James P. Lee

Quantum photonics hybrid integration platform

Fundamental to integrated photonic quantum computing is an on-chip method for routing and modulating quantum light emission. We demonstrate a hybrid integration platform consisting of arbitrarily designed waveguide circuits and single photon sources. InAs quantum dots (QD) embedded in GaAs are bonded to an SiON waveguide chip such that the QD emission is coupled to the waveguide mode. The waveguides are SiON core embedded in a SiO2 cladding. A tuneable Mach Zehnder modulates the emission between two output ports and can act as a path-encoded qubit preparation device. The single photon nature of the emission was verified by an on-chip Hanbury Brown and Twiss measurement.

A semiconductor photon-sorter

Obtaining substantial nonlinear effects at the single-photon level is a considerable challenge that holds great potential for quantum optical measurements and information processing. Of the progress that has been made in recent years one of the most promising methods is to scatter coherent light from quantum emitters, imprinting quantum correlations onto the photons. We report effective interactions between photons, controlled by a single semiconductor quantum dot that is weakly coupled to a monolithic cavity. We show that the nonlinearity of a transition modifies the counting statistics of a Poissonian beam, sorting the photons in number. This is used to create strong correlations between detection events and to create polarization-correlated photons from an uncorrelated stream using a single spin. These results pave the way for semiconductor optical switches operated by single quanta of light.

Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘dropletepitaxy’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at ~1550 nm, with mean FSS 4x smaller than for StranskiKrastanow QDs.

Surface acoustic wave modulation of a coherently driven quantum dot in a pillar microcavity

We report the efficient coherent photon scattering from a semiconductor quantum dot embedded in a pillar microcavity. We show that a surface acoustic wave can periodically modulate the energy levels of the quantum dot but has a negligible effect on the cavity mode. The scattered narrow-band laser is converted into a pulsed single-photon stream, displaying an anti-bunching dip characteristic of single-photon emission. Multiple phonon sidebands are resolved in the emission spectrum, due to the absorption and emission of vibrational quanta in each scattering event.

Ramsey interference in a multilevel quantum system

We report Ramsey interference in the excitonic population of a negatively charged quantum dot measured in resonant fluorescence. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing the laser detuning to reveal two-, three-, and four-level system behavior. We show that under finite field the phase-sensitive control of two optical pulses from a single laser can be used to prepare both population and spin states simultaneously. We also demonstrate the coherent optical manipulation of a trapped spin in a quantum dot in a Faraday geometry magnetic field.

Independent indistinguishable quantum light sources on a reconfigurable photonic integrated circuit

We report a compact, scalable, quantum photonic integrated circuit realised by combining multiple, independent InGaAs/GaAs quantum-light-emitting-diodes (QLEDs) with a silicon oxynitride waveguide circuit. Each waveguide joining the circuit can then be excited by a separate, independently electrically contacted QLED. We show that the emission from neighbouring QLEDs can be independently tuned to degeneracy using the Stark Effect and that the resulting photon streams are indistinguishable. This enables on-chip Hong-Ou-Mandel-type interference, as required for many photonic quantum information processing schemes.

Growth scheme for quantum dots with low fine structure splitting at telecom wavelengths

Quantum dots based on InAs/InP hold the promise to deliver entangled photons with wavelength suitable for the conventional telecom window around 1550 nm [1]. This makes them predestined to be used in future quantum networks applications based on existing fiber optics infrastructure. A prerequisite for the efficient generation of such entangled photons is a small fine structure splitting (FSS) in the quantum dot excitonic eigenstates [2], as well as the ability to integrate the dot into photonic structures to enhance and direct its emission. Using optical spectroscopy, we show that a growth strategy based on droplet epitaxy can simultaneously address both issues.

Cavity-enhanced coherent quantum emitters

Semiconductor quantum dots embedded in micropillar cavities are a promising system for use in quantum photonic technologies. We demonstrate cavity-enhanced coherent scattering, on-chip superresolving phase measurements and photon sorting.

Magneto-Thermo-Triboelectric Generator (MTTG) for thermal energy harvesting

We present a novel thermal energy harvesting system using triboelectric effect. Recently, there has been intensive research efforts on energy harvesting using triboelectric effect, which can produce surprising amount of electric power (when compared to piezoelectric materials) by rubbing or touching (i.e, electric charge by contact and separation) two different materials together. Numerous studies have shown the possibility as an attractive alternative with good transparency, flexibility and low cost abilities for its use in wearable device and smart phone applications markets. However, its application has been limited to only vibration source, which can produce sustained oscillation with maintaining contact and separation states repeatedly for triboelectric effect. Thus, there has been no attempt toward thermal energy source. The proposed approach can convert thermal energy into electricity by pairing triboelectric effect and active ferromagnetic materials The objective of the research is to develop a new manufacturing process of design, fabrication, and testing of a Magneto-Thermo-Triboelectric Generator (MTTG). The results obtained from the approach show that MTTG devices have a feasible power energy conversion capability from thermal energy sources. The tunable design of the device is such that it has efficient thermal capture over a wide range of operation temperature in waste heat.

Integrated photonics with quantum emitters: a new hybrid integration platform(Conference Presentation)

The creation of a quantum photonic integrated circuit, bringing together quantum light sources; detectors; and elements for routing and modulating the photons; is a fundamental step towards a compact and self-contained quantum information processor. Here we report on the realisation of a new hybrid integration platform for InAs Quantum Dot-based quantum light sources and waveguide-based photonic circuits. In this scheme, GaAs devices containing embedded quantum dots are bonded to a silicon oxynitride waveguide circuit such that the quantum dot emission is coupled to the waveguide mode. The output from the waveguide element is coupled into optical fibre (also bonded to the waveguide chip) and the whole assembly is cooled to cryogenic temperatures. Integrated tuneable Mach-Zehnder interferometers permit on-chip photon routing to be achieved and allow the device to act as a path-encoded qubit preparation device. By utilising one such interferometer as a reconfigurable beam splitter, the single photon nature of the emission was confirmed by a Hanbury Brown and Twiss measurement on chip.