Sharif Digital Repository

A method is described to solve the nonlinear Langevin equations arising from quadratic interactions in quantum mechanics. While the zeroth order linearization approximation to the operators is normally used, here, first and second order truncation perturbation schemes are proposed. These schemes employ higher-order system operators, and then approximate number operators with their corresponding mean boson numbers only where needed. Spectral densities of higher-order operators are derived, and an expression for the second-order correlation function at zero time-delay has been found, which reveals that the cavity photon occupation of an ideal laser at threshold reaches √6 - 2, in good... 

We enquire into the quasi many-body localization in topologically ordered states of matter, revolving around the case of Kitaev toric code on the ladder geometry, where different types of anyonic defects carry different masses induced by environmental errors. Our study verifies that the presence of anyons generates a complex energy landscape solely through braiding statistics, which suffices to suppress the diffusion of defects in such clean, multicomponent anyonic liquid. This nonergodic dynamics suggests a promising scenario for investigation of quasi many-body localization. Computing standard diagnostics evidences that a typical initial inhomogeneity of anyons gives birth to a glassy... 

We present a detailed calculation of the linear and nonlinear optical response of four types of monolayer two-dimensional (2D) transition-metal dichalcogenides (TMDCs), having the formula MX2 with M = Mo, W and X = S, Se. The calculations are based on 6-band tight-binding model of TMDCs, and then performing a semi-classical perturbation analysis of response functions. We numerically calculate the linear and nonlinear surface susceptibility tensors with ωΣ = ωr + ωs + ωt. Both non-degenerate and degenerate cases are studied for third-harmonic generation and nonlinear refractive index, respectively. Computational results obtained with no external fitting parameters are discussed regarding two... 

We describe a quantum interface between an optical and a microwave field based on their common interaction with a nano-mechanical resonator. This is an effective source of two-mode squeezing with an optical idler and a microwave signal, enabling continuous variable teleportation  

We have studied the simplest optomechanical system close to and in the bistable regime. We find that Optomechanical entanglement is particularly strong in this regime for large enough detuning. The robustness of entanglement against temperature is also studied  

We investigate the possibility of correcting errors occurring on a multipartite system through a feedback mechanism that acquires information through partial access to the environment. A partial control scheme of this type might be useful in dealing with correlated errors. In fact, in such a case, it could be enough to gather local information to decide what kind of global recovery to perform. Then, we apply this scheme to the depolarizing and correlated errors and quantify its performance by means of entanglement fidelity  

We investigate the Fano profile in an optomechanical system with two vibrating mirrors. The detuning between their mechanical frequency, leads to the occurrence of double Fano resonances. The Fano profile can be controlled by adjusting the cavity parameters  

Cavity quantum electrodynamics of multipartite systems are studied in depth, which consists of an arbitrary number of emitters in interaction with an arbitrary number of cavity modes. The governing model is obtained by taking the full field-dipole and dipole-dipole interactions into account, and is solved in the Schrödinger picture with assumption of vanishing field and dipole interactions at high energies. An extensive code is developed that is able to solve the system and track its evolution in time, while maintaining sufficient degrees of arbitrariness in setting up the initial conditions and interacting partitions. Using this code, we have been able to numerically evaluate various... 

Using coherent states in optical quantum process tomography is a practically relevant approach. Here we develop a framework for complete characterization of quantum-optical processes in terms of normally-ordered moments by using coherent states as probes. We derive the associated superoperator tensors for several optical processes. We also show that our technique can be used to determine nonclassicality features of quantum-optical states and processes. Furthermore, we investigate identification of multimode Gaussian processes and show that the number of necessary probe coherent states scales linearly with the number of modes. © 2017 American Physical Society  

In this paper we analyze a disk-like quantum dot embedded in an engineered two-dimensional (2D) photonic crystal cavity as an artificial atom. In this quantum dot electron and hole form an exciton where photon and electron-hole bound state can interact. Within the engineered electromagnetic vacuum of the PBG material, the exciton can emit and reabsorb a photon. If the exciton energy lies near the photonic band gap edge the exciton level splits into two levels. This is similar to the interaction of field and an atom in a high Q cavity. Here excitonic states are evaluated taking in to account the band mixing for holes. Also energy associated with dressed exction is evaluated  

