state, we will show that in a range of the related parameter, while the CHSH inequality is violated, the fidelity, which indicates the amount of success of quantum teleportation, is lower than the upper bound of the corresponding fidelity for states that can be simulated with a local hidden variable theory. Meanwhile, we will see that for Gisin's state with hidden nonlocality, filtering, which leads to the appearance of nonlocality and more specifically leads to the violation of the CHSH inequality, also increases the fidelity.

In this study, the detection method of entanglement of 3-mode spin coherence states is investigated. For this purpose, by properly defining the computational codes, these states are mapped to a three qubit quantum state, and then, by using the Mermin- Kalyshko inequality, the entanglement of these states is studied and an analytical relationship to determine the entanglement region is presented. The results of numerical analysis of the extracted inequality show that the entanglement region of these states depends on the coherence parameters, the amount of spin as well as the phase of the states. For example, in the case where the values of the coherence parameters are equal but with opposite signs, by adjusting the coherence phase, the degree of entanglement can be controlled such that the maximum entanglement occurs . More important, as the value of spin increases, the allowable range of the coherence parameter for entanglement detection increases. These results are consistent with the data reported in the study of the degree of entanglement of 2-mode superposition of spin coherent states using other measures and criteria of entanglement.  The findings of this study can be used in the study of non-classical and quantum systems and quantum correlations in quantum information science.