Fabian Laudenbach

Passive ROADM Flexibility in Optical Access With Spectral and Spatial Reconfigurability

An energy-aware solution for physical-layer reconfigurability in metro-access networks is presented. The dynamicity of optical switching is introduced in nodes that are perceived as fully-passive by the network. Energy scavenging at low optical feed level of -10dBm supports field-deployment without local electrical power supply. Two types of network nodes are demonstrated experimentally. First, a resilience node is evaluated for fast protection switching in 10.7 ms at the feeder segment. Optical switching is further exploited for the purpose of dynamic allocation of spectral slices and routing in a new class of reconfigurable optical add-drop multiplexer. The spectral bandwidth of drop segments can be extended on demand while intranetwork communication among different segments of the access network is also enabled. Finally, we discuss the potential for realizing self-powering by means of tapping optical signals traversing the access network rather than utilizing a dedicated pump source.

Continuous-Variable Quantum Key Distribution with Gaussian Modulation-The Theory of Practical Implementations

Quantum key distribution using weak coherent states and homodyne detection is a promising candidate for practical quantum-cryptographic implementations due to its compatibility with existing telecom equipment and high detection efficiencies. However, despite the actual simplicity of the protocol, the security analysis of this method is rather involved compared to discrete-variable QKD. In this article we review the theoretical foundations of continuous-variable quantum key distribution (CV-QKD) with Gaussian modulation and rederive the essential relations from scratch in a pedagogical way. The aim of this paper is to be as comprehensive and self-contained as possible in order to be well intelligible even for readers with little pre-knowledge on the subject. Although the present article is a theoretical discussion of CV-QKD, its focus lies on practical implementations, taking into account various kinds of hardware imperfections and suggesting practical methods to perform the security analysis subsequent to the key exchange. Apart from a review of well known results, this manuscript presents a set of new original noise models which are helpful to get an estimate of how well a given set of hardware will perform in practice.

Modelling parametric down-conversion yielding spectrally pure photon pairs.

Pair creation by spontaneous parametric down-conversion (SPDC) has become a reliable source for single-photon states, used in many kinds of quantum information experiments and applications. In order to be spectrally pure, the two photons within a generated pair should be as frequency-uncorrelated as possible. For this purpose most experiments use narrow bandpass filters, having to put up with a drastic decrease in count rates. This article elaborates (theoretically and by numerical evaluation) the alternative method to engineer a setup such that the SPDC-generated quantum states are intrinsically pure. Using pulsed pump lasers and periodically poled crystals this approach makes bandpass filtering obsolete and allows for significantly higher output intensities and therefore count rates in the detectors. After numerically scanning all common wavelength regimes, polarisation configurations and three different non-linear crystals, we present a broad variety of setups which allow for an implementation of this method.

A novel single-crystal & single-pass source for polarisation- and colour-entangled photon pairs

We demonstrate a new generation mechanism for polarisation- and colour-entangled photon pairs. In our approach we tailor the phase-matching of a periodically poled KTP crystal such that two downconversion processes take place simultaneously. Relying on this effect, our source emits entangled bipartite photon states, emerging intrinsically from a single, unidirectionally pumped crystal with uniform poling period. Its property of being maximally compact and luminous at the same time makes our source unique compared to existing photon-entanglement sources and is therefore of high practical significance in quantum information experiments.

Numerical Investigation of Photon-Pair Generation in Periodically Poled MTiOXO4 ( M=K , Rb, Cs; X=P , As)

We present a detailed numerical investigation of five nonlinear materials and their properties regarding photon-pair creation using parametric downconversion. Periodic poling of ferroelectric nonlinear materials is a convenient way to generate collinearly propagating photon pairs. Most applications and experiments use the well-known potassium titanyl phosphate (KTiOPO4, ppKTP) and lithium niobate (LiNbO3, ppLN) crystals for this purpose. In this article we provide a profound discussion on the family of KTP-isomorphic nonlinear materials, including KTP itself but also the much less common CTA (CsTiOAsO4), KTA (KTiOAsO4), RTA (RbTiOAsO4) and RTP (RbTiOPO4). We discuss in which way these crystals can be used for creation of spectrally pure downconversion states and generation of crystal-intrinsic polarisation- and frequency entanglement. The investigation of the new materials disclosed a whole new range of promising experimental setups, in some cases even outperforming the established materials ppLN and ppKTP.

