<title>Quantum harmonic oscillator state synthesis and analysis</title>

We propose a new scheme for generating the superposition and entanglement of the coherent states and squeezed states by considering N superconducting charge qubits (or artificial two-level atoms) interacting with photons in a high finesse cavity on a chip, assisted by a strong driving field. By virtue of the parameters of this system, we can generate novel quantum states, for example, multiparty entangled states and Schrodinger cat states among the superconducting qubits, coherent states and squeezed states of the cavity. These states, whose amplitudes are about two orders greater than those from the atomic quantum electrodynamics in classical cavity, are important for understanding the boundary between quantum and classical behavior and can be utilized in experimental studies on decoherence. This device may be an architecture for future solid-state quantum computation and communication.

We define Schrodinger cat states as superpositions of q-deformed Barut–Girardello su(1, 1) coherent states with an adjustable angle φ in a q-deformed Fock space. We study the statistical properties of the q-deformed Barut–Girardello su(1, 1) coherent states and Schrodinger cat states. The statistical properties of photons are always sub-Poissonian for q-deformed Barut–Girardello su(1, 1) coherent states. For Schrodinger cat states in the cases φ = 0, π/2, π, the statistical properties of photons are always sub-Poissonian if φ = π/2, and the other cases are hard to determine because they depend on the parameters q and k. Moreover, we find some interesting properties of Schrodinger cat states in the limit |z| → 0, where z is the parameter of those states. We also derive that the statistical properties of photons are sub-Poissonian in the undeformed case where π/2 ≤ φ ≤ 3π/2.

We calculate the second-order moments, the Robertson-Schrödinger uncertainty product, and the Mandel factor for various superpositions of coherent phase states with opposite arguments, comparing the results with similar superpositions of the usual (Klauder-Glauber-Sudarshan) coherent states. We discover that the coordinate variance in the analog of even coherent states can show the most strong squeezing effect, close to the maximal possible squeezing for the given mean photon number. On the other hand, the Robertson-Schrödinger (RS) uncertainty product in superpositions of coherent phase states increases much slower (as function of the mean photon number) than in superpositions of the usual coherent states. A nontrivial behavior of the Mandel factor for small mean photon numbers is discovered in superpositions with unequal weights of two components. An exceptional nature of the even and odd superpositions is demonstrated.

The nearest Hilbert-Schmidt distance of (pure) states to coherent states is discussed as a quantitative measure of nonclassicality of states in quantum optics or quantum mechanics of harmonic oscillators. Fock states, squeezed coherent states and finite superpositions of coherent states (Schrodinger cat states) are considered as examples. Some other approaches to nonclassicality of states in quantum optics are shortly discussed.

Recently several methods have been proposed for generation of superposition (Schrödinger cat) states in microwave cavities. At microwave frequencies thermal photons can significantly affect statistical properties of superposition states. In the present letter we study the influence of a thermal heatbath on non-classical properties of quantum superposition states. We show that at non-zero temperatures the loss of quantum coherences is much faster than at zero temperature and that the sensitivity of the quantum coherence to the presence of thermal photons can lead to some difficulties in the preparation of Schrödinger cat states in microwave cavities unless the temperature of the microwave cavity is sufficiently low.

We study statistical properties of quantum superposition states (Schrodinger-cat states) amplified by phase-sensitive (squeezed) amplifiers. We show that the phase-sensitive amplifier with a properly chosen phase can preserve quantum coherences and nonclassical behavior of the Schrodinger-cat-state input even for a gain factor G larger than 2. In particular, we show that for an even coherent state (CS) phase-sensitive amplifiers can preserve squeezing for G>2 but simultaneously in the process of amplification the noise added by the amplifier leads to a rapid increase of fluctuations in the photon number. Because of the finite maximum degree of squeezing obtainable for the even CS the maximum gain factor G m for which squeezing can still be observed in the output state is finite

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

Theory and numerical simulation of field-induced director dynamics in confined nematics investigated by nuclear magnetic resonance

In a recent experiment the progressive decoherence of a mesoscopic superposition of two coherent field states in a high-Q cavity, known as Schrodinger cat state, has been measured for the first time [Brune et al., Phys. Rev. Lett. 77, 4887 (1996)]. Here, the full master equation governing the coupled dissipative dynamics of the atom-field system studied in the experiment is formulated and solved numerically for the experimental parameters. The model simulated avoids the approximations underlying an analytically solvable model which is based on a harmonic expansion of the energies of the dressed atomic states and on a treatment of their dynamics within the adiabatic approximation. In particular, the numerical simulations reveal that the coupling of the cavity field mode to its environment causes important decoherence effects already during the initial preparation phase of the Schrodinger cat state. This phenomenon is investigated in detail with the help of a measure for the purity of states. Moreover, the Hilbert-Schmidt distance of the intended target state, the Schrodinger cat, to the state that is actually prepared in the experiment is determined.

