Guilherme P. Temporão

Full polarization control for fiber optical quantum communication systems using polarization encoding

A real-time polarization control system employing two non-orthogonal reference signals multiplexed in either time or wavelength with the data signal is presented. It is shown, theoretically and experimentally, that complete control of multiple polarization states can be attained employing polarization controllers in closed-loop configuration. Experimental results on the wavelength multiplexing setup show that negligible added penalties, corresponding to an average added optical Quantum Bit Error Rate of 0.044%, can be achieved with response times smaller than 10 ms, without significant introduction of noise counts in the quantum channel.

Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40??rad?s?1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

Real-time monitoring of single-photon detectors against eavesdropping in quantum key distribution systems

By employing real-time monitoring of single-photon avalanche photodiodes we demonstrate how two types of practical eavesdropping strategies, the after-gate and time-shift attacks, may be detected. Both attacks are identified with the detectors operating without any special modifications, making this proposal well suited for real-world applications. The monitoring system is based on accumulating statistics of the times between consecutive detection events, and extracting the afterpulse and overall efficiency of the detectors in real-time using mathematical models fit to the measured data. We are able to directly observe changes in the afterpulse probabilities generated from the after-gate and faint after-gate attacks, as well as different timing signatures in the time-shift attack. We also discuss the applicability of our scheme to other general blinding attacks.

WDM-PON Monitoring With Tunable Photon Counting OTDR

We explore the performance of a tunable optical time domain reflectometer based on single photon detection for fault location and quantification in wavelength division multiplexing (WDM)-passive optical networks based on a 32-channel cyclic arrayed waveguide grating using the S- and C-bands for upstream (156 Mb/s) and downstream (2.5 Gb/s) transmission and the L-band for monitoring. Downstream Rayleigh and Raman scattering contributions to the dark count noise were evaluated and properly filtered out with a total WDM + filter rejection ratio of 90 dB. Upstream power was found to generate the greatest contribution to the noise floor and is the limiting factor to the upstream transmission data rate. In-service monitoring experiments with full upstream and downstream power show that the proposed scheme is able to achieve dynamic range levels of up to 32 dB with 5-m spatial resolution and no measurable penalty in data transmission.

Automatic Fault Detection in WDM-PON With Tunable Photon Counting OTDR

An automatic fiber-optical fault analysis system making use of a tunable photon counting optical time-domain reflectometry (OTDR) (ν-OTDR) is proposed and demonstrated in a passive optical network testbed. The employment of the ℓ trend filter as a signal processing tool enables the minimization of the intrinsic coherent random noise impact on the acquired data and also an automatic identification of fiber faults. A feedback loop between an FPGA-based acquisition unit and the filters selections yields highly accurate automatic results and minute monitoring periods with ITU-T grid DWDM tunability, spatial resolution of 6 m and a measured 32 dB dynamic range.

Detection of fiber faults in passive optical networks

Wider deployment of fiber in the last mile is driven by increased customer needs for broadband communication services. This deployment requires solutions that reduce operational expenditures for the operator. A cost-efficient fully reliable and accurate monitoring solution supporting fault detection, identification, and localization in different fiber access architectures will be essential. In this article, we present a fast, automatic, and precise monitoring method applicable to both power-splitter- and wavelength-router-based passive optical networks through the combined techniques of optical time domain reflectometry and optical transceiver monitoring. The description of the architecture, components, and process flow is followed by tests on setups with live data transmission.

Spreading remote lab usage a system — A community — A Federation

Experiments have been at the heart of scientific development and education for centuries. From the outburst of Information and Communication Technologies, virtual and remote labs have added to hands-on labs a new conception of practical experience, especially in Science, Technology, Engineering and Mathematics education. This paper aims at describing the features of a remote lab named Virtual Instruments System in Reality, embedded in a community of practice and forming the spearhead of a federation of remote labs. More particularly, it discusses the advantages and disadvantages of remote labs over virtual labs as regards to scalability constraints and development and maintenance costs. Finally, it describes an actual implementation in an international community of practice of engineering schools forming the embryo of a first world wide federation of Virtual Instruments System in Reality nodes, under the framework of a project funded by the Erasmus+ Program.

Spectral characterization of weak coherent state sources based on two-photon interference

We demonstrate a method for characterizing the coherence function of coherent states based on two-photon interference. Two states from frequency mismatched faint laser sources are fed into a Hong–Ou–Mandel interferometer, and the interference pattern is fitted with the presented theoretical model for the quantum beat. The fitting parameters are compared to the classical optical beat when bright versions of the sources are used. The results show the equivalence between both techniques.

Scattering Effects on QKD Employing Simultaneous Classical and Quantum Channels in Telecom Optical Fibers in the C‐band

Almost all Quantum Key Distribution experiments in optical fibers performed so far have used the so called “dark fibers,” which are exclusively used as the quantum channel. As quantum communications make its way towards commercial applications, there is a clear advantage in using fibers with simultaneous classical signals, via wavelength multiplexing, for that purpose. We show what is the noise impact of transmitting classical optical signals alongside a quantum channel for a fixed fiber length.

Few-photon heterodyne spectroscopy.

We perform a high-resolution Fourier transform spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram, between test and reference sources, it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques.