Quantum information processing in optical images

Optical images can be used to transport, store and process information in a parallel way. We discuss different results obtained in the domain of ‘quantum imaging’, aiming at exploiting at the same time the quantum properties of optical images and their intrinsic parallelism. We define the notion of standard quantum limit (SQL) in optical resolution, set by the quantum noise of usual coherent light, and show that it can be much lower than the diffraction limit. We also prove that this limit can be circumvented by especially designed nonclassical and multimode light. We present an experiment showing that OPOs oscillating inside an exactly confocal cavity actually produce such transverse multimode nonclassical light. We finally describe another experiment which has surpassed the SQL in the case of beam positioning, both in the 1D and 2D cases.     

Four modes of optical parametric operation for squeezed state generation

We report a versatile instrument, based on a monolithic optical parametric amplifier, which reliably generates four different types of squeezed light. We obtained vacuum squeezing, low power amplitude squeezing, phase squeezing and bright amplitude squeezing. We show a complete analysis of this light, including a full quantum state tomography. In addition we demonstrate the direct detection of the squeezed state statistics without the aid of a spectrum analyser. This technique makes the nonclassical properties directly visible and allows complete measurement of the statistical moments of the squeezed quadrature.Received: 10 March 2003, Published online: 26 August 2003PACS: 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurementsU.L. Andersen: Present address: Institute of Optics, Information and Photonics, Max-Planck Research Group, University of Erlangen-Nuernberg, 91058 Erlangen, Germany.     

Optical experiments beyond the quantum limit: Squeezing,entanglement, and teleportation

Results of experiments recently performed are reported, in which two optical parametric amplifiers were set up to generate two independently quadrature squeezed continuous wave laser beams. The transformation of quadrature squeezed states into polarization squeezed states and into states with spatial quantum correlations is demonstrated. By utilizing two squeezed laser beams, a polarization squeezed state exhibiting three simultaneously squeezed Stokes operator variances was generated. Continuous variable polarization entanglement was generated and the Einstein-Podolsky-Rosen paradox was observed. A pair of Stokes operators satisfied both the inseparability criterion and the conditional variance criterion. Values of 0.49 and 0.77, respectively, were observed, with entanglement requiring values below unity. The inseparability measure of the observed quadrature entanglement was 0.44. This value is sufficient for a demonstration of quantum teleportation, which is the next experimental goal of the authors.     

Noiseless independent signal and power amplification

We present a noiseless optical amplifier comprising a signal-amplifying feed-forward loop and a power-amplifying injection-locked laser. We demonstrate that the signal amplifier can attain a signal-transfer coefficient limited solely by the quantum efficiency of our in-loop photodetector and that we can independently amplify the optical power while leaving the normalized intensity-noise spectral density of the input field unchanged.     

Understanding and controlling laser intensity noise

We present unified theoretical expressions for laser intensity noise in the presence of injection locking and feedback. We discuss optimum control strategies for different configurations and frequency regions. We illustrate the various effects with experimental results from Nd : YAG non-planar ring oscillator lasers.     

Investigation of polarisation effects in injection locked lasers

Received: 14 June 1996/Revised version: 6 November 1996     

Quantum study of information delay in electromagnetically induced transparency

Using electromagnetically induced transparency (EIT), it is possible to delay and store light in atomic ensembles. Theoretical modeling and recent experiments have suggested that the EIT storage mechanism can be used as a memory for quantum information. We present experiments that quantify the noise performance of an EIT system for conjugate amplitude and phase quadratures. It is shown that our EIT system adds excess noise to the delayed light that has not hitherto been predicted by published theoretical modeling. In analogy with other continuous-variable quantum information systems, the performance of our EIT system is characterized in terms of conditional variance and signal transfer.     

Quantum measurements of spatial conjugate variables: displacement and tilt of a Gaussian beam

We consider the problem of measurement of optical transverse profile parameters and their conjugate variable. Using multimode analysis, we introduce the concept of detection noise modes. For Gaussian beams, displacement and tilt are a pair of transverse-profile conjugate variables. We experimentally demonstrate the optimal encoding and detection of these variables with a spatial homodyning scheme. Using higher-order spatial mode squeezing, we show the sub-shot-noise measurements for the displacement and tilt of a Gaussian beam.     

Effects of photodynamic therapy with topical application of 5-aminolevulinic acid on normal skin of hairless guinea pigs.

Photodynamic therapy (PDT) is a relatively new approach to the treatment of neoplasms which involves the use of photoactivatable compounds to selectively destroy tumors. 5-Aminolevulinic acid (ALA) is an endogenous substance which is converted to protoporphyrin IX (PpIX) in the synthetic pathway to heme. PpIX is a very effective photosensitizer. The goal of this study was to evaluate the effect of PDT using topical ALA on normal guinea pig (g.p.) skin and g.p. skin in which the stratum corneum was removed by being tape-stripped (TS). Evaluation consisted of gross examination, PpIX fluorescence detection, reflectance spectroscopy, and histology. There was no effect from the application of light or ALA alone. Normal non-TS g.p. skin treated with ALA and light was unaffected unless high light and ALA doses were used. Skin from which the stratum corneum was removed was highly sensitive to treatment with ALA and light: 24 h after treatment, the epidermis showed full thickness necrosis, followed by complete repair within 7 d. Time-dependent fluorescence excitation and emission spectra were determined to characterize the chromophore and to demonstrate a build-up of the porphyrin in the skin. These data support the view that PDT with topical ALA is a promising approach for the treatment of epidermal cutaneous disorders.