Search

Atomic nonlinear interferometry has wide applications in quantum metrology and quantum information science. Here we propose a nonlinear time-reversal interferometry scheme with high robustness and metrological gain based on the spin squeezing generated by arbitrary quadratic collective-spin interaction, which could be described by the Lipkin–Meshkov–Glick (LMG) model. We optimize the squeezing process, encoding process, and anti-squeezing process, finding that the two particular cases of the LMG model, one-axis twisting and two-axis twisting outperform in robustness and precision, respectively. Moreover, we propose a Floquet driving method to realize equivalent time reverse in the atomic system, which leads to high performance in precision, robustness, and operability. Our study sets a benchmark for achieving high precision and high robustness in atomic nonlinear interferometry. [ABSTRACT FROM AUTHOR]

Neural networks are becoming ubiquitous in various areas of physics as a successful machine learning (ML) technique for addressing different tasks. Based on ML technique, we propose and experimentally demonstrate an efficient method for state reconstruction of the widely used Sagnac polarization-entangled photon source. By properly modeling the target states, a multi-output fully connected neural network is well trained using only six of the sixteen measurement bases in standard tomography technique, and hence our method reduces the resource consumption without loss of accuracy. We demonstrate the ability of the neural network to predict state parameters with a high precision by using both simulated and experimental data. Explicitly, the mean absolute error for all the parameters is below 0.05 for the simulated data and a mean fidelity of 0.99 is achieved for experimentally generated states. Our method could be generalized to estimate other kinds of states, as well as other quantum information tasks. [ABSTRACT FROM AUTHOR]

We present a magnetic scanning microscope equipped with a nitrogen-vacancy (NV) center scanning probe that has the ability to mechanically tune the strain of soft matter in-situ. The construction of the microscope and a continuous strain-tuning sample holder are discussed. An optically detected magnetic resonance protocol utilized in the imaging is described. In order to show the reliability of this microscope, the strain conduction is estimated with finite element simulation, and x-ray diffraction is required for calibration when freestanding crystal films are under consideration. A magnetic imaging result is displayed to demonstrate the nano-scale imaging capability. The microscope presented in this work is helpful in studying strain-coupled magnetic physics such as magnetic phase transition under strain and strain-tuned cycloidal orientation tilting. [ABSTRACT FROM AUTHOR]

In an optomechanical system with double-sided cavities, it was shown that squeezing and entanglement can be dramatically enhanced by a suitable periodically-modulated field driving either or both ends of the cavity. But, whether two-fold amplitude modulations can have interference effects and further enhance the desired effect are still not known. Here, we revisit this question and show that the relative phase of the two external lasers plays an important role in controlling the classical dynamics of the optical and mechanical modes, leading to irregular oscillating behaviours. Furthermore, the asymptotic dynamics of the mechanical mean values can transit from one-period oscillation to period-doubling oscillation, which is a nonlinear process associated with chaos. Except for changes in the oscillation period, the oscillation amplitudes of the classical mechanical mean values can be apparently enhanced by increasing the relative phase between two modulation driving lasers. The enlarged oscillation amplitudes of the classical mechanical mean values could destroy the condition of the effective detuning to generate entanglement. As a consequence, the entanglement between two cavity fields gradually decreases and disappears entirely when the relative phase between two modulation driving lasers is large enough. Although the entanglement between two cavity fields can be hardly enhanced by the cooperative effect of two-fold modulations even when the relative phase is chosen properly, this investigation may provide a new approach for harnessing optomechanical nonlinearity to manipulate rich dynamics. [ABSTRACT FROM AUTHOR]

We introduce four new entangled quantum states which are generated by applying symmetric photon operations to two-mode entangled coherent states. As a comparison, we investigate quantum statistical properties, squeezing, entanglement affected by photon operations. Through our numerical and theoretical analysis, it is interesting to find that the symmetric photon-addition operation enhances the squeezing and weakens the entanglement of two-mode entangled coherent states. [ABSTRACT FROM AUTHOR]

In this paper we design a new joint remote state preparation (JRSP) scheme for a single qubit by introducing a mentor whose role is to initiate and fix the protocol after which it is for the rest of the parties to follow some prescribed instructions. There is no initial entangled connection between the senders and the receiver. The scheme has eight branchings depending on an act of measurement by the mentor. The protocol has advantages in terms of secrecy of the information to be transmitted. [ABSTRACT FROM AUTHOR]

