Malte Avenhaus

Fiber-assisted single-photon spectrograph

For many experiments in quantum optics like homodyne detection or heralded state preparation it is essential to learn about the spectral structure of the quantum mechanical states under study [1,2]. Typically, in these experiments the requirement of very low losses imposes the main constraint on the usability of conventional grating spectrometers from classical optics. Furthermore, the high cost of detectors sensitive to single photons - like avalanche photo diodes (APDs) - prohibits the use of detector arrays required in spectrographs. We present an implementation of a fibre integrated spectrograph for characterizing the spectrum of an ultrafast optical pulse at the single photon level [3].

Accessing higher order correlations in quantum optical states by time multiplexing.

We experimentally measured higher order normalized correlation functions (nCF) of pulsed light with a time-multiplexing-detector. We demonstrate excellent performance of our device by verifying unity valued nCF up to the eighth order for coherent light, and factorial dependence of the nCF for pseudothermal light. We applied our measurement technique to a type-II parametric downconversion source to investigate mutual two-mode correlation properties and ascertain nonclassicality.

Photon Number Statistics of Multimode Parametric Down-Conversion

We experimentally analyze the complete photon number statistics of parametric down-conversion and ascertain the influence of multimode effects. Our results clearly reveal a difference between single-mode theoretical description and the measured distributions. Further investigations assure the applicability of loss-tolerant photon number reconstruction and prove strict photon number correlation between signal and idler modes.

Experimental verification of high spectral entanglement for pulsed waveguided spontaneous parametric down-conversion

We study the spectral properties of Spontaneous Parametric Down Conversion in a periodically poled waveguided structure of KTP crystal pumped by ultra-short pulses. Our theoretical analysis reveals a strongly multimode and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. Experimental evidence, based on Hong-Ou-Mandel polarization interference with narrowband filtering, confirms this result.

How colors influence numbers: Photon statistics of parametric down-conversion

Parametric down-conversion (PDC) is a technique of ubiquitous experimental significance in the production of nonclassical, photon-number-correlated twin beams. Standard theory of PDC as a two-mode squeezing process predicts and homodyne measurements observe a thermal photon number distribution per beam. Recent experiments have obtained conflicting distributions. In this article, we explain the observation by an a priori theoretical model solely based on directly accessible physical quantities. We compare our predictions with experimental data and find excellent agreement.

Direct probing of the Wigner function by time-multiplexed detection of photon statistics

We investigate the capabilities of loss-tolerant quantum state characterization using a photon-number resolving, time-multiplexed detector (TMD). We employ the idea of probing the Wigner function point-by-point in phase space via photon parity measurements and displacement operations, replacing the conventional homodyne tomography. Our emphasis lies on reconstructing the Wigner function of non-Gaussian Fock states with highly negative values in a scheme that is based on a realistic experimental set-up. In order to establish the concept of loss-tolerance for state characterization, we show how losses can be decoupled from the impact of other experimental imperfections, i.e. the non-unity transmittance of the displacement beamsplitter and non-ideal mode overlap. We relate the experimentally accessible parameters to effective ones that are needed for an optimized state reconstruction. The feasibility of our approach is tested by Monte Carlo simulations, which provide bounds resulting from statistical errors that are due to limited data sets. Our results clearly show that high losses can be accepted for a defined parameter range, and moreover, that—in contrast to homodyne detection—mode mismatch results in a distinct signature, which can be evaluated by analysing the photon number oscillations of the displaced Fock states.

Bright integrated photon-pair source for practical passive decoy-state quantum key distribution

We report on a bright, nondegenerate type-I parametric down-conversion source, which is well suited for passive decoy-state quantum key distribution. We show the photon-number-resolved analysis over a broad range of pump powers and we prove heralded higher-order

Characteristics of displaced single photons attained via higher order factorial moments

n

Spectral overlap in direct measurements of displaced single‐photon states

-photon states up to

Characterizing single photons by photon counting

n=4