QPMoptics: a novel tool to simulate and optimise photon pair creation

We present a new scientific software which is able to simulate any experimental setup based on photon pair creation with spontaneous parametric down-conversion in periodically poled crystals. Moreover, our software is able to optimise experimental parameters for the sake of high down-conversion efficiency and quantum performance. Given the optional input of any relevant individual experimental setup, the large number of output parameters and plots and the user-friendly intuitive graphical user interface, we believe that our software can be a helpful tool to any experimentalist who uses the quasi-phase-matching technique to generate collinearly propagating photon pairs.

Thermal effects of the quantum states generated from the isomorphs of PPKTP crystal

Abstract We theoretically and numerically investigate the temperature-dependent properties of the biphotons generated from four isomorphs of periodically poled KTiOPO 4 (PPKTP): i.e., PPRTP, PPKTA, PPRTA and PPCTA. It is discovered that the first type of group-velocity-matched (GVM) wavelength is decreased by 6.4, 1.2, 8.9, 25.6 and 6.3 nm, while the phase-matched wavelength is decreased by 4.4, −0.4, −1.2, 29.1 and 59.5 nm for PPKTP, PPRTP, PPKTA, PPRTA and PPCTA, respectively, when the temperature is increased from 20 °C to 120 °C. Although the maximal spectral purity of the heralded single photons is not changed at different temperature, the Hong-Ou-Mandel (HOM) interference shows different patterns due to a shift of the joint spectral amplitude. These thermal effects are very important for precise control of the quantum state for the future applications in quantum information processing, for example, in quantum interference or spectroscopy.

Visible-Light Multi-Gb/s Transmission Based on Resonant Cavity LED With Optical Energy Feed

Multi-Gb/s visible-light communication is demonstrated using a commercial off-the-shelf resonant-cavity light emitting diode (LED), which is originally rated for 150 Mb/s. By applying analog frequency response equalization and multi-carrier modulation schemes, a transmission capacity of up to 4.3 Gb/s is obtained over a single wavelength in a close-proximity scenario. Nyquist-shaped multi-band modulation and orthogonal-frequency-division multiplexing are applied with high spectral sub-carrier efficiencies of up to 8 b/symbol. The transmission rate is experimentally investigated as a function of the loss budget and further related to link reach based on free-space measurements under clear weather conditions. Analog signal transmission is also validated using real-time signal (de-)modulation with a high-definition video payload. We further demonstrate that on/off keying in combination with simpler baseband modulation can be facilitated for data rates of up to 750 Mb/s. This proves that commercially available LEDs can serve as a versatile low-cost transmitter. Finally, the joint transmission of energy and data has been validated. A power feed with an irradiance of 240 W/m2 is experimentally shown to enable a remotely supplied optical burst receiver for periodic access to Gb/s data rates.

Multi-wavelength photon pair source assisted by a silicon-on-insulator micro-ring resonator

An integrated photon pair source in the 1550-nm wavelength region is demonstrated. Spontaneous four-wave mixing is facilitated through a silicon-on-insulator micro-ring filter with 125-GHz spaced resonances. Coincidences in the pair emission are observed with a 95% visibility at spectral channels equidistant to the pump wavelength.

A novel compact and efficient source of photonic entanglement

Up to this point, photon-entanglement sources suffered from the compromise of being either compact or efficient, but not both at the same time [1-6]. Our new concept of creating entanglement [7] combines most of the advantages of the various existing schemes. The setup offers high coupling efficiencies due to its collinear design and is still as compact and simplistic as a BBO source [1], requiring only one crystal with uniform poling periodicity, pumped from only one direction.