We analyse the entanglement of the light produced by a three-level laser whose cavity contains a parametric amplifier, with the cavity mode driven by the coherent light and coupled to a squeezed vacuum reservoir. Employing stochastic differential equations associated with the normal ordering resulted from the pertinent master equation, the quadrature variances and entanglement of the two-mode light are obtained and photon statistics of the two-light modes is discussed. It is found that the three-level laser generates squeezed light under certain conditions, with maximum intracavity squeezing being 89.7% below the coherent state level. We also show that the parametric amplifier enhances the mean photon number and photon number correlation. [ABSTRACT FROM AUTHOR]

In this manuscript, we present a system consisting of a three-level atom interacting with optical field. We investigate qualitatively the entanglement and atomic (field) geometric phase under the effect of cavity damping. The atom–field entanglement is measured by the negativity. We show that these quantifiers depend strongly on the variations of the initial settings of the atom, and this exhibits substantial phenomena that depend on the cavity damping effect. Finally, we explore the link between the entanglement and atomic (field) geometric phase of different physical parameters within the presence and absence of the cavity damping effect. [ABSTRACT FROM AUTHOR]

We propose an optimal protocol for remotely preparing a four-qubit entangled state. In this protocol, a sender and two potential receivers share a ten-qubit entangled state. The sender performs a two-qubit state measurement based on the information of the quantum state that needs to be prepared. According to the measurement results of the sender, any potential receiver can obtain the quantum state that the sender needs to prepare by using the corresponding unitary transformation. [ABSTRACT FROM AUTHOR]

In this paper, we study detection of the state non-classicality for a quantum harmonic oscillator by a qubit in the presence of dissipation effects. We show that dissipation can enhance the effectiveness of the method in case of using the corrected form of the related nonclassicality witness. Such an improvement is attributed to the fact that dissipation leads to probing a surface, instead of a curve, of the complex plane for non-classicality condition on normally-ordered characteristic function. [ABSTRACT FROM AUTHOR]

We propose a scheme of controlled teleportation of an arbitrary three-qubit state by using a five-qubit cluster state. In our scheme, the sender Alice performs five sets of single-qubit measurements and the controller Charlie needs only take a single-qubit measurement. The receiver Bob can reconstruct the arbitrary three-qubit state by performing the single-qubit unitary operations and a controlled-NOT operation if he obtains the help of the controller. We show that the efficiency in this scheme approaches 100% [ABSTRACT FROM AUTHOR]

We have studied the case in which one mode of the light field in the two-mode squeezed vacuum state evolves in a diffusion channel. By virtue of thermo-entangled state representation and the technique of integration within an ordered product, the evolution formula of the field density operator is given. Its non-classical properties, such as squeezing effect, antibunching effect, the violation of Cauchy–Schwartze inequality and the entanglement property between two modes, are studied. The influences of the squeezing parameter and the dissipation time on the non-classical properties are discussed. The results obtained by the numerical method show that its non-classical properties are all weakened with the dissipation. On the other hand, its squeezing effect and the entanglement property between two modes are strengthened, but its antibunching effect and the violation of Cauchy–Schwartze inequality are weakened with the increase of the squeezing parameter. [ABSTRACT FROM AUTHOR]

We propose a scheme of controlled cyclic quantum teleportation of an arbitrary two-qubit entangled state by using a ten-qubit entangled state. In our scheme, Alice can teleport an arbitrary two-qubit entangled state to Bob, Bob can teleport an arbitrary two-qubit entangled state to Charlie and at the same time Charlie can teleport an arbitrary two-qubit entangled state to Alice with the help of the controller David. [ABSTRACT FROM AUTHOR]

We consider the case that the two-level atom off-resonantly interacts with cavity field via a one-photon hopping in double Jaynes-Cummings system. Geometrical quantum discord (GQD) is adopted to quantify the degree of correlation between two subsystems. GQD dynamics between two atoms, between cavity fields, as well as between atom and cavity fields are studied. The influences of detuning on GQD are discussed. The results obtained using the numerical method show that GQD between two atoms, between cavity fields, and between atom and cavity fields all display periodic oscillations. Furthermore, their periodic oscillations frequencies increase with the increase of detuning. On the other hand, GQD between two cavity fields and that between atom and cavity are weakened with increase of detuning. [ABSTRACT FROM AUTHOR]

In this paper, we consider the situation that three identical two-level atoms are separately trapped in the three single-mode cavities. Each atom resonantly interacts with cavity via a one-photon hopping. The dynamics of nonlocality in the system is investigated via Mermin-Ardehali-Belinksii-Klyshko inequality. The results show that when three atoms are initially in W state and three-cavity fields are in vacuum states both the quantum state of three atoms and that of three cavities all display nonlocality On the other hand, when three atoms are initially in Greenberger-Horne-Zeilinger state and three-cavity fields are in vacuum states, the quantum state of three atoms and that of three cavities all do not display nonlocality. [ABSTRACT FROM AUTHOR]

In this paper we consider the situation that four identical two-level atoms are separately trapped in four single-mode optical cavities, which are placed at the vertices of a tetrahedron and coupled by six fibers. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the schrödinger equation when the total excitation number of the system equals one. The geometrical quantum discords between atoms and between cavities are investigated. The effects of cavity-fiber coupling coefficient on the geometrical quantum discords between atoms and between cavities are discussed. The results obtained using the numerical method indicate that the geometrical quantum discords between atoms and between cavities are all weakened with increase of cavity-fiber coupling coefficient. [ABSTRACT FROM AUTHOR]

We propose a scheme to prepare W state for three atoms in a cavity via adiabatic passage and quantum Zeno dynamics. Appropriate Rabi frequencies of the classical fields are selected to realize the present scheme. Numerical analysis is showed, which indicates that the scheme is robust against the floating of the pulse delay and laser intensity, and the atomic spontaneous radiation and the cavity decay are efficiently suppressed by engineering adiabatic passage. Moreover, the scheme is more achievable in experiment than other existing schemes. Based on the current experimental technology, this scheme for generation of high-fidelity W state for three atoms can be achieved. [ABSTRACT FROM AUTHOR]

In this paper, we consider the situation that four identical two-level atoms are separately trapped in separated tetrahedral structure single-mode optical cavities, which are placed at the vertices of a tetrahedron and are coupled by four fibres. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the Schrödinger equation when the total excitation number of the system equals one. Negativity is adopted to quantify the degree of entanglement between two subsystems. The entanglement dynamics between atoms and between cavities is studied. The influences of atom-cavity coupling coefficient on the entanglement between atoms and that between cavities are discussed. The results obtained using the numerical method show that the atom-atom entanglement and the cavity-cavity entanglement are all strengthened with increase of atom-cavity coupling coefficient. [ABSTRACT FROM AUTHOR]

In this paper the interaction between two two-level atoms with a single-mode quantized field is studied. To achieve exact information about the physical properties of the system, one should take into account various sources of dissipation such as photon leakage of cavity, spontaneous emission rate of atoms, internal thermal radiation of cavity and dipole-dipole interaction between the two atoms. In order to achieve the desired goals, we obtain the time evolution of the associated density operator by solving the time-dependent Lindblad equation corresponding to the system. Then, we evaluate the temporal behavior of total population inversion and quantum entanglement between the evolved subsystems, numerically. We clearly show that how the damping parameters affect on the dynamics of considered properties. By analyzing the numerical results, we observe that increasing each of the damping sources leads to faster decay of total population inversion. Also, it is observed that, after starting the interaction, the entanglement between one atom with other parts of the system as well as the entanglement between “atom-atom” subsystem and the “field”, tend to some constant values very soon. Moreover, the stable values of entanglement are reduced via increasing the damping factor ΓA (ΓA(1)=ΓA(2)=ΓA) where ΓA is the spontaneous emission rate of each atom. In addition, we find that by increasing the thermal photons, the entropies (entanglements) tend sooner to some increased stable values. Accordingly, we study the atom-atom entanglement by evaluating the concurrence under the influence of dissipation sources, too. At last, the effects of dissipation sources on the genuine tripartite entanglement between the three subsystems include of two two-level atoms and a quantized field are numerically studied. Due to the important role of stationary entanglement in quantum information processing, our results may provide useful hints for practical protocols which require some appropriate mechanisms to prevent or at least minimize the influence of decoherence phenomenon. [ABSTRACT FROM AUTHOR]

