Degree Of Squeezing Research Articles

Continuous-variable quantum teleportation of even and odd coherent states through varied gain channels

This paper has investigated quantum teleportation of even and odd coherent states in terms of the EPR entanglement states for continuous variables. It discusses the relationship between the fidelity and the entanglement of EPR states, which is characterized by the degree of squeezing and the gain of classical channels. It shows that the quality of teleporting quantum states also depends on the characteristics of the states themselves. The properties of teleporting even and odd coherent states at different intensities are investigated. The difference of teleporting two such kinds of quantum states are analysed based on the quantum distance function.

Cloning of Gaussian states by linear optics

We analyze in details a scheme for cloning of Gaussian states based on linear optical components and homodyne detection recently demonstrated by U. L. Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated for a generic (pure or mixed) Gaussian state taking into account the effect of non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in most quantum information protocols the covariance matrix of the set of input states is not perfectly known, we evaluate the average cloning fidelity for classes of Gaussian states with the degree of squeezing and the number of thermal photons being only partially known.

Conditions for spin squeezing in a cold87Rb ensemble

We study the conditions for generating spin squeezing via a quantum non-demolition measurement in an ensemble of cold87Rb atoms. By considering the interaction of atoms in the 5S1/2(F = 1) ground state with probe light tuned near the D2 transition, we show that, for large detunings, this system is equivalent to a spin-1/2 system when suitable Zeeman substates and quantum operators are used to define a pseudo-spin. The degree of squeezing is derived for the rubidium system in the presence of scattering causing decoherence and loss. We describe how the system can decohere and lose atoms, and predict as much as 75% spin squeezing for atomic densities typical of optical dipole traps.

The effect of finite bandwidth squeezed light on entanglement creation in the Dicke model

We analyse the relation between local two-atom and total multi-atom entanglements in the Dicke system composed of a large number of atoms. We use concurrence as a measure of entanglement between two atoms in the multi-atom system, and the spin squeezing parameter as a measure of entanglement in the whole n-atom system. In addition, the influence of the squeezing phase and bandwidth on entanglement in the steady-state Dicke system is discussed. It is shown that the introduction of a squeezed field leads to a significant enhancement of entanglement between two atoms, and the entanglement increases with increasing degree of squeezing and bandwidth of the incident squeezed field. In the presence of a coherent field the entanglement exhibits a strong dependence on the relative phase between the squeezed and coherent fields, that can jump quite rapidly from unentangled to strongly entangled values when the phase changes from zero to π. We find that the jump of the degree of entanglement is due to a flip of the spin squeezing from one quadrature component of the atomic spin to the other component when the phase changes from zero to π. We also analyse the dependence of the entanglement on the number of atoms and find that, despite the reduction in the degree of entanglement between two atoms, a large entanglement is present in the whole n-atom system and the degree of entanglement increases as the number of atoms increases.

Squeezed-light source for superresolving microscopy.

We propose a source of multimode squeezed light that can be used for superresolving microscopy. This source is an optical parametric amplifier with a properly chosen diaphragm on its output and a Fourier lens. We demonstrate that such an arrangement produces squeezed prolate spheroidal waves that are the eigenmodes of the optical imaging scheme used in microscopy and discuss the conditions of the degree of squeezing and of the number of spatial modes in illuminating light.

Experimental Simulation of Spin Squeezing by Nuclear Magnetic Resonance

We report on the experimental simulation of spin squeezing using a liquid-state nuclear magnetic resonance (NMR) quantum information processor. This was done by identifying the energy levels within the symmetric subspace of a system of n spin-1/2 nuclei with the energy levels of the simulated spin-(n/2) system. The results obtained for our simulations of spin-1 and spin-3/2 systems are consistent with earlier theoretical studies of spin squeezing, and illustrate interesting relations between the degree of squeezing and the strength of the correlations among the underlying spin-1/2 particles. PACS: 03.65.Bz; 03.67.-a; 03.67.Lx

Squeezing close to the stability boundaries of the Paul trap

We propose theoretically and prove numerically that an arbitrarily high degree of squeezing is obtained for the wave function of a charged particle in an ideal Paul trap if the trap is operated close to the stability boundaries.

Application of quantum inequalities to quantum optics

We establish a connection between quantum inequalities (known from quantum field theory on curved spacetimes) and the degree of squeezing in quantum-optical experiments. We prove an inequality which binds the reduction of the electric-field fluctuations to their duration. The bigger the level of fluctuations-suppression the shorter its duration. As an example of the application of this inequality in the case of squeezed light whose phase is controlled with 1% accuracy we derive a limit of -15dB on the allowed degree of squeezing.

Dynamic squeezing in a single-mode boson field interacting with two-level system

We studied the time evolution of a two-level electron system interacting with a single-mode bosonic field (i.e. photons, phonons). We found that in the adiabatic limit (i.e. electron motion fast and boson motion slow) it is possible to obtain a reduction in fluctuation for the position coordinate of boson and momentum coordinate as well, depending on the adiabatic potential. If the system is on a lower adiabatic sheet we obtain a reduction in momentum fluctuations. However, if the system is on an upper sheet the fluctuation in position coordinate (associated with the bosonic field) is reduced. The maximum reduction is for a range of parameters given by g 2/ v≈1, where g is the electron–photon (phonon) coupling strength and 2 v is the electron energy level splitting. Moreover, it is possible to generate squeezed light at a frequency different from the pump frequency. The output frequency is mainly given by the curvature of the adiabatic potential. The system is interesting because it can be implemented in nanoscale systems like a single or double quantum well system interacting with a laser field of radiation. The degree of squeezing can be improved either by time modulating the energy of the electron levels inside the quantum well or by modulating the laser field.

A theoretical and experimental study of fluctuations of the optical parametric oscillator

Twin beams generated by OPOs show quantum noise reduction. For type I crystals they reduce in the degenerate case to a single mode, which below threshold exhibits some degree of squeezing. In the nondegenerate case squeezing occurs in the spectrum of their intensity difference. After a theoretical analysis of their spectral properties in both non- and degenerate cases, measured intensity noise correlation and single beam spectra are discussed for pump powers up to 14 times the threshold value.

A quasi-energy operator in the Jaynes-Cummings model in a polychromatic classical field

A perturbation theory is developed for constructing the quasi-energy operator Q of the Tavis-Cummings Hamiltonian, which includes the interaction of atoms with a classical quasi-monochromatic field. The operator Q of the first order in the interaction δ of an atom with a resonator mode has a form of the generalized Tavis-Cummings Hamiltonian (in the interaction representation) to which the oppositely rotating terms with a changed interaction constant are added. Such a Hamiltonian has a singularity in the dimensionless amplitude σ of a classical field. In the vicinity of this singularity, the Hamiltonian spectrum tends to a continuous one, while the degree of squeezing of field quadratures (in its eigenstates) increases infinitely. In the case of one atom and the biharmonic perturbation, the operator Q is obtained up to the third order of the perturbation theory. The spectral problem for Q is studied. The features of the dependence of the quasi-energy spectrum on σ are explained by the presence of an efficient barrier between the regions of the “coordinate” space. It is found that the above-mentioned singularity corresponds to the beginning of the parametric resonance zone. Analytic expressions for the top and bottom of this zone in the δσ plane are presented.

Quantum teleportation using quantum nondemolition technique

We propose a new scheme and protocol for quantum teleportation of a single-mode field state, based on entanglement produced by quantum non-demolition interaction. We show that the recently attained results in QND technique allow to perform the teleportation in quantum regime. We also show that applying QND coupling to squeezed fields will significantly improve the quality of teleportation for a given degree of squeezing.

Coherent states in the bilayer fractional quantum Hall ferromagnet

We find that we can use a ‘squeezed-vacuum’ state conceived in quantum optics to give the ground-state wave function for the bilayer quantum Hall system at the total Landau-level filling of ν=1/ m ( m: odd integer). This is derived in the boson approximation, where a particle–hole pair creation across the symmetric–antisymmetric gap, Δ SAS, is regarded as a boson. The degree of squeezing is given as a function of the layer separation and Δ SAS.

