Coincidence fractional Fourier transform implemented with partially coherent light radiation

We introduce the coincidence fractional Fourier transform (FRT) implemented with incoherent and partially coherent light radiation. Optical systems for implementing the coincidence FRT are designed. The results show that the visibility and quality of the coincidence FRT of an object are closely related to the light source's transverse size, coherence, and spectral width. As an example, we numerically study the coincidence FRT of a single slit.     

Single photon detection of degenerate photon pairs at 1.55 μm from a periodically poled lithium niobate parametric downconverter

Abstract

The performance of a communication system that uses 1.55 μm correlated photon pairs is analysed experimentally in terms of achievable coincidence rates, optimal pump rates, and the performance of custom-built photon-counting detectors at 1.55 μm. The testbed considered in this study uses standard telecom fibre, twin photons, and photon-counting detectors. Degenerate cw time-frequency entangled photon pairs are produced via quasiphase-matched spontaneous parametric downconversion in bulk periodically poled lithium niobate. The photon pairs are efficiently collected into a single-mode fibre and are sent to a pair of custom-built InGaAs photon-counting avalanche photodiodes that are passively quenched, gated in Geiger mode, and thermoelectrically cooled to temperatures as low as - 60°C. Reliable photoncounting operation with a quantum efficiency of 20% at a dark count probability of 0.04% per gate (20 ns) and negligible afterpulses is reported.     

宣布式纠缠单光子源及量子化光辐射量值溯源体系研究

针对fJ(飞焦)至单光子极弱能量测量及溯源需求,提出一种纠缠单光子源.通过基于冷原子团的自发四波混频效应产生波长795 nm的宣布式纠缠光子对,采用背景辐射抑制与实时补偿、高精度激光稳频、时序脉冲精密同步获得窄线宽、高质量的单光子源.在此基础上,研究了纠缠光子源参数校准方法,进一步建立以量子基准为基础的全新光辐射量值溯...     

Anisotropic narrowing of biphoton wave packet distribution in spontaneous parametric down-conversion with biaxial crystal

We investigate theoretically the strong anisotropy in the wave packet distribution of biphoton pairs generated via spontaneous parametric down-conversion (SPDC) process with biaxial crystals. From the angle relationship in biaxial crystals, we deduce the wave function of type I SPDC by considering the longitudinal detuning under paraxial approximation. Taking BiB₃O₆ (BIBO) as an example, we numerically simulate the single-particle and coincidence distributions in two observation planes under the optimum phase-matching scheme. In contrast to uniaxial crystals, a stronger anisotropy effect and a higher entanglement of the photon pair states are obtained, which are attributed to the complex structure of biaxial crystals. These results are important for the generation of highly entangled biphoton pairs, especially for multi-photon pairs generation.     

Optical absorption measurements with parametric down-converted photons

It has been known for some time that correlated detection of pairs of photons generated by parametric down-conversion can eliminate several sources of error that occur in single-beam measurements. In the correlated photon measurements, the down-converted photons are separated into two beams with one photon of a pair in each beam. The absolute detection efficiency of a detector in one beam can be determined from the count rate of a detector in the other beam and the coincidence rate for the two detectors. These ideas can be used to measure the optical absorbance of a sample placed in front of one of the detectors. Errors due to stray light and dark counts are substantially reduced and fluctuations in pump intensity largely eliminated.     

Imaging detector of temporally coherent radiation

We present an optical architecture and image processor capable of detecting and locating temporally coherent radiation that may be dominated by incoherent background radiation. The optical architecture makes use of a coherent light modulator that modulates light of sufficient coherence length while it leaves light of short coherence length unmodulated. The design of the coherent light modulator offers a substantially wider field of view than did past designs, permitting its application within an imaging system. The image processor synchronously detects the modulation imposed on coherent light while it rejects incoherent light fluctuations. Results of a laboratory test are presented. The system tested in the laboratory had a 26 degrees field of view and was able to detect and locate coherent radiation >30 dB below the background incoherent light level.     

Electric field induced removal of the biexciton binding energy in a single quantum dot

We control the electrostatic environment of a single InAsP quantum dot in an InP nanowire with two contacts and two lateral gates positioned to an individual nanowire. We empty the quantum dot of excess charges and apply an electric field across its radial dimension. A large tuning range for the biexciton binding energy of 3 meV is obtained in a lateral electric field. At finite lateral electric field the exciton and biexciton emission overlap within their optical line width resulting in an enhancement of the observed photoluminescence intensity. The electric field dependence of the exciton and biexciton is compared to theoretical predictions and found to be in good qualitative agreement. This result is promising toward generating entangled photon pairs on demand without the requirement to remove the anisotropic exchange splitting from asymmetric quantum dots.     