By virtue of the integration technique within ordered product of operators and Dirac’s representation theory we find a new formula for normally ordering coordinate-operator functions, that is f ( Q ˆ ) = : exp ( 1 4 ∂ 2 ∂ Q ˆ 2 ) f ( Q ˆ ) : , where Q ˆ is the coordinate operator, the symbol : : denotes normal ordering. Using this formula we can arrange a given quantum operator function f ( Q ˆ ) in its normal ordering conveniently and fast. Furthermore, we can derive a series of new relations about Hermite polynomial and Laguerre polynomial, as well as some new differential relations. [ABSTRACT FROM AUTHOR]

We study maximum simultaneous 2 n th-order Hong and Mandel's squeezing of the most general the Hermitian operator, X θ ≡ X 1 cos θ + X 2 sin θ , and n th-order sub-Poissonian photon statistics (connected with expectation value of 2 n th power of number fluctuation), in the most general superposition state, ψ = Z 1 α + Z 2 β of two coherent states α and β . Here Hermitian operators X 1,2 are defined by X 1 + iX 2 = a , the annihilation operator, complex numbers Z 1,2 , α , β are all arbitrary and the only restriction on these is the normalization condition of the state. We find a linear relationship between these two non-classical effects when the angle θ = arg( α + β ) and α + β ≫ α − β . We show that maximum simultaneous 2 n th-order Hong and Mandel's squeezing for the operator X θ and n th-order sub-Poissonian photon statistics in the state occur for an infinite combinations with α − β = A 2 n exp ± i 1 / 2 π + i θ ; A 2 n is a constant which depends on the order of squeezing, Z 2 / Z 1 = exp 1 / 2 ( β α * − β * α ) and with θ = arg( α + β ) and α + β ≫ α − β . We also illustrate analytically and numerically by an example that both effects occur simultaneously for high-intensity field but for observation of such effects, sensitive settings of the parameters are required. [ABSTRACT FROM AUTHOR]

In this paper, we study the decoherence of quantum excitation (photon-added) even /odd coherent states, $((\hat a{^{\dagger }})^{m} \left | {\alpha _ \pm } \right \rangle )$, in a thermal environment by investigating the variation of negative part of the Wigner quasidistribution function vs. the rescaled time. For this purpose, at first we obtain the time-dependent Wigner function corresponding to the mentioned states in the framework of standard master equation. Then, the time evolution of the Wigner function associated with photon-added even /odd coherent states, as well as the number of added photons m are analysed. It is shown that, in both states, the negative part of the Wigner function decreases with time. By deriving the threshold value of the rescaled time for single photon-added even /odd coherent states, it is also found that, if the rescaled time exceeds the threshold value, the associated Wigner function becomes positive, i.e., the decoherence occurs completely. [ABSTRACT FROM AUTHOR]

We mainly study on the effect of decoherence of the multiple photon annihilation-then-creation coherent state (MPACCS) in two different channels, i.e., amplitude decay and phase damping, by virtue of the time-evolution of the Wigner functions (WFs) of such state. Based on deriving the analytical expression of MPACCS's normalization factor related to the Bell-polynomial, the time-evolution of its WFs in each channel is derived analytically and discussed numerically. After undergoing the amplitude decay channel, the partial negativity of WFs diminishes gradually and the WFs have no negative region when the decay time exceeds a threshold value, while the negativity of WFs in phase damping channel varies more slowly than that of the amplitude decay. Especially, at long times WF evolves into the case of vacuum state in amplitude decay channel and a sum of weight WFs of all the Fock states in phase damping channel. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the higher-order nonclassical properties of a particular class of generalized coherent states namely the deformed photon-added nonlinear coherent states (DPACS) A † m |α, f, m 〉. To achieve this purpose we pay attention to higher-orders of squeezing (both Hillery- and Hong–Mandel-types), sub-Poissonian statistics and anti-bunching of the mentioned states with a well-known nonlinearity function. It is shown that for enough large values of field intensity (|α| 2 ) for a fixed N (the order of squeezing) by increasing m (the order of excitation) the degree of squeezing evaluated by Hillery and Hong–Mandel approaches increases, while for a chosen fixed value of m , by increasing N for Hillery (Hong–Mandel) type of squeezing the strength of squeezing decreases (increases). Similarly, the degree of higher-order sub-Poissonian statistics (with fixed K ) becomes lower when m increases, while (with fixed m ) it gets greater values when the order of sub-Poissonian K increases. At last, higher-order anti-bunching of the DPACS is evaluated, by which we established that its (always) negative values increase with increasing m , α and l (the order of anti-bunching) individually, i.e. the degree of anti-bunching increases. [ABSTRACT FROM AUTHOR]

