Tedros Tsegaye

Single quantum dots emit single photons at a time : Antibunching experiments

We have studied the photoluminescence correlation from a single InAs/GaAs self-assembled Stranski–Krastanow quantum dot under continuous, as well as under pulsed excitation. Under weak continuous excitation, where the single dot luminescence is due primarily to single exciton recombinations, antibunching is observed in the single dot emission correlation. Under weak pulsed excitation, the number of photons emitted by the quantum dot per pulse is close to one. We present data obtained under both conditions and are able to show that devices based on single quantum dots can be used to generate single photons.

Experiments on long wavelength (1550 nm) "plug and play" quantum cryptography systems.

An interferometric quantum cryptographic system at 1550nm wavelength using gated InGaAs Avalanche Photo Diodes as single-photon receivers is demonstrated for a transmission distance up to 40 km.

Complementarity and the uncertainty relations

We formulate a general complementarity relation starting from any Hermitian operator with discrete nondegenerate eigenvalues. We then elucidate the relationship between quantum complementarity and the Heisenberg-Robertson uncertainty relation. We show that they are intimately connected. Finally we exemplify the general theory with some specific suggested experiments.

Full-duplex DOCSIS/WirelessDOCSIS fiber-radio network employing packaged AFPM-based base-stations

The simultaneous wireline (600 MHz) and wireless (5.5 GHz) transmission of data over cable service interface specification (DOCSIS) signals in a hybrid fiber-radio access network is presented. The DOCSIS signal wireless replica is generated by means of an optical harmonic up-conversion technique based on a dual-drive Mach-Zehnder modulator. The optical signal is fed to a base station (BS) where a packaged asymmetric Fabry-Pe/spl acute/rot modulator/detector acts simultaneously as a photodetector and an optical modulator. At the BS, the DOCSIS signals can be fed either to a wireline or a wireless access network, in a highly flexible approach. Full-duplex operation has been demonstrated for both access types, including indoor wireless transmission.

Full-Duplex DOCSIS/WirelessDOCSIS Fiber–Radio Network Employing Packaged AFPMs as Optical/Electrical Transducers

A hybrid fiber-radio access network architecture for simultaneous wireline and wireless transmissions of data-over-cable service interface specification (DOCSIS) signals is presented. An all-optical harmonic up-conversion technique using a dual-drive Mach-Zehnder modulator provides the downstream optical signal modulated not only at the intermediate frequency in the 600- to 900-MHz band for wireline transmission but also at the up-converted frequency in the 5.45- to 5.75-GHz band for wireless transmission. An InGaAsP/InGaAsP multiple-quantum-well asymmetric Fabry-Perot modulator/detector has been designed, fabricated, and packaged and has been employed in the base station (BS) as an optical/electrical transducer, simultaneously providing the functions of optical intensity modulation and photodetection. At the BS, the DOCSIS signals are recovered at the wireline and wireless frequencies for the respective feeding of a cable access network or a fixed wireless access network in a highly flexible approach. Full-duplex operation has been demonstrated for both access types in an indoor laboratory environment. In a subsequent small-scale field trial, real-life Internet traffic provided by a local community antenna television system operator has been transported over the present hybrid fiber-radio access network architecture, and simultaneous transmission of both DOCSIS and digital television signals has also been performed.

Experimental demonstration of the relative phase operator

We have experimentally demonstrated a realization of the two-mode relative phase operator introduced by Luis and Sanchez-Soto (Luis A and Sanchez-Soto L L 1993 Phys. Rev. A 48 4702). The relative phase distribution function was measured for a weakly excited relative phase eigenstate and weakly excited two-mode coherent states. The experiment is also (using the eigenstates) a demonstration of Heisenberg-limited interferometry.

Comprehensive experimental test of quantum erasure

Abstract:In an interferometer, path information and interference visibility are incompatible quantities. Complete determination of the path will exclude any possibility of interference, rendering zero visibility. However, it is, under certain conditions, possible to trade the path information for improved (conditioned) visibility. This procedure is called quantum erasure. We have performed such experiments with polarization-entangled photon pairs. Using a partial polarizer, we could vary the degree of entanglement between the object and the probe. We could also vary the interferometer splitting ratio and thereby vary the a priori path predictability. This allowed us to test quantum erasure under a number of different experimental conditions. All experiments were in good agreement with theory.

High-Speed 1.56-μm Multiple Quantum Well Asymmetric Fabry-Perot Modulator/Detector (AFPMD) for Radio-Over-Fibre Applications

We have designed and fabricated air-bridged optical modulator/detectors with modulation bandwidth exceeding 15GHz, the highest yet reported for InGaAsP/InGaAsP MQW AFPMDs. Wavelength dependence of the modulated output power at microwave frequencies is reported for the first time and the wide optical bandwidths (> 17nm) so measured suggest that the AFPMD can be used with low-cost uncooled CWDM lasers in radio-over-fibre applications.

Quantum phase resolution and phase distribution

We discuss generation and measurement strategies for maximum-likelihood estimation of small phase shifts. We demonstrate that optimal encoding of small phase shifts and optimal measurements of small phase shifts require different states. We also show that states with identical phase resolution may have rather different Pegg-Barnett phase distributions. Hence, the connection between phase distribution and quantum phase resolution is rather weak.

Experimental Demonstration of Full-Duplex DOCSIS Signal Transmissions over a Wireline/Wireless-Fibre Access Network

A fibre-radio access network architecture for simultaneous wireline and wireless transmissions of data over cable service interface specification (DOCSIS) signals is presented. An all-optical harmonic up-conversion technique using a dual-drive Mach-Zehnder modulator (DD-MZM) is employed at the central station while an InGaAsP/InGaAsP multiple quantum well (MQW) asymmetric Fabry-Perot modulator/detector (AFPM) functions as a single optical/electrical transducer performing optical intensity modulation and photodetection simultaneously at the base station. Full-duplex operation is demonstrated for both access types in an indoor laboratory environment as well as in a small-scale outdoor field trial