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Based on the idea of transition from classical optics to quantum optics we deduce the natural expressions of optical beam splitter (BS) and 2-cascaded BS operators in coherent state representation and also obtain their normally ordered forms. Moreover, we theoretically prepare some desired quantum entangled states and investigate several carried experiments. Finally, our work shall be focused on the following two aspects: the quantum-optical catalysis of the BS and quantum state truncation of the 2-cascaded BS.

Optical tomogram of a quantum state can serve as an alternative to its density matrix since it contains all the information on this state. In this paper we use the explicit expression of the intermediate state |q, f, g〉(that is, an eigenvector of the coordinate and momentum operator fQ + gP) and the Radon transform between the Wigner operator Δ(p, q) and the pure-state density operator |q, f, g〉〈q, f, g| to calculate the specific tomograms for multiple-photon-added coherent states (MPACSs), thermal states (MPATSs) and displaced thermal states (MPADTSs). Their analytical tomograms are actually the finite-summation representations related to single-variable Hermite polynomials. Besides, the numerical results indicate that the larger photon-addition number can lead to the multi-peak tomographic distributions for these three states, and the measurable tomograms of MPACSs and MPADTSs exhibit π-periodic features but MPATSs can’t.

Employing the integration technique within normal products of bosonic operators, we present normal product representations of thermal-state superpositions and investigate their nonclassical features, such as quadrature squeezing, sub-Poissonian distribution, and partial negativity of the Wigner function. We also analytically and numerically investigate their evolution law and decoherence characteristics in an amplitude-decay model via the variations of the probability distributions and the negative volumes of Wigner functions in phase space. The results indicate that the evolution formulas of two thermal component states for amplitude decay can be viewed as the same integral form as a displaced thermal state ρ ( V , d ) , but governed by the combined action of photon loss and thermal noise. In addition, the larger values of the displacement d and noise V lead to faster decoherence for thermal-state superpositions.

The quantization scheme between double enhanced charge phase-slip (ECPS) qubits is given. In this system, there exist quantum entanglement phenomenona. The entanglement characteristic is discussed by employing the concurrence. An interesting conclusion is obtained, i.e., if one desire to change the entanglement between two ECPS qubits, the mutual inductance should be considered first, but it is also a good choice to tune symmetric and asymmetric combination of two gate voltages.

We theoretically introduce two new photon-modulated atomic coherent states (ACSs) via using the Schwinger bosonic representation of the angular momentum operators (the sequential operations J ± n ) on an ACS, and investigate their nonclassicality using the Wigner distribution, photon number distribution, and entanglement entropy. It is found that photon-modulated ACSs possess more stronger nonclassicality than the original ACS in certain regions of τ, the nonclassicality enhances with increasing number n of the operations J ± and the operation J + ( − ) n enhances the entanglement in the region of small (large) τ.

A new kind of nonlinear coherent states, i.e. the k eigenstates of the powers B^k (k ≥ 3) of the generalized annihilation operator B^=a^†-1f(N^) of f-oscillators, are constructed using the inverse operators of the boson annihilation and creation operators. The set of these states may form a complete quantum mechanics representation. Using newly defined higher order squeezing and an antibunching, the properties of the Mth-order squeezing and the antibunching effect of the k states are investigated. The results show that the Mth-order [M = (n + 1/2)k; n = 0, 1, 2, …] squeezing effects exist in all of the k states when k is even and the antibunching effect exists in all of the k states.

Extending the recent work completed by Fan et al. [Front. Phys. 9(1), 74 (2014)] to a two-mode case, we investigate how a two-mode squeezed vacuum evolves when it undergoes a two-mode amplitude dissipative channel, with the same decay rate κ, using the continuous-variable entangled state approach. Our analytical results show that the initial pure-squeezed vacuum state evolves into a definite mixed state with entanglement and squeezing, decaying over time as a result of amplitude decay. We also investigate the time evolutions of the photon number distribution, the Wigner function, and the optical tomogram in this channel. Our results indicate that the evolved photon number distribution is related to Jacobi polynomials, the Wigner function has a standard Gaussian distribution (corresponding to the vacuum) at long periods, losing its nonclassicality due to amplitude decay, and a larger squeezing leads to a longer decay time.

