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M. B. Ward

Publications and source records attributed to M. B. Ward.

11 recordsLinked to original sources
arXiv2019

A tuneable telecom-wavelength entangled light emitting diode

Entangled light emitting diodes based on semiconductor quantum dots are promising devices for security sensitive quantum network applications, thanks to their natural lack of multi photon-pair generation. Apart from telecom wavelength emission, network integrability of these sources ideally requires electrical operation for deployment in compact systems in the field. For multiplexing of entangled photons with classical data traffic, emission in the telecom O-band and tuneability to the nearest wavelength channel in compliance with coarse wavelength division multiplexing standards (20 nm channel spacing) is highly desirable. Here we show the first fully electrically operated telecom entangled light emitting diode with wavelength tuneability of more than 25nm, deployed in an installed fiber network. With the source tuned to 1310.00 nm, we demonstrate multiplexing of true single entangled photons with classical data traffic and achieve entanglement fidelities above 95% on an installed fiber in a city.

quant-ph↗
arXiv2017

Quantum-dot based telecom-wavelength quantum relay

The development of quantum relays for long haul and attack-proof quantum communication networks operating with weak coherent laser pulses requires entangled photon sources at telecommunication wavelengths with intrinsic single-photon emission for most practical implementations. Using a semiconductor quantum dot emitting entangled photon pairs in the telecom O-band, we demonstrate for the first time a quantum relay fulfilling both of these conditions. The system achieves a maximum fidelity of 94.5 % for implementation of a standard 4-state protocol with input states generated by a laser. We further investigate robustness against frequency detuning of the narrow-band input and perform process tomography of the teleporter, revealing operation for arbitrary pure input states, with an average gate fidelity of 83.6 %. The results highlight the potential of semiconductor light sources for compact and robust quantum relay technology, compatible with existing communication infrastructures.

quant-ph↗
arXiv2016

Resonance fluorescence from a telecom-wavelength quantum dot

We report on resonance fluorescence from a single quantum dot emitting at telecom wavelengths. We perform high-resolution spectroscopy and observe the Mollow triplet in the Rabi regime--a hallmark of resonance fluorescence. The measured resonance-fluorescence spectra allow us to rule out pure dephasing as a significant decoherence mechanism in these quantum dots. Combined with numerical simulations, the experimental results provide robust characterisation of charge noise in the environment of the quantum dot. Resonant control of the quantum dot opens up new possibilities for on-demand generation of indistinguishable single photons at telecom wavelengths as well as quantum optics experiments and direct manipulation of solid-state qubits in telecom-wavelength quantum dots.

cond-mat.mes-hall↗
arXiv2015

Interference with a Quantum Dot Single-Photon Source and a Laser at Telecom Wavelength

The interference of photons emitted by dissimilar sources is an essential requirement for a wide range of photonic quantum information applications. Many of these applications are in quantum communications and need to operate at standard telecommunication wavelengths to minimize the impact of photon losses and be compatible with existing infrastructure. Here we demonstrate for the first time the quantum interference of telecom-wavelength photons from an InAs/GaAs quantum dot single-photon source and a laser; an important step towards such applications. The results are in good agreement with a theoretical model, indicating a high degree of indistinguishability for the interfering photons.

quant-ph↗
arXiv2015

Interference of short optical pulses from independent gain-switched laser diodes for quantum secure communications

Since the introduction of the decoy-state technique, phase-randomised weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultra-fast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose of whether the same laser could be employed to achieve high-speed measurement-device-independent-QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging highvisibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.

quant-ph↗
arXiv2012

Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron-hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent. Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them. Near the minimum splitting the eigenstates rotate in the plane of the sample, being orientated at 45° when the splitting is smallest. Using this system we show direct control over the exciton states in one quantum dot, leading to the generation of entangled photon pairs.

cond-mat.mes-hall↗
arXiv2007

Quantum key distribution using a triggered quantum dot source emitting near 1.3 microns

We report the distribution of a cryptographic key, secure from photon number splitting attacks, over 35 km of optical fiber using single photons from an InAs quantum dot emitting ~1.3 microns in a pillar microcavity. Using below GaAs-bandgap optical excitation, we demonstrate suppression of multiphoton emission to 10% of the Poissonian level without detector dark count subtraction. The source is incorporated into a phase encoded interferometric scheme implementing the BB84 protocol for key distribution over standard telecommunication optical fiber. We show a transmission distance advantage over that possible with (length-optimized) uniform intensity weak coherent pulses at 1310 nm in the same system.

quant-ph↗
arXiv2007

Surface-acoustic-wave-driven luminescence from a lateral p-n junction

The authors report surface-acoustic-wave-driven luminescence from a lateral p-n junction formed by molecular beam epitaxy regrowth of a modulation doped GaAs/AlGaAs quantum well on a patterned GaAs substrate. Surface-acoustic-wave-driven transport is demonstrated by peaks in the electrical current and light emission from the GaAs quantum well at the resonant frequency of the transducer. This type of junction offers high carrier mobility and scalability. The demonstration of surface-acoustic-wave luminescence is a significant step towards single-photon applications in quantum computation and quantum cryptography.

cond-mat.other↗
arXiv2006

Single-photon-emitting diodes: a review

Compact and reliable sources of non-classical light could find many applications in emerging technologies such as quantum cryptography, quantum imaging and also in fundamental tests of quantum physics. Single self-assembled quantum dots have been widely studied for this reason, but the vast majority of reported work has been limited to optically excited sources. Here we discuss the progress made so far, and prospects for, electrically driven single-photon-emitting diodes (SPEDs).

quant-ph↗
arXiv2002

High-performance planar light-emitting diode

Planar light-emitting diodes (LEDs) fabricated within a single high-mobility quantum well are demonstrated. Our approach leads to a dramatic reduction of radiative lifetime and junction area with respect to conventional vertical LEDs, promising very high-frequency device operation. Devices were fabricated by UV lithography and wet chemical etching starting from p-type modulation-doped AlGaAs/GaAs heterostructures grown by molecular beam epitaxy. Electrical and optical measurements from room temperature down to 1.8 K show high spectral purity and high external efficiency. Time-resolved measurements yielded extremely short recombination times of the order of 50 ps, demonstrating the relevance of the present scheme for high-frequency device applications in the GHz range.

cond-mat.mtrl-sci↗