Mixing the squeezed vacuum and coherent state in a beam splitter generates NOON states of the electromagnetic field, but with fidelity less than unity. In this article it is shown that the output of the beam splitter generates the NOON states with unit fidelity if superposed coherent states are used instead of the squeezed vacuum. Also, the post-selection probability of NOON states is shown to be higher. [ABSTRACT FROM AUTHOR]

In this paper, we consider two classes of nonlinear squeezed states: squeezed vacuum and squeezed one-photon states, which according to their generation processes, essentially include respectively even and odd bases of Fock space. Then, we produce their general superposition with respective phase φ which consists of all bases of Fock space. As a physical realization of the presented approach, we will use the q -deformation nonlinearity function to produce the corresponding nonlinear squeezed states and their superposition. In the continuation, due to extensive interest in nonclassicality features, we discuss about their main nonclassical properties such as sub-Poissonian statistics, normal and amplitude squared squeezing, number and phase squeezing and Husimi quasi-probability function of the obtained states and compare with those of the original constituent components. Meanwhile, we also pay attention to linear squeezed states (vacuum and one-photon) and particularly their superposition, too. Accordingly, the nonclassicality features of the introduced states are evidently established. At last, simple schemes for the generation of such states are presented. [ABSTRACT FROM AUTHOR]

Quantum state tomography (QST) is an essential tool for characterizing an unknown quantum state. Recently, QST has been performed for entangled qudits based on orbital angular momentum, time-energy uncertainty, and frequency bins. Here, we propose a QST for time-bin qudits, with which the number of measurement settings scales linearly with dimension d . Using the proposed scheme, we performed QST for a four-dimensional time-bin maximally entangled state with 16 measurement settings. We successfully reconstructed the density matrix of the entangled qudits, with which the average fidelity of the state was calculated to be 0.950.

The interaction of light and matter is often described by the exchange of single excitations. When the coupling strength is a significant fraction of the system frequencies, the number of excitations are no longer preserved and that simple picture breaks down. This regime is known as the ultrastrong coupling regime and is characterized by non-trivial light–matter eigenstates and by complex dynamics. In this work, we propose to use an array Josephson junctions to increase the impedance of the light mode enabling ultrastrong coupling to a transmon qubit. We show that the resulting dynamics can be generated and probed by taking advantage of the multi-mode structure of the junction array. This proposal relies on the frequency tunability of the transmon and, crucially, on the use of a low frequency mode of the array, which allows for non-adiabatic changes of the ground state.

We develop a coherent control theory for multimode quantum light. It allows us to examine a fundamental problem in quantum optics: what is the optimal pulse form to drive a two-photon-transition? In formulating the question as a coherent control problem, we show that—and quantify how much—the strong frequency quantum correlations of entangled photons enhance the transition compared to shaped classical pulses. In ensembles of collectively driven two-level systems, such enhancement requires nonvanishing interactions.

On-demand creation of entanglement between distant qubits is a necessary ingredient for distributed quantum computation. We propose an entanglement scheme that allows for single-shot deterministic entanglement creation by detecting a single photon passing through a Mach–Zehnder interferometer with one transmon qubit in each arm. The entanglement production essentially relies on the fact that superconducting microwave structures allow one to achieve strong coupling between the qubit and the photon. By detecting the photon via a photon counter, a parity measurement is implemented and the wave function of the two qubits is projected onto a maximally entangled state. Most importantly, the entanglement generation is heralded such that our protocol is not susceptible to photon loss due to the indivisible nature of single photons.

We suggest and demonstrate a tomographic method to characterise homodyne detectors at the quantum level. The positive operator measure associated with the detector is expanded in a quadrature basis and probed with a set of coherent states. The coefficients of the expansion are then retrieved using a least squares algorithm. Our model is general enough to describe different implementations of the homodyne setup, and it has proven capable of effectively describing the detector response to different tomographic sets. We validate the reconstructed operator measure on nonclassical states and exploit results to estimate the overall quantum efficiency of the detector.