We theoretically analyze the nonclassicality and entanglement of two new non-Gaussian entangled states generated by applying multiple-photon addition and subtraction to a two-mode binomial state. The nonclassical properties are investigated in terms of the partial negativity of the Wigner functions, whose results show that their nonclassicality can be enhanced via one-mode even-number photon operations and two-mode symmetrical operations for the initial two-mode binomial state. We also find that there exists some enhancement in the entanglement properties in certain parameter ranges via one-mode photon-addition and two-mode symmetrical operations.

A class of finite-dimensional even and odd pair coherent states, which may be generated via the vibrational motion of a trapped ion in two-dimensional harmonic potential in experiment, is constructed and then some of its statistical properties are discussed. The study shows that the new states exhibit remarkably different statistical properties for different parameters q and ξ .

Using the technique of integration within an ordered product (IWOP) of operators we derive a new kind of bipartite entangled state |β 1,β 2〉, which is perfectly different from the preceding entangled states in construction. We also present main properties, generation scheme of |β 1,β 2〉, superposition of |β 1,β 2〉, Wigner function and some applications of |β 1,β 2〉 in quantum optics.

A new bipartite coherent-entangled state (CES) is found which exhibits both coherent state and entangled state properties. The set of CESs constitute a complete and partly nonorthogonal representation. Using an asymmetric beamsplitter a experimental scheme to produce the CES is proposed. Applications of CESs in quantum optics are also presented.

Starting from the classical equation of the motion of a domain wall in the ferromagnetic systems, the quantum energy levels of the wall and the corresponding eigenfunctions in the case of considering damping term are given by using the canonical quantization method and unitary transformation. The quantum fluctuations of displacement and momentum of the moving wall has also been given as well as the uncertain relation.

A new kind of bipartite coherent-entangled state (CES) is introduced in the two-mode Fock space which exhibits the properties of both coherent state and entangled state. The set of CESs make up a complete and partly nonorthogonal representation. A simple experimental scheme to produce the CES is proposed by using a beam splitter. Some applications of the CES are also presented in quantum optics.

The new coherent-entangled state |z,x;θ〉 is proposed in the two-mode Fock space, which exhibits both the properties of coherent and entangled states. The completeness relation of |z,x;θ〉 is proved by virtue of the technique of integral within an ordered product of operators. A generalized Hadamard-Fresnel complementary transformation derived by virtue of the coherent-entangled state |z,x;θ〉, which is unitary. The new unitary operator plays the role of both Hadamard transformation for \((\hat{a}_{1}\sin\theta -\hat{a}_{2}\cos\theta)\) and Fresnel transformation for \((\hat{a}_{1}\cos\theta +\hat{a}_{2}\sin\theta)\), respectively.

In this paper we construct a new type of finite-dimensional pair coherent states | ξ , q 〉 as realizations of SU(2) Lie algebra. Using the technique of integration within an ordered product of operator, the nonorthogonality and completeness properties of the state | ξ , q 〉 are investigated. Based on the Wigner operator in the entangled state | τ 〉 representation, the Wigner function of | ξ , q 〉 is obtained. The properties of | ξ , q 〉 are discussed in terms of the negativity of its Wigner function. The tomogram of | ξ , q 〉 is calculated with the aid of the Radon transform between the Wigner operator and the projection operator of the entangled state | η , κ 1 , κ 2 〉. In addition, using the entangled state | τ 〉 representation of | ξ , q 〉 to show that the states | ξ , q 〉 are just a set of energy eigenstates of time-independent two coupled oscillators.

This paper examines the quantization for the capacitance coupling inductance-capacitance (LC) mesoscopic circuit. By virtue of the “invariant eigen-operator” (IEO) method, it deduces the selection rules of energy-level transition when the system is disturbed by the external electromagnetic field.

A new method of quantizing the mesoscopic RLC circuit is proposed, i.e., such a circuit can be equivalent to a changing mass simple harmonic oscillator. By virtue of the thermal entangled state representation and the generalized Hellmann-Feynman Theorem, the thermal effect for the system is investigated.