Quantum dot (QD) systems containing electron spins may hold a role in a future photonic quantum circuit as a means of storing a quantum state in a spin superposition. In general, the spin superposition state maps directly to a photon emitted out of the plane (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> ) with the photon in a polarization superposition. In waveguides, however, this is more difficult: one requires a waveguide mode that is able to transmit both TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> modes in coherent superposition. We have already demonstrated a cross-waveguide design, as shown in Fig 1(a), that converts spin superposition states to a path encoding when the QD is located close to the waveguide centre.

Liquid crystal is a representative soft matter, which has physical properties between those of conventional liquid and those of crystal in a temperature range above a melting point. A liquid-crystal display (LCD) employs the response of the liquid-crystal alignment to the electric field and is a key device of an information display. For common LCDs, the precise control of the initial alignment of LC molecules is needed so that a good dark state, thus a high contrast ratio, can be obtained. If the birefringence can be induced in the liquid phase by the application of electric field, it is of great use as a material for the LCD application. In this study, we will report a unique property of dichiral azobenzene liquid crystals: an electric induction of birefringence in a liquid phase of an antiferroelectric dichiral azobenzene liquid crystal. The optically isotropic texture changes into the homogenous birefringent texture by the application of the in-plane electric field above the clearing temperature of the liquid crystal. We find that one of the possible reasons of the induction of the birefringence in the isotropic phase is the electrically-induced increase of the phase transition temperature between the antiferroelectric liquid-crystalline and "liquid" phases, i.e., increase in the clearing temperature. The resulting birefringence can be disappeared by the irradiation of UV light, due to the photoinduced isomerization of the azobenzene compound, thus dual control of the birefringent structure, by the irradiation of light and/or by the application of the electric field, is achieved.

Conventional Glauber coherent states (CS) can be defined in several equivalent ways, e.g., by displacing the vacuum or, explicitly, by their infinite Poissonian expansion in Fock states. It is well known that these definitions become inequivalent if applied to finite $d$-level systems (qudits). We present a comparative Wigner-function description of the qudit CS defined (i) by the action of the truncated displacement operator on the vacuum and (ii) by the Poissonian expansion in Fock states of the Glauber CS truncated at $(d\ensuremath{-}1)$-photon Fock state. These states can be generated from a classical light by its optical truncation using nonlinear and linear quantum scissors devices, respectively. We show a surprising effect that a macroscopically distinguishable superposition of two qudit CS (according to both definitions) can be generated with high fidelity by displacing the vacuum in the qudit Hilbert space. If the qudit dimension $d$ is even (odd), then the superposition state contains Fock states with only odd (even) photon numbers, which can be referred to as the odd (even) qudit CS or Schr\"odinger's cat state. This phenomenon can be interpreted as an interference of a single CS with its reflection from the highest-energy Fock state of the Hilbert space, as clearly seen via phase-space interference of the Wigner function. We also analyze nonclassical properties of the qudit CS including their photon-number statistics and nonclassical volume of the Wigner function, which is a quantitative parameter of nonclassicality (quantumness) of states. Finally, we study optical tomograms, which can be directly measured in the homodyne detection of the analyzed qudit cat states and enable the complete reconstructions of their Wigner functions.

The detection of microwave states is complicated by strong thermal noise, which is inevitably introduced by linear amplifiers. We show how to extract from measured data normally or anti-normally ordered moments of photon creation and annihilation operators, the set of which contains complete information on the quantum state of an electromagnetic field. Equations for the evolution of the quantum state are derived in terms of moments. Using this approach, we consider in detail issues of decoherence and thermalization of microwave quantum states. Results are illustrated using the examples of Fock, coherent, squeezed, thermal, and even and odd coherent states (Schrodinger cat states).

Coupled ferromagnetic and ferroelectric materials, known as multiferroics, are an important class of materials that allow magnetism to be manipulated through the application of electric fields. Bismuth ferrite, BiFeO3, is the most-studied intrinsic magnetoelectric multiferroic because it maintains both ferroelectric and magnetic ordering to well above room temperature. Here we report the use of epitaxy-free wet chemical methods to create strained nanoporous BiFeO3. We find that the strained material shows large changes in saturation magnetization on application of an electric field, changing from 0.04 to 0.84 μb per Fe. For comparison, non-porous films produced using analogous methods change from just 0.002 to 0.01 μb per Fe on application of the same electric field. The results indicate that nanoscale architecture can complement strain-layer epitaxy as a tool to strain engineer magnetoelectric materials.

The p-Ge hot hole laser is as yet the only solid state tunable laser with a strong emission in the THz frequency range. Monte Carlo simulations have shown that modelocking of the laser on the intervalence band transition should be possible by gain modulation through the application of an appropriate rf electric field. Recently we did observe for the first time the generation of 200 picosecond pulses in the high frequency (approximately equals 100 cm -1 ) emission range. Now also pulses as short as 100 ps have been observed in the low frequency regime (approximately equals 50 cm -1 ). A detailed study of the wavelength dependent optical output of the laser has been started now for (normal) pulsed -- as well as for active mode locked operation. Results on pulse shape and small signal gain in the low frequency (equals low magnetic field) regime are given.