Experimental entanglement distillation of mesoscopic quantum states

The distribution of entangled states between distant parties in an optical network is crucial for the successful implementation of various quantum communication protocols such as quantum cryptography, teleportation and dense coding [1-3]. However, owing to the unavoidable loss in any real optical channel, the distribution of loss-intolerant entangled states is inevitably inflicted by decoherence, which causes a degradation of the transmitted entanglement. To combat the decoherence, entanglement distillation, which is the process of extracting a small set of highly entangled states from a large set of less entangled states, can be used [4-14]. Here we report on the mesoscopic distillation of deterministically prepared entangled light pulses that have undergone non-Gaussian noise. The entangled light pulses [15-17] are sent through a lossy channel, where the transmission is varying in time similarly to light propagation in the atmosphere. By employing linear optical components and global classical communication, the entanglement is probabilistically increased.Comment: 13 pages, 4 figures. It's the first submitted version to the Nature Physics. The final version is already published on Nature Physics vol.4, No.12, 919 - 923 (2008

Predicting Fading in Free Space Communication Channel Using Deep Learning

Free space optical communication plays a role in daily communications and has the advantage that it does not need a huge infrastructure of cables. Due to that advantage it can be used to deliver the internet to urban as well as remote locations, in the communication with drones, etc. However, the optical signal propagating through the atmosphere gets distorted due to fluctuations in weather parameters such as temperature and wind speed, resulting in optical turbulence, which impacts the strength of the optical signal that is received. In our work, we will use a deep learning algorithm to predict when these distortions could happen based on optical turbulence and weather data. Deep learning algorithms will be trained on the weather data as an input and the intensity of the signal as an output. Knowing the potential fading in the signal can help us to prevent losing the connection with the receiver. For instance, if we control a drone with an optical communication channel then it is important to know the potential fading in the signal, since this can be helpful for the controller to take action to prevent losing the connection with the drone.https://ecommons.udayton.edu/stander_posters/3739/thumbnail.jp

Performance analysis of atmospheric field conjugation adaptive arrays

System configurations based on single monolithic-apertures that are immune to atmospheric fluctuations are being developed. Main goal is the improvement of the performance achievable in coherent, free-space optical communication systems using atmospheric compensation techniques such as adaptive optics. As an alternative to a single monolithicaperture coherent receiver with a full-size collecting area, a large effective aperture can be achieved by combining the output signal from an array of smaller receivers. We study the communication performance of field conjugation adaptive arrays applied in synchronous laser communication through the turbulent atmosphere. We assume that a single information-bearing signal is transmitted over the atmospheric fading channel, and that the adaptive array coherent receiver combines multiple dependent replicas to improve detection efficiency. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various atmospheric parameters and the number of branches combined at the receiver.Postprint (published version

Variance In Fade-time Of A Gamma-gamma Distributed Irradiance Signal

Free-space optical communications are predominantly hindered by optical turbulence, an effect caused by temperature and pressure variations within the atmosphere. The result is an optical wave interfering with itself due to multipath propagation via tiny refractive-index fluctuations across the wave-front. Optical communication systems are affected when the channel conditions induce fading in the irradiance signal that is received at the detector. The nature of optical interference imparted by the atmosphere is a random process and therefore the received irradiance signal is often characterized by an appropriate probability density function (PDF). Data collected during past free-space optical experiments in the atmosphere support the gammagamma distribution as a practical PDF model for received irradiance fluctuations, although the irradiance fluctuations do occasionally tend towards a lognormal distribution. Utilization of the gamma-gamma irradiance PDF allows for calculation of statistical moments of the irradiance threshold level-crossing distribution. Presented analysis focuses on the results of the gamma-gamma irradiance PDF. Previously, expressions were developed for the expected number of gamma-gamma distributed irradiance threshold level-crossings. Expressions for the mean square number of gamma-gamma distributed irradiance threshold level-crossings are derived and presented. The derived expressions lead to the mean and variance of signal fade time. Outcomes of the derived expressions are presented in relation to free-space optical communication system performance. iii Comparisons are made between the theoretical analysis and experimental data taken at the Innovative Science and Technology Facility (ISTEF) located at the Kennedy Space Center in Cape Canaveral, Florida. The strength of the atmospheric turbulence is often characterized by three measurable parameters: the refractive index structure constant �� 2 , the inner scale �0 , and the outer scale �0 . The optical path (�~1��) was instrumented such that direct comparisons could be drawn between the measured atmospheric turbulence parameters and the parameters of the gamma-gamma irradiance model. Variance of fade time data were found to agree well for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of these fade statistics

Atmospheric channel effects on terrestrial free space optical communication links

Abstract. This paper illustrates the challenges imposed by the atmospheric channel on the design of a terrestrial laser communication link. The power loss due to scattering effect is described using the Kim/Kruse scattering model while the effect and the penalty imposed by atmospheric turbulence is highlighted by considering the bit error rate (BER) of an On-Off Keying modulated link in an optical Poisson channel. The power loss due to thick fog can measure over 100 dB/km while snow and rain result in much lower attenuation. We show that non-uniformity in the atmospheric temperature also contributes to performance deterioration due to scintillation effect. At a BER of 10-4, for a channel with a turbulence strength of>0.1, the penalty imposed by turbulence induced fading is over 20 photoelectron counts in order to achieve the same level of performance as a channel with no fading. The work reported here is part of the EU COST actions and EU projects.

Current optical technologies for wireless access

The objective of this paper is to describe recent activities and investigations on free-space optics (FSO) or optical wireless and the excellent results achieved within SatNEx an EU-framework 6th programme and IC 0802 a COST action. In a first part, the FSO technology is briefly discussed. In a second part, we mention some performance evaluation criterions for the FSO. In third part, we briefly discuss some optical signal propagation experiments through the atmosphere by mentioning network architectures for FSO and then discuss the recent investigations in airborne and satellite application experiments for FSO. In part four, we mention some recent investigation results on modelling the FSO channel under fog conditions and atmospheric turbulence. Additionally, some recent major performance improvement results obtained by employing hybrid systems and using some specific modulation and coding schemes are presented

Atmospheric Channel Characteristics for Quantum Communication with Continuous Polarization Variables

We investigate the properties of an atmospheric channel for free space quantum communication with continuous polarization variables. In our prepare-and-measure setup, coherent polarization states are transmitted through an atmospheric quantum channel of 100m length on the roof of our institute's building. The signal states are measured by homodyne detection with the help of a local oscillator (LO) which propagates in the same spatial mode as the signal, orthogonally polarized to it. Thus the interference of signal and LO is excellent and atmospheric fluctuations are autocompensated. The LO also acts as spatial and spectral filter, which allows for unrestrained daylight operation. Important characteristics for our system are atmospheric channel influences that could cause polarization, intensity and position excess noise. Therefore we study these influences in detail. Our results indicate that the channel is suitable for our quantum communication system in most weather conditions.Comment: 6 pages, 4 figures, submitted to Applied Physics B following an invitation for the special issue "Selected Papers Presented at the 2009 Spring Meeting of the Quantum Optics and Photonics Section of the German Physical Society