In the preceding Comment [Jian-Zhong Du, Su-Juan Qin, Qiao-Yan Wen, and Fu-Chen Zhu, Phys. Rev. A 74, 016301 (2006)], it has been shown that in a quantum secret sharing protocol proposed in [S. Bagherinezhad and V. Karimipour, Phys. Rev. A 67, 044302 (2003)], one of the receivers can cheat by splitting the entanglement of the carrier and intercepting the secret, without being detected. In this reply we show that a simple modification of the protocol prevents the receivers from this kind of cheating. © 2006 The American Physical Society  

We show that networks of spin-1 particles connected in a special geometry and subject to Affleck-Kennedy-Lieb-Tasaki (AKLT) interaction are capable of perfectly transferring states of particles (qubits and qutrits) if we also allow a global control of the network in predetermined time intervals. The geometry can be one, two, and three dimensional. The strengths of the couplings have the same modulus, and only their signs differ on various bonds. Any particle which is routed in the network acquires relative phase shifts which can be corrected after it is extracted from the network. An advantage of this protocol is that one can route more than one particle through the network simultaneously.... 

In this paper we derive a new upper bound on the quantum key distillation capacity. This upper bound is an extension of the classical bound of Gohari and Anantharam on the source model problem. Our bound strictly improves the quantum extension of reduced intrinsic information bound of Christandl et al. Although this bound is proposed for quantum settings, it also serves as an upper bound for the special case of classical source model, and may improve the bound of Gohari and Anantharam. The problem of quantum key distillation is one in which two distant parties, Alice and Bob, and an adversary, Eve, have access to copies of quantum systems A, B, E respectively, prepared jointly according to... 

We propose an exactly solvable waveguide lattice incorporating an inhomogeneous coupling coefficient. This structure provides classical analogs to the squeezed number and squeezed coherent intensity distribution in quantum optics where the propagation length plays the role of a squeezed amplitude. The intensity pattern is obtained in a closed form for an arbitrary distribution of the initial beam profile. We have also investigated the phase transition to the spatial Bloch-like oscillations by adding a linear gradient to the propagation constant of each waveguide (α). Our analytical results show that the Bloch-like oscillations appear above a critical value for the linear gradient of the... 

A scheme for deterministic secure and high bit-rate direct communication without resorting to a distinct control interval is proposed. It utilizes three entangled qubits, and presents higher bit transfer rate of information and higher security. The security of protocol is asserted by introducing a security control for each transferred bit. The protocol is investigated for a class of individual attacks and it is explicitly showed that the protocol has a high security even in the presence of channel loss  

How a proposed quantum nonlocal phenomenon could be incompatible with the requirements of special relativity is studied. To show this, the least set of assumptions about the formalism and the interpretation of non-relativistic quantum theory is considered. Then, without any reference to the collapse assumption or any other stochastic processes, an experiment is proposed, involving two quantum systems, that interacted at an arbitrary time, with results which seem to be in conflict with requirements of special relativity  

We study quantum states generated by a sequence of nearest neighbor bipartite entangling operations along a one-dimensional chain of spin qubits. After a single sweep of such a set of operations, the system is effectively described by a matrix product state (MPS) with the same virtual dimension as the spin qubits. We employ the explicit form of the MPS to calculate expectation values and two-site correlation functions of local observables, and we use the results to study fluctuations of collective observables. Through the so-called macroscopicity and the squeezing properties of the collective spin variables they witness the quantum correlations and multiparticle entanglement within the... 

We show that two parties far apart can use shared entangled states and classical communication to align their coordinate systems with a very high fidelity. Moreover, compared with previous methods proposed for such a task, i.e., sending parallel or antiparallel pairs or groups of spin states, our method has the extra advantages of using single-qubit measurements and also being secure, so that third parties do not extract any information about the aligned coordinate system established between the two parties. The latter property is important in many other quantum information protocols in which measurements inevitably play a significant role. © 2017 American Physical Society  

A system scheme is presented, which allows non-reciprocal wave transmission or directional amplification of electromagnetic signals, using a boxed four-node method. Edges represent strong hopping interactions and diagonals stand for weak parametric interactions. Using careful optimization of values for design parameters, we are able to obtain non-reciprocity in excess of 12 and 130 dB for intrinsic and extrinsic configurations at identical input/output frequencies. For the directional amplification, an isolation as high as 40 dB is demonstrated with the forward/backward gains of ±20 dB. Cascading two such systems potentially can offer high isolations at high gains. © 1965-2012 IEEE