We demonstrate both additive and subtractive manufacture of integrated spiral phase plases (SPPs) on top of photonic crystal surface emitting lasers (PCSELs) for the generation of orbital angular momentum(OAM) for quantum communication using qudits. We find that both additive and subtractive techniques allow us to create SPPs with higher resolution than existing lithography based techniques, paving the way for integrated sources of OAM in higher dimensional Hilbert spaces for satellite based communications.

The interaction of quantum objects is of fundamental nature in the exploration of the quantum properties of the world and for a wide range of quantum technologies. Specifically, the interactions between a photon or a coherent state and a quantum emitter are among the most well-studied areas. The interactions between two qubits are generally simplified into two quantum objects in static well defined states. In this work we explore a fundamentally new dynamic type of spin-photon interaction. We show the imprinting of the dynamics from a single electron spin in the ground state of a quantum dot. A quantum modulation of the output phase (either 0 or pi but no values in between) is observed, forming a novel quantum state of light that cannot be described classically. This state of light can directly witness the underlying dynamics of a spin in a quantum emitter.

Quantum dots in micropillars are one of the most promising options for a bright, deterministic single photon source. While highly efficient devices (>95%) have been designed, there remains a significant bottleneck that impacts the overall system efficiency: the large numerical aperture of the output mode. This leads to inefficient coupling of emitted photons into single-mode fiber (SMF), thus limiting practical integration into quantum computing and communication architectures. We show that with the addition of a well designed aspheric SiO2 microlens we can decrease the mode-matching losses to a SMF from 83.1% to <0.1(0.1)%. This can result in a single photon source design with 96.4(0.1)% end-to-end efficiency, paving the way for scalable photonic quantum technologies.

Spin lasers leverage electron spin-polarisation to control photon polarisation, offering the potential for lower thresholds, rapid modulation, and all-optical data processing. We report successful spin injection into a commercial vertical cavity surface emitting laser (VCSEL) using optical pumping at wavelengths of 794 nm and 810 nm. Maximum circular polarisation achieved was about 20% at 794 nm, whereas this falls to 5% with exciting at 810 nm. We attribute this to reduced spin injection due to the longer excitation wavelength in line with previous measurements on quantum wells under optical orientation. We extend the spin-flip model to account for realistic excitation conditions and find our theoretical analysis accurately reproduces experimental trends, surveying spin injection in commercial VCSELs.

Quantum dots have the potential to be the brightest deterministic single photon source with plausible high end applications in quantum computing and cluster state generation. In this work, we re-examine the design of simple micropillars by meticulously examining the structural effects of the decay into leaky channels beyond the atom-like cavity estimation. We show that precise control of the side losses with the diameter and avoidance of propagating Bloch modes in the DBR structure can result in easy to manufacture broadband (Q≈750-2500) micropillars and demonstrate extremely high internal efficiency (90.5%-96.4%). We also demonstrate that such cavities naturally decouple from the phonon sideband, with the phonon sideband reducing by a factor of 5-33 allowing us to predict that the photons should show 99.2%-99.8% indistinguishability.

We study the resonance fluorescence properties of an optically active spin 1/2 system, elucidating the effects of a magnetic field on the coherence of the scattered light. We derive a master equation model for this system that reproduces the results of a two level system (TLS) while also being applicable to a spin system with ground state coupling. This model is then solved analytically in the weak excitation regime. The inclusion of spin dynamics in our model alters the properties of the coherently scattered light at a fundamental level. For a TLS the coherence properties are known to be determined by the input laser. We show that spin scattered light inherits the coherence properties of the spin. This mapping allows us to measure spin dynamics and coherence time through direct measurement of the scattered fields. Furthermore, we show the ability to resolve sub-natural linewidth zeeman splittings. Along with representing an invaluable tool for spin spectroscopy understanding the coherence properties of the spin-scattered field will be vital for spin-photon based quantum technologies.

Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarization (C points), with elliptical polarizations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles, and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor ∼30), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.

We present a novel device capable of enhanced and broadband collection of pairs of photons emitted by a single semiconductor quantum dot.

We present a novel device capable of enhanced and broadband collection of pairs of photons emitted by a single semiconductor quantum dot.

The generation of photon pairs in quantum dots is in its nature deterministic. However, efficient extraction of photon pairs from the high index semiconductor material requires engineering of the photonic environment. We report on a micropillar device with 69.4(10)% efficiency that features broadband operation suitable for extraction of photon pairs. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85%.

We determine the propagation loss of GaAs photonic crystal waveguides by spectral imaging of the spontaneous emission from the embedded InAs/GaAs quantum dots. The results are compared with the loss obtained by imaging the near field of the out-of-plane radiation of the waveguide mode propagating within the light cone. From the corresponding far field, we furthermore measure the mode wavevector, from which we determine the waveguide dispersion. Additionally, we show that spectral imaging allows to determine the relative efficiencies of the couplers. Using the same experiment, and detailed photonic simulations, we have determined the beta factor and the directionality of the emission of the QDs, finding beta factors up to 99% and high directionalities.

