Quantum imaging and inverse scattering

We consider the inverse scattering problem that arises in two-photon quantum imaging with interferometric measurements. We show that the two-point correlation function of the field contains information about the scattering medium at a spatial frequency of twice the Rayleigh bandwidth. The linearized inverse problem, however, yields reconstructions with a resolution of λ/2, where λ is the wavelength of light.     

Spatial reshaping of a squeezed state of light

Reshaping the spatial profile, or mode, of a quantum state of light is one of the challenges in many quantum optics applications. We test the noise properties of a universal programmable mode converter and demonstrate that it can reshape the spatial mode of a beam while retaining its quantum properties. No detectable amount of noise is added to the light and only the standard transmission losses through conventional optical elements are found to affect the non-classical nature of the transformed light.     

Controlling single-photon Fock-state propagation through opaque scattering media

The control of light scattering is essential in many quantum optical experiments. Wavefront shaping is a technique used for ultimate control over wave propagation through multiple-scattering media by adaptive manipulation of incident waves. We control the propagation of single-photon Fock states through opaque scattering media by spatial phase modulation of the incident wavefront. We enhance the probability that a single photon arrives in a target output mode with a factor 30. Our proof-of-principle experiment shows that the propagation of quantum light through multiple-scattering media can be controlled, with prospective applications in quantum communication and quantum cryptography.     

四元裂解位相调制实现相干光通过散射介质聚焦

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.     

A nonlinear optical-fiber interferometer for nondemolitional measurement of photon number

A nondemolitional measurement scheme to determine the photon number was realised using an optical fiber as the optical Kerr medium. The φ⁽³⁾ value of an optical fiber for the optical Kerr effect was measured using this scheme. A classical correlation between the measured light intensity and the outgoing light intensity was observed experimentally. The goal of the measurement accuracy to obtain a quantum correlation is also discussed aiming at the quantum nondemolition (QND) measurement of the photon number.     

Geometrical optics in Fresnel quantum holography

The geometrical optics in Fresnel quantum holography for quantum entangled two-photon source and classical thermal light source based on the second-order correlation measurement has been discussed. We theoretically prove that the Fresnel quantum hologram of the detected object can be obtained through second-order correlation measurement and the optical reconstruction process can be accomplished by making use of a point light source.     

Probing the quantum state of a 1D Bose gas using off-resonant light scattering

We present a theoretical treatment of coherent light scattering from an interacting 1D Bose gas at finite temperatures. We show how this can provide a nondestructive measurement of the atomic system states. The equilibrium states are determined by the temperature and interaction strength, and are characterized by the spatial density-density correlation function. We show how this correlation function is encoded in the angular distribution of the fluctuations of the scattered light intensity, thus providing a sensitive, quantitative probe of the density-density correlation function and therefore the quantum state of the gas.     

Spatial statistics of a partially coherent wave in a randomly inhomogeneous cubic medium

The spatial statistics of a partially coherent light wave in a randomly inhomogeneous medium is considered on the basis of perturbation theory. The equations for the spatial coherence function of the light wave in a nonlinear irregular medium were solved. The variation in the fluctuation field dispersion, in its amplitude and phase, as well as the behavior of the radius of spatial correlation for various perturbations of the incident radiation, were investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 27, No. 1, pp. 56–64, January, 1984.     

Can two-photon correlation of chaotic light be considered as correlation of intensity fluctuations?

Two-photon correlation phenomena, including the historical experiment of Hanbury Brown and Twiss, may have to be described quantum mechanically, regardless of whether the source of radiation is classical or quantum. Supporting this point, we present a ghost imaging type of second-order spatial correlation experiment of chaotic light to show that the classical understanding based on the concept of statistical intensity fluctuations does not give a correct interpretation for the observation. From a practical point of view, this experiment demonstrates the possibility of having high contrast lensless two-photon imaging with chaotic light, suggesting imaging applications for radiations for which no effective lens is available.     

Long-range correlations of the polarized-light intensity in disordered samples

The spatial correlation function of intensity fluctuations in a speckle formed by polarized light multiply scattered in a disordered sample has been calculated. The dependence of the long-range spatial correlations on the polarization state of incident light and the depolarizing properties of the medium has been determined for the cases of transmission and reflection.     

Improvement of the optical image reconstruction based on multiplexed quantum ghost images

Ghost imaging allows one to obtain information on an object from the spatial correlation function between photons propagating through or reflected from the object and photons of the reference arm. In this case, detection in the object arm is performed over the entire aperture of the beam and, therefore, it does not give information on the object. The reference beam does not interact with the object, but is recorded with a scanning point detector or a CCD array permitting the measurement of the spatial correlation function of photons in two arms. The use of multimode entangled quantum light beams by illuminating the object by one beam and orienting other beams to reference arms makes it possible to obtain simultaneously several ghost images (GIs). Cross correlations of multiplexed GIs (MGIs) are determined by eighth-order field correlation functions. A special algorithm is developed for calculating higher-order correlations of Bose operators. The presence of GI cross correlations is used for improving the quality of the reconstructed object’s image by their processing using the measurement reduction method. An example of the computer simulation of the image reconstruction by MGIs formed in the field of four-frequency entangled quantum states is considered. It is found that in this case the reduced GI has a signal-to-noise ratio several times higher than that of GIs.     