We introduce a new kind of nonclassical mixed state generated by adding arbitrary photons to a displaced thermal state, i.e., the photon-added displaced thermal state (PADTS), and obtain the normalization factor, which is simply related to two-variable Hermite polynomials. We also discuss the nonclassicality of the PADTS by considering quantum phase-space distributions. The results indicate that the value of the photon count statistics is maximum when the number of detected photons is equal to the number of added photons, and that the photon-added operation has a similar modulation effect with increasing displacement. Moreover, the negative volume of the Wigner function for the PADTS takes a maximal value for a specific photon-added number.

In the newly constructed entangled state representation embodying quantum entanglement of Einstein, Podolsky and Rosen, the usual wave function of atomic coherent state ∣τ〉 = exp (μJ+-μ*J-)∣j, -j〉 turns out to be just proportional to a single-variable ordinary Hermite polynomial of order 2j, where j is the spin value. We then prove that a two-dimensional time-independent anisotropic harmonic oscillator in a uniform magnetic field possesses energy eigenstates which can be classified as the states ∣τ〉 in terms of the spin values j.

The mesoscopic single RLC (resistance-inductance-capacitance) circuit and the RLC circuit including complicated coupling are quantized by employing Dirac's standard canonical quantization method. The thermal effects for the systems are investigated by virtue of GHFT (the generalized Hellmann–Feynman theorem). The results distinctly show the effect of temperature on the quantum fluctuation.

Based on the entangled state representation and Feynman’s idea that “electron pairs are bosons, … , a bound pair acts as a Bose particle”, we present a quantization scheme for a mutual-inductance-coupled L C circuit including Josephson junctions. Then we use the Heisenberg equation of motion to obtain the modified operator Josephson equations, which may be useful for quantum control. The variation of the phase-difference operator of the junction with time is also investigated.

The generalized continuous variable two-mode entangled state is proposed by using the technique of integration within an ordered product (IWOP) of operators. The characteristics of this new entangled representation and its application in quantum teleportation are analyzed in detail. These results indicate that such real parameters indeed make up a new entangled representation owing to its completeness and orthogonal relation. By employing the as quantum transmission channel, the teleportation of any single-mode quantum state can be realized by a unitary transformation.

The properties of the even and odd negative binomial states are investigated. Mainly we concentrate on the statistical properties for such states and we consider the squeezing phenomenon by examining the variation in the quadrature variances. Using the Pegg–Barnett formalism of phase operator, the phase probability distributions of the states are discussed, and also the phase and number squeezing with different parameters are studied. The numerical computation results show that the phase probability distributions for the even and odd NBSs can clearly exhibit the different features of quantum effects.

We show that for quantum tomography there exist two mutually conjugating intermediate coordinate-momentum entangled states | η 1 , η 2 〉 λ , ν and | ς 1 , ς 2 〉 σ , τ . The Radon transforms of the Wigner operators are just the pure-state density matrices | η 1 , η 2 〉 λ , ν 〈 λ , ν η 1 , η 2 | and | ς 1 , ς 2 〉 σ , τ 〈 σ , τ ς 1 , ς 2 | , respectively. As a result, the tomogram of quantum states is the module-square of their wave function in these representations. A new convenient formalism of quantum tomogram is thus established.

By introducing the entangled state representation and following Feynman's assumption that "electron pairs are bosons … A bound pair acts as a Bose particle", we construct a Hamiltonian operator for the Josephson transmission line (JTL) detector, then we use the Heisenberg equation of motion to derive the modified Josephson equation. Based on the modified Josephson equation, the measurement dynamics of JTL detector is investigated, which will be helpful for us to analyze the quantum measurement of JTL detector.

The coherent-entangled state |α, x; λ> with real parameters λ is proposed in the two-mode Fock space, which exhibits the properties of both the coherent and entangled states. The completeness relation of |α, x; λ> is proved by virtue of the technique of integral within an ordered product of operators. The corresponding squeezing operator is derived, with its own squeezing properties. Furthermore, generalized P-representation in the coherent-entangled state is constructed. Finally, it is revealed that superposition of the coherent-entangled states may produce the EPR entangled state.