Experimental observation of truncated fractional Fourier transform for a partially coherent Gaussian Schell-model beam

The truncated fractional Fourier transform (FRT) is applied to a partially coherent Gaussian Schell-model (GSM) beam. The analytical propagation formula for a partially coherent GSM beam propagating through a truncated FRT optical system is derived by using a tensor method. Furthermore, we report the experimental observation of the truncated FRT for a partially coherent GSM beam. The experimental results are consistent with the theoretical results. Our results show that initial source coherence, fractional order, and aperture width (i.e., truncation parameter) have strong influences on the intensity and coherence properties of the partially coherent beam in the FRT plane. When the aperture width is large, both the intensity and the spectral degree of coherence in the FRT plane are of Gaussian distribution. As the aperture width decreases, the diffraction pattern gradually appears in the FRT plane, and the spectral degree of coherence becomes of non-Gaussian distribution. As the coherence of the initial GSM beam decreases, the diffraction pattern for the case of small aperture widths gradually disappears.     

Electrooptic modulation in thin film barium titanate plasmonic interferometers

We demonstrate control of the surface plasmon polariton wavevector in an active metal-dielectric plasmonic interferometer by utilizing electrooptic barium titanate as the dielectric layer. Arrays of subwavelength interferometers were fabricated from pairs of parallel slits milled in silver on barium titanate thin films. Plasmon-mediated transmission of incident light through the subwavelength slits is modulated by an external voltage applied across the barium titanate thin film. Transmitted light modulation is ascribed to two effects, electrically induced domain switching and electrooptic modulation of the barium titanate index.     

Tests of a Two-Photon Technique for Measuring Polarization Mode Dispersion With Subfemtosecond Precision

An investigation is made of a recently introduced quantum interferometric method capable of measuring polarization mode dispersion (PMD) on sub-femtosecond scales, without the usual interferometric stability problems associated with such small time scales. The technique makes use of the extreme temporal correlation of orthogonally polarized pairs of photons produced via type-II phase-matched spontaneous parametric down-conversion. When sent into a simple polarization interferometer these photon pairs produce a sharp interference feature seen in the coincidence rate. The PMD of a given sample is determined from the shift of that interference feature as the sample is inserted into the system. The stability and resolution of this technique is shown to be below 0.2 fs. We explore how this precision is improved by reducing the length of the down-conversion crystal and increasing the spectral band pass of the system.     

Experimental observation of fractional Fourier transform for a partially coherent optical beam with Gaussian statistics

We report the experimental observation of the fractional Fourier transform (FRT) for a partially coherent optical beam with Gaussian statistics [i.e., partially coherent Gaussian Schell-model (GSM) beam]. The intensity distribution (or beam width) and the modulus of the square of the spectral degree of coherence (or coherence width) of a partially coherent GSM beam in the FRT plane are measured, and the experimental results are analyzed and agree well with the theoretical results. The FRT optical system provides a convenient way to control the properties, e.g., the intensity distribution, beam width, spectral degree of coherence, and coherence width, of a partially coherent beam.     

On the extinction of radiation by a homogeneous but spatially correlated random medium

Exponential extinction of incoherent radiation intensity in a random medium (sometimes referred to as the Beer-Lambert law) arises early in the development of several branches of science and underlies much of radiative transfer theory and propagation in turbid media with applications in astronomy, atmospheric science, and oceanography. We adopt a stochastic approach to exponential extinction and connect it to the underlying Poisson statistics of extinction events. We then show that when a dilute random medium is statistically homogeneous but spatially correlated, the attenuation of incoherent radiation with depth is often slower than exponential. This occurs because spatial correlations among obstacles of the medium spread out the probability distribution of photon extinction events. Therefore the probability of transmission (no extinction) is increased.     

A Fabry–Perot-like two-photon interferometer for high-dimensional time-bin entanglement

We generate high-dimensional time-bin entanglement using a mode-locked laser and analyse it with a two-photon Fabry–Perot interferometer. The dimension of the entangled state is limited only by the phase coherence between subsequent pulses and is practically infinite. In our experiment a picosecond mode-locked laser at 532 nm pumps a non-linear potassium niobate crystal to produce photon pairs by spontaneous parametric down-conversion (SPDC) at 810 and 1550?nm.     

Numerical feasibility study of the fabrication of subwavelength structure by mask lithography

Electromagnetic diffraction of a light wave by a single aperture of subwavelength width and subsequent propagation in a lossy medium are numerically investigated. This diffraction problem simulates exposure of a resist with an amplitude mask. It is found that there is the possibility of fabricating a lambda/2 structure on a resist of lambda/4 thickness, where lambda is the wavelength of the exposing light in vacuum, by conventional contact or by proximity lithography. It is also found that an air gap between a mask and a resist of up to lambda/2 does not have a significant effect on resolution. This approach permits easy and cost-effective fabrication of subwavelength structures and leads to wide availability of diffractive optical elements in the nonscalar domain.     

Spatial and temporal resolution in entangled ghost imaging

ABSTRACT

We show that, when the integration time of the single photon detectors is longer than the correlation time of the biphoton, the attainable spatial resolution in ghost imaging with entangled signal-idler pairs generated in type II spontaneous parametric down conversion is limited by the angular spread of single-frequency-signal-idler pairs. If, however, the detector integration time is shorter than the biphoton correlation time, the transverse k-vectors of different spectral components combine coherently in the image, improving the spatial resolution.     

Bell's inequality tests: from photons to B-mesons

We analyse the recent claim that a violation of a Bell's inequality has been observed in the B-meson system [A. Go, J. Mod. Opt. 51 991 (2004)]. The results of this experiment are a convincing proof of quantum entanglement in B-meson pairs similar to that shown by polarization entangled photon pairs. However, we conclude that the tested inequality is not a genuine Bell's inequality and thus cannot discriminate between quantum mechanics and local realistic approaches.     

Transferring the spatial state of an entangled photon pair to a single photon by photon detection

We show theoretically that the spatial state of entangled photons generated by parametric down-conversion can be transferred to the spatial state of an idler photon by signal photon detection. This study considered the general condition with an arbitrary pump field profile and the detection of a signal photon at an arbitrary distance from a nonlinear crystal where the entangled photons are generated. Upon the detection of a signal photon, the two-photon state function of the entangled state can be transferred to a single-photon state function of the idler field due to the EPR type correlation between the signal and idler fields. The spatial state of the idler field contains more information on the original two-photon state.     

Schrödinger wave equation for confinement of photon in a waveguide

In the present work, electromagnetic field confinement in a subwavelength waveguide structure is obtained using concepts of quantum mechanics and uncertainty principle. Semi-macroscopic considerations of field interaction at the dielectric interfaces are used in this work. The modal field profile in the subwavelength waveguide is obtained by considering the photon as a particle in the waveguide having finite probability of tunneling. Thus, uncertainty of position is assigned to it. The momentum uncertainty is calculated from position uncertainty. Schrödinger wave equation for the photon is written by incorporating position-momentum uncertainty. The equation is solved and the field distribution in the waveguide is obtained. The field distribution and power confinement is compared with conventional waveguide theory. They were found in good agreement with each other.     

Entanglement distillation from Gaussian input states by coherent photon addition

The entanglement between Gaussian entangled states can be increased by non-Gaussian operations. We design a new scheme, named coherent photon addition, which can coherently add one photon generated by a spontaneous parametric down-conversation process to Gaussian quadrature-entangled light pulses created by a non-degenerate optical parametric amplifier. This operation can increase the entanglement of input two-mode Gaussian states as an entanglement distillation, and provides us with a new method of non-Gaussian operation. This scheme can also help us to study the decoherence of adding one- to two-mode Gaussian states from coherent photon addition to normal photon addition.     

Quantum state measurement of any arbitrary entangled field-state via Ramsey interferometry

Multipartite entangled states are the key resource and play a crucial role in latest applications of quantum mechanics. We propose a scheme for the measurement of quantum state of multimode entangled field state trapped in multiple cavities. The scheme is based on the measurement of photon statistics of the displaced entangled field state in Ramsey type set-up. In this set-up, the atoms undergo a dispersive phase shift when they pass through the off-resonant entangled field in cavities. By measuring the internal states of the atoms, the photon statistics and the Wigner function can be reconstructed.