When a periodically modulated many-body quantum system is weakly coupled to an environment, the combined action of these temporal modulations and dissipation steers the system towards a state characterized by a time-periodic density operator. To resolve this asymptotic non-equilibrium state at stroboscopic instants of time, we use the dissipative propagator over one period of modulations, ‘Floquet map’, and evaluate the stroboscopic density operator as its invariant. Particle interactions control properties of the map and thus the features of its invariant. In addition, the spectrum of the map provides insight into the system relaxation towards the asymptotic state and may help to understand whether it is possible (or not) to construct a stroboscopic time-independent Lindblad generator which mimics the action of the original time-dependent one. We illustrate the idea with a scalable many-body model, a periodically modulated Bose–Hubbard dimer. We contrast the relations between the interaction-induced bifurcations in a mean-field description with the numerically exact stroboscopic evolution and discuss the characteristics of the genuine quantum many-body state vs the characteristics of its mean-field counterpart.

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector. [ABSTRACT FROM AUTHOR]

The unitary equivalence of different bipartite entangled states with continuous variables under unitary transformations are investigated. With the help of the technique of integration within an ordered product of operators, the corresponding unitary operators are also derived. These results may deepen people's understanding to the various bipartite entangled states, and enrich the representations and transformations theory in quantum mechanics. [ABSTRACT FROM AUTHOR]

We present an analysis of a system of three two-level atoms interacting with one another through dipole-dipole interaction. The interaction manifests between the excited state of one of the atoms and the ground state of its nearest neighbour. Steady-state populations of the density matrix elements are presented and are compared with a situation when only two atoms are present. It can be noticed that the third atom modifies the behaviour of the three atoms. Two configurations are analysed, one in which the three atoms are in a line, with no interaction between atoms at the end points and the other in which the atoms form a closed loop with one atom interacting with both its neighbours. [ABSTRACT FROM AUTHOR]

Abstract: Transient response of nearly equispaced three-level ladder-type atomic system with a broad-band squeezed vacuum (SV) is investigated. We focus our attention in the interplay between the quantum interference and the squeezed field on the population distribution. It is shown that an atomic population inversion can be attained on one of the optical transitions due to the SV. Additionally, we show, with the proper value of the relative phase, the SV can also lead to unexpected population inversion on the transition between two different levels. [Copyright &y& Elsevier]

Spontaneous emission spectrum in gravitational Jaynes-Cummings model (JCM) beyond rotating wave approximation is studied. First, in gravitational JCM with respect to counter-rotating terms, effective Hamiltonian of Schrödinger equation in interaction picture is obtained. Then, by transforming the Schrödinger equation to two differential equations, the amplitudes of probability are obtained. In this case, influence of certain parameters on spontaneous emission spectrum is investigated. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we consider the model which consists of a degenerate Raman process involving two degenerate Rydberg energy levels of an atom interacting with a single-mode cavity field. The influence of the atomic coherence on the von Neumann entropy of the atom and the atomic inversion is investigated. It is shown that the atomic coherence decreases the amount of atom-field entanglement. It is also found that the collapse and revival times are independent of the atomic coherence, while the amplitude of the revivals is sensitive to this coherence. Moreover, the Q function and the entropy squeezing of the field are examined. Some new conclusions can be obtained. [Copyright &y& Elsevier]

Abstract: Applying the operator a + a + to the superposed coherent states several times, superpositions of the excited–deexcited coherent states are obtained. Compared with the original superposed coherent states, these new states can have stronger squeezing and anti-bunching effects. The operation a + a + can also induce squeezing or antibunching effect if the original states do not possess these properties. Calculations about the phase properties, the Q function and the Wigner function reflect the non-classical character of the excited–deexcited states from different aspects. [Copyright &y& Elsevier]

Abstract: The effects on squeezing and sub-poissonian of light in fourth harmonic generation (FHG) are investigated based on the fully quantum mechanically up to the first order Hamiltonian interaction in gt, where g is the coupling constant between the modes per second and t is the interaction time between the waves during the process in a nonlinear medium. FHG is a process in which an incident laser beam of the fundamental frequency ω interacts with a nonlinear medium to produce the harmonic frequency at 4ω. The coupled Heisenberg equations of motion involving real and imaginary parts of the quadrature operators are established. The occurrence of amplitude squeezing effects in both the quadratures of the radiation field in the fundamental mode is investigated and found to be dependent on the selective phase values of the field amplitude. The photon statistics of the pump mode in this process have also been investigated and found to be sub-poissonian in nature. It is found that there is no possibility to produce squeezed light in the harmonic mode up to first-order interaction in gt. Further, we have found the case up to second-order Hamiltonian interaction in gt that the normal squeezing in the harmonic mode is directly depends upon the fourth-order squeezing of the initial pump field. This gives a method of converting higher-order (fourth-order) squeezing into normal squeezing in the harmonic mode and vice versa. [Copyright &y& Elsevier]

Nonclassical states of macroscopic objects are promising for ultrasensitive metrology as well as testing quantum mechanics. In this work, we investigate dissipative mechanical quantum state engineering in an optically levitated nanodiamond. First, we study single-mode mechanical squeezed states by magnetically coupling the mechanical motion to a dressed three-level system provided by a nitrogen-vacancy center in the nanoparticle. Quantum coherence between the dressed levels is created via microwave fields to induce a two-phonon transition, which results in mechanical squeezing. Remarkably, we find that in ultrahigh vacuum quantum squeezing is achievable at room temperature with feedback cooling. For moderate vacuum, quantum squeezing is possible with cryogenic temperature. Second, we present a setup for two mechanical modes coupled to the dressed three levels, which results in two-mode squeezing analogous to the mechanism of the single-mode case. In contrast to previous works, our study provides a deterministic method for engineering macroscopic squeezed states without the requirement for a cavity.

We propose and analyse a feasible experimental scheme for a quantum force sensor based on the elimination of backaction noise through coherent quantum noise cancellation (CQNC) in a hybrid atom-cavity optomechanical setup assisted with squeezed vacuum injection. The force detector, which allows for a continuous, broadband detection of weak forces well below the standard quantum limit (SQL), is formed by a single optical cavity simultaneously coupled to a mechanical oscillator and to an ensemble of ultracold atoms. The latter acts as a negative-mass oscillator so that atomic noise exactly cancels the backaction noise from the mechanical oscillator due to destructive quantum interference. Squeezed vacuum injection enforces this cancellation and allows sub-SQL sensitivity to be reached in a very wide frequency band, and at much lower input laser powers.

Important tasks in cavity quantum electrodynamics include the generation and control of quantum states of spatially separated particles distributed in different cavities. An interesting question in this context is how to prepare entanglement among particles located in different cavities, which are important for large-scale quantum information processing. We here consider a multi-cavity system where cavities are coupled to a superconducting (SC) qubit and each cavity hosts many SC qubits. We show that all intra-cavity SC qubits plus the coupler SC qubit can be prepared in an entangled Greenberger–Horne–Zeilinger (GHZ) state, by using a single operation and without the need of measurements. The GHZ state is created without exciting the cavity modes; thus greatly suppressing the decoherence caused by the cavity-photon decay and the effect of unwanted inter-cavity crosstalk on the operation. We also introduce two simple methods for entangling the intra-cavity SC qubits in a GHZ state. As an example, our numerical simulations show that it is feasible, with current circuit-QED technology, to prepare high-fidelity GHZ states, for up to nine SC qubits by using SC qubits distributed in two cavities. This proposal can in principle be used to implement a GHZ state for an arbitrary number of SC qubits distributed in multiple cavities. The proposal is quite general and can be applied to a wide range of physical systems, with the intra-cavity qubits being either atoms, NV centers, quantum dots, or various SC qubits.

We derive a general expression that quantifies the total entanglement production rate in continuous variable systems, where a source emits two entangled Gaussian beams with arbitrary correlators. This expression is especially useful for situations where the source emits an arbitrary frequency spectrum, e.g. when cavities are involved. To exemplify its meaning and potential, we apply it to a four-mode optomechanical setup that enables the simultaneous up- and down-conversion of photons from a drive laser into entangled photon pairs. This setup is efficient in that both the drive and the optomechanical up- and down-conversion can be fully resonant.