Light squeezing at the transition to quantum chaos

We investigate theoretically the dynamics of squeezed state generation in nonlinear systems possessing a transition from regular to chaotic dynamics in the limit of a large number of photons. As an example, the model of a kicked Kerr oscillator is considered. We show that at the transition to quantum chaos the maximum possible degree of squeezing increases exponentially in time, in contrast to the regular dynamics, where the degree of squeezing increases in time only power-wise. We demonstrate the one-to-one correspondence of the degree of squeezing and the value of the local Lyapunov instability rate in corresponding classical chaotic system.

Spin-squeezed ground states in the bilayer quantum Hall ferromagnet

A "squeezed-vacuum" state considered in quantum optics is shown to be realized in the ground-state wavefunction for the bilayer quantum Hall system at the total Landau level filling of $\nu=1/m$ (m: odd integer). This is derived in the boson approximation, where a particle-hole pair creation across the symmetric-antisymmetric gap, $\Delta_{SAS}$, is regarded as a boson. In terms of the pseudospin describing the layers, the state is a spin-squeezed state, where the degree of squeezing is controlled by the layer separation and $\Delta_{SAS}$. An exciton condensation, which amounts to a rotated spin-squeezed state, has a higher energy due to the degraded SU(2) symmetry for $\Delta_{SAS} \neq 0$.

Deep squeezing of coherent light in the intensity-dependent Jayne–Cummings model

In this paper we have investigated the amplitude squeezing of the intensity-dependent Jayne-Cummings model. An analytic function has been obtained for large [nbar] (mean photon number) to express the light squeezing. We found that the detuning Δ has a favourable effect on the squeezed state. Slightly increasing the detuning can significantly increase the degree of squeezing. The most striking result is that the field can stay in a deep-squeezed state for a long time when Δ/[nbar] is large. Also, for some ranges of the detuning, the maximum squeezing of the field almost does not vary with the change in detuning.

Interference of squeezed light produced by two independent optical parametric oscillators

The interference of the squeezed light produced by two independent, identical optical parametric oscillators below threshold is discussed in the context of semiclassical theory. It is shown that interference exists in the usual sense between the output fields from two optical parametric oscillators with coherent injected signals. The degree of squeezing and the intensity and noise fluctuation of the interference field are calculated in detail.

Rumpled graphite basal plane as a model heterogeneous carbon surface

A new model is suggested for the heterogeneous surfaces of nongraphitized carbon adsorbents. It may be called the rumpled graphite basal plane (RGBP). The atomic structure of RGBP can be obtained by squeezing a graphite basal plane in a molecular dynamics computer simulation under a random distribution of initial atomic velocities. The empirical Tersoff potential describes the carbon–carbon interactions. The degree of squeezing is chosen to reproduce the main features of the x-ray interference function of nongraphitized carbon blacks. Grand canonical ensemble Monte Carlo simulation of the isotherms of adsorption of N2 on RGBP reproduce experimental isotherms on these carbon blacks reasonably well, especially in the BET region of relative pressures.

DIPOLE SQUEEZING OF THE ATOM IN A TWO-ATOM SYSTEM: EFFECTS OF THE SUPERPOSITION STATES

Dipole squeezing of the atom in the presence of another atom is investigated for the one-photon and two-photon transition mechanism with the initial atoms in the two-atom squeezed state and the field in a vacuum, a coherent, or a squeezed state. For a vacuum input, the degree of squeezing is shown to depend on the photon multiplicity 'm' and the superposition angles of the atoms, θ1 and θ2. One of the quadratures of the atomic polarization is found to exhibit permanent squeezing only for some nonzero values of θ1, as well as θ2. The effect of θ2 on the dipole squeezing, however, is found to be negligible for the initial field in a coherent or a squeezed state. A comparison with the dipole squeezing of a single atom is also presented.

Three-level Atom in a Squeezed Vacuum II

Abstract The resonance fluorescence of a three-level system is studied in each of its three possible configurations—ladder, lambda and vee—when it also interacts with broad band squeezed vacua and the applied fields are resonant. We obtain general expressions which apply for any configuration and do not make use of the secular approximation. Detailed results are presented which depend upon the degree of squeezing and the squeezing phase, in addition to the intensities of the applied fields. It is shown that asymmetric spectra may be obtained in certain circumstances. These are interpreted in terms of coherent and incoherent transitions induced by the squeezed fields. We also take the opportunity to assess the accuracy of the secular approximation.