Self-tuned quantum dot gain in photonic crystal lasers

We demonstrate that very few (2-4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.     

Time correlations of coherent radiation propagating through a randomly inhomogeneous layer

The intensity and the time correlation function of radiation transmitted through a layer of a randomly inhomogeneous medium are determined by numerical simulation of the Bethe-Salpeter equation for scalar and electromagnetic fields. The results of simulation agree well with the available experimental data. The domain of validity is determined numerically for the hypothesis of the diffusion approximation on the replacement of the extinction length by the transport length as a characteristic spatial scale for systems with the anisotropic scattering cross section. The polarized and depolarized components of the time correlation function of the transmitted light are calculated for the first time for different anisotropy parameters. It is shown that, to describe the residual polarization of transmitted light taking into account anisotropy, it is insufficient to perform the scale replacement of the extinction length by the transport length.     

Amplification of entangled light upon reflection from a cavity with nonlinear medium

The kinetic equation for the case of reflection of a broadband entangled light of a nondegenerate parametric source from a cavity is derived with the use of the technique of stochastic differential Ito equations. Statistical properties of the light reflected from a cavity with a nonlinear transparent medium, in which the process of special parametric interaction occurs, are considered on the basis of the obtained equation. Due to the integrals of motion, the entangled light, which is an EPR pair of continuous variables, can be amplified without destruction of the quantum correlation.     

The role of spatial dispersion of an electromagnetic wave in its penetration through a quantum well

The theory of light penetration through a quantum well in a strong magnetic field perpendicular to the well plane is developed under the conditions where interband transitions occur in the well. The light wavelength is assumed to be comparable to the well width. The relationships for the reflection, absorption, and transmission are derived with due regard for the spatial dispersion of a monochromatic light wave and the difference between the refractive indices of the quantum well and the barrier. The normal incidence of light with respect to the well plane is considered, and one excited level is taken into account. It is demonstrated that the above two factors most strongly affect the reflection, because the reflection from the well boundaries appears in addition to the reflection caused by interband transitions in the quantum well. The most radical changes in the reflection are observed in the case when the reciprocal radiative lifetime of the excited state in the quantum well is short compared to the reciprocal nonradiative lifetime. In the range of large well widths, the applicability of the theory is limited by the existence condition of quantum well levels.     

Quantum images in non degenerate optical parametric oscillators

Quantum images, that is inhomogeneous field distributions purely generated by quantum fluctuations, persist when passing from the degenerate to the non-degenerate case of optical parametric oscillators (OPO). Below the threshold for parametric oscillation where the near-field distributions are homogeneous both in intensity and phase, appropriate spatial correlation functions anticipate the transverse spatial pattern that appears above threshold. In particular, the angular dependence of the far field spatial correlation function is able to reveal the travelling-wave nature of the phase pattern above threshold typical of nondegenerate OPOs. Cross-correlation functions between signal and idler intensities show clear evidence of the non-classical nature of the output light. Received 8 October 1999     

Elastic scattering of light from quantum-well exciton-polarization fluctuations in a microcavity

A model of exciton polarization fluctuations in a quantum well of a randomly variable lateral width is proposed. The stochastic part of the nonlocal susceptibility of quasi-two-dimensional excitons is expressed through random functions of the shape of quantum well boundaries. A theory of elastic light scattering from a quantum well placed in a Fabry-Perot cavity or a semiconductor microcavity is constructed in the lowest (Born) approximation in interface roughness height. The scattering cross section is calculated for an arbitrary statistics of interface roughness. The spectral and angular dependences of the intensity of light scattered by a quantum well have been studied using Gaussian correlation functions of the interface shape. It follows from numerical estimates that elastic resonant scattering in quantum wells should be observed at an rms roughness height of the order of the atomic monolayer thickness.     

Teaching a laser beam to go straight

In classical physics a beam of light propagates in a perfectly straight line and this means that we can measure small displacements with unlimited accuracy. However, this is not correct for real laser beams when we take the quantum properties of light into account. Spatial measurements will be limited by quantum noise, similar to the limitations for optical communication and sensing. Here we derive the spatial quantum noise limit and show how to measure it. Next we demonstrate that we can use specially prepared light with quantum correlations, so-called squeezed light, to improve spatial measurements to below this quantum limit. In this way we prepare a beam which goes in a straighter line than the output of any conventional laser.     

光场-原子BEC相互作用系统的量子相关性质

运用全量子理论,在Bogoliubov近似下研究了光场与二能级原子玻色-爱因斯坦凝聚体(BEC)相互作用系统的量子相关性质,结果表明:光场和原子玻色-爱因斯坦凝聚体在相互作用过程中,其量子相关性质保持不变.