We investigate the effect of nuclear spins on the phase shift and polarisation rotation of photons scattered off a quantum dot-cavity system. We show that as the phase shift depends strongly on the resonance energy of an electronic transition in the quantum dot, it can provide a sensitive probe of the quantum state of nuclear spins that broaden this transition energy. By including the electron-nuclear spin coupling at a Hamiltonian level within an extended input-output formalism, we show how a photon scattering event acts as a nuclear spin measurement, which when rapidly applied leads to an inhibition of the nuclear spin dynamics via the quantum Zeno effect, and a corresponding stabilisation of the optical resonance. We show how such an effect manifests in the intensity autocorrelation $g^{(2)}(\tau)$ of scattered photons, whose long-time bunching behaviour changes from quadratic decay for low photon scattering rates (weak laser intensities), to ever slower exponential decay for increasing laser intensities as optical measurements impede the nuclear spin evolution.

The authors present here designs for tuneable confined Tamm plasmons (CTPs) resonant at 1.3m, consisting of an AlAs/GaAs distributed Bragg reflector and gold disc. Using numerical methods they explored the effect of disc diameter on the CTP resonance and position of a dipole source (modelling a quantum dot) on emission through the disc. They found decreasing disc diameter resulted in a blue-shifted fundamental mode and that a dipole positioned at the centre of the disc emitted with an angular distribution that collected 90% of the transmitted power within a numerical aperture of 0.7. They also explore the Purcell enhancement under the CTP as a function of dipole position.

Summary form only given. There has recently been great interest in chiral waveguides, where the polarisation of the forwards and backwards propagating light fields are oppositely circular at certain positions in space (C-points) [1]. This results in new phenomena, such as the one way emission of light from spin-transitions in quantum dots (QDs) or atoms (simulation, fig. b). The emitted photon's direction is determined by the spin state of the emitter.To date calculations on this topic have either been restricted to certain special cases, or have relied on simplifying “rate forwards/backwards” models [2]. Using a Green's function method we have generalised to the case of any polarisable dipole at a point supporting any polarisation. Several interesting physical effects arise here that could not have been predicted using the previous models. For example in the figure we show how perfect chiral behaviour can be recovered at points of elliptical polarisation by using equally elliptical dipoles stretched along the orthogonal axis. This may be realised in QDs with a vector magnetic field.We further generalise our approach to include the population state of a multi-level system. This allows us to calculate the dynamics of complicated multi-transition “quantum dipoles”, such as a Λ-system, recovering results consistent with experiment [3].

By performing a full analysis of the projected local density of states (LDOS) in a photonic crystal waveguide, we show that phase plays a crucial role in the symmetry of the light-matter interaction. By considering a quantum dot (QD) spin coupled to a photonic crystal waveguide (PCW) mode, we demonstrate that the light-matter interaction can be asymmetric, leading to unidirectional emission and a deterministic entangled photon source. Further we show that understanding the phase associated with both the LDOS and the QD spin is essential for a range of devices that can be realized with a QD in a PCW. We also show how suppression of quantum interference prevents dipole induced reflection in the waveguide, and highlight a fundamental breakdown of the semiclassical dipole approximation for describing light-matter interactions in these spin dependent systems.

Self-assembled quantum dots (QDs), nanosized semiconductors, are often known as artificial atoms due to their atomic-like spectra. For this reason they have long been proposed as a means to mediate interactions between single photons, a useful capability for photonic quantum information technology. I will describe the role of QDs in the latest developments in photonic quantum information technology (QIT), and highlight some of our progress in combining the atomic-like properties of QDs with photonic structures to perform a variety of functionalities.

We introduce the concept of entanglement enhanced interferometry from the viewpoint of the detected photons. The standard quantum limit is achieved when sequentially detected photons are assumed to be in an uncorrelated product state. However when we access the correlations between the detected photons that existed before the interferometer it becomes clear that entanglement enhanced measurement beyond the quantum limit could be achieved independent of loss. We describe possible realisations of this post-measurement entanglement detection using a small array of spin photon entangling gates. We then describe a proof of principle experiment using only linear optics resources.

We present a scheme to demonstrate loophole-free Bell inequality violation where the entanglement between photon pairs is transferred to solid state (spin) qubits mediated by cavity QED interactions. As this transfer can be achieved in a heralded way, our scheme is basically insensitive to losses on the channel, and works also in the weak coupling regime. We consider potential experimental realizations using single atom, colour centre and quantum dot cavity systems. Finally our scheme appears to be promising for implementing quantum information protocols based on non-locality. Here we discuss a possible implementation of device-independent quantum key distribution.