Using the entangled state representation of Wigner operator and some formulae related to the two-variable Hermite polynomials, the Wigner function of the squeezed pair coherent state (SPCS) and its two marginal distributions are derived. Based on the entangled Husimi operator introduced by Fan et al. (Phys. Lett. A 358:203, 2006) and the Weyl ordering invariance under similar transformations, we also obtain the Husimi function of the SPCS and its marginal distribution functions. The comparison between the two quasibability functions shows that, for the same amount of information included in two functions, the solving process of the Husimi function is simpler than that of the Wigner function.

By means of the Weyl correspondence and Wigner theorem the marginal distribution of Wigner function in mesoscopic RLC circuit at finite temperature was discussed. Here we endow the Wigner function with a new physical meaning, i.e., its marginal distributions’ statistical average for q2/(2C) and p2/(2L) are the temperature-related energy stored in capacity and in inductance of the mesoscopic RLC circuit, respectively.

Via the Hamilton dynamical approach we have constructed Hamiltonian for the mutual inductance coupling magnetic flux qubits. The entangled state representation is used to propose Cooper-pair number-phase quantization and the Hamiltonian operator for the whole system. The dynamical evolution of the phase difference operator and the Cooper-pairs number operator is investigated by virtue of Heisenberg equations.

We find that when a single Josephson junction is inserted into a mesoscopic LC circuit, the operator Josephson equation is modified accompanying with the modification of Farady equation describing the inductance. By virtue of the entangled state representation and using the appropriate phase operator in bosonic form we derive the modified equations.

Using the coherent state representation of Wigner operator and the technique of integration within an ordered product (IWOP) of operators, the Wigner functions of the even and odd negative binomial states (NBSs) are obtained. We concentrate our analysis on the fact that the even and odd NBSs interpolate between the even and odd coherent states and the even and odd quasi-thermal states depending on the values of the parameters involved. Moreover, the tomograms of the even and odd NBSs are calculated by virtue of intermediate coordinate-momentum representation in quantum optics.

Using the coherent state representation of Wigner operator and the technique of integration within an ordered product (IWOP) of operators, the Wigner functions of the Klauder-Perelomov coherent states (KP-CSs) for the pseudoharmonic oscillator (PHO) are obtained and the variations of the Wigner functions with the parameters k and z are discussed. Moreover, the tomograms of the KP-CSs for the PHO are calculated by virtue of intermediate coordinate-momentum representation in quantum optics.

We derive the energy-level and wave functions for two moving charged particles with elastic coupling, using two mutually conjugate entangled state representations.

Mesoscopic damped mutual inductance coupled double resonance RLC circuit is quantized by the method of damped harmonic oscillator quantization and linear transformation. The energy levels of this circuit are given. By thermo-field dynamics (TFD), the quantum fluctuations of the current and voltage of each loop are researched in the thermal vacuum state, thermal coherent state and thermal squeezed state. It is shown that the quantum fluctuations of the current and voltage are related not only to the circuit inherent parameter and coupled magnitude of mutual inductance, but also squeezed coefficients, squeezed angle, environmental temperature and damped resistance. Furthermore, because of environmental temperature and damped resistance, the quantum fluctuations increase with the increase of temperature and decay along with time.

Based on Feynman's explanation about Cooper pair that "a bound pair acts as a Bose particle" and the bosonic operator Hamiltonian of the Josephson junction, we realize that the quantum state of the Josephson junction is a Cooper pair number-phase entangled state constructed by the phase operator across the junction. Its Schmidt decomposition is derived. The Cooper pair number-phase squeezed state's projection onto this entangled state leads to a geometric distribution.

The time evolvement of quantum state has been researched by the schemes of Louisell’s common canonical quantization and Gauss Propagator for mesoscopic coupled double resonance circuit with electrical sources. Moreover, the formulae of time evolvement wave function and transition probability of quantum state of this circuit are given. It is more significant for the quantitative analysis quantum character of mesoscopic circuit.

Using the Pegg-Barnett formalism of phase operator, we obtain phase probability distributions of new even and odd nonlinear coherent states. It is shown that the distributions for the states are rather different, and unlike the case of ordinary even and odd coherent states the Pegg-Barnett distribution clearly reflects the different character of quantum interference in the case of the new even and odd coherent states.

Starting from the Kirchhoff's equation for electric circuits and in reference of damped harmonic oscillator quantization and thermo-field dynamics (TFD), the quantization of damped double-resonance mesoscopic RLC circuit involving complicated coupling is proposed. The quantum fluctuations of charge and current of each loop are calculated in thermal squeezed state, thermal coherent state and thermal vacuum state, respectively. The results not only depend on the circuit proper parameters and coupled magnitude, but also rely on the squeezing coefficients, environmental temperature and damped resistance. The fluctuations increase with temperature rising and decay with time.

In this paper we shall introduce two q-analogues of the squeezed states in terms of the technique of integration within an ordered product of operators and the properties of the inverses of q-deformed annihilation and creation operators, and some nonclassical properties of the states are examined. Furthermore, we obtain some new completeness relations composed of the bra and ket which are not mutually Hermitian conjugate.

The quantization scheme of a single-qubit structure with Superconducting Quantum Interference Device (SQUID) is given. By introducing a unitary matrix and by means of spectral decomposition, the Hamiltonian operator of the system is exactly formulated in compact forms in spin-1/2 notation. The eigenvalues and eigenstates of the system are discussed.

Wigner function in phase space has its physical meaning as marginal probability distribution in coordinate space and momentum space respectively, here we endow the Wigner function with a new physical meaning, i.e., its marginal distributions’ statistical average for q2/(2C) and p2/(2L) are the energy stored in capacity and in inductance of a mesoscopic L-C circuit at finite temperature, respectively.

Using the entangled state representation of Wigner operator and the technique of integration within an ordered product (IWOP) of operators, the Wigner function of the pair coherent state is derived. The variations of the Wigner function with the parameters α and q in the ρ – γ phase space are discussed. The physical meaning of the Wigner function for the pair coherent state is given by virtue of its marginal distributions. The tomogram of the pair coherent state is calculated with the help of the Radon transform between the Wigner operator and the projection operator of the entangled state | η 1 , η 2 , τ 1 , τ 2 〉 .

By using of the invariant theory, we have studied phase of Bose-Einstein condensate in a double-well potential interacting with a time-dependent single-mode traveling-wave laser field, the dynamical and geometric phases are presented respectively. The Aharonov-Anandan phase is also obtained under the cyclical evolution.

The excited squeezed light (ESL) can be the outcome of interaction between squeezed light probe and excited atom, which can explore the status and the structure of the atom. We calculate the Wigner function and tomogram of ESL that may be comparable to the experimental measurement of quadrature-amplitude distribution for the light field obtained using balanced homodyne detection. The method of calculation seems new.

On the basis of Feynman's explanation about Cooper pair that “a bound pair act as a Bose particle” and the bosonic operator Hamiltonian of Josephson junction [H.-Y. Fan, Phys. Lett. A 289 (2001) 172] as well as the entangled state representation we introduce characteristic function and Wigner function in the sense of Cooper-pair number–phase. We employ the correlated-amplitude–number state representation to present it. The Cooper-pair number distribution and phase distribution are the marginal distributions of this Wigner distribution, respectively.

Mesoscopic damped double resonance mutual capacitance coupled RLC circuit is quantized by the method of damped harmonic oscillator quantization. The Hamiltonian is diagonalized by unitary transformation. The eigenenergy spectra of this circuit are given. The quantum fluctuations of the charges and current of each loop are researched in excitation state of the squeezed vacuum state, the squeezed vacuum state and in vacuum state. It is show that, the quantum fluctuations of the charges and current are related to not only circuit inherent parameter and coupled magnitude, but also quantum number of excitation, squeezed coefficients, squeezed angle and damped resistance. And, because of damped resistance, the quantum fluctuation decay along with time.

Based on Feynman's explanation about the Cooper pair that "a bound pairs act as a Bose particle" and the bosonic operator Hamiltonian of Josephson junction (H.-Y. Fan, Int. J. Mod. Phys. B17 (2003) 2599) as well as the entangled state representation, we establish a possible number–phase squeezing mechanism for the Cooper-pair and the phase difference between the two sides of the junction. We find that when an extra energy (e.g. microwave radiation) is provided to the junction, then this squeezing mechanism can happen.