Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

The first quantum technology that harnesses quantum mechanical effects for its core operation has arrived in the form of commercially available quantum key distribution systems. This technology achieves enhanced security by encoding information in photons such that an eavesdropper in the system can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, which promise exponentially greater computational power for particular tasks. Photonics is destined to have a central role in such technologies owing to the high-speed transmission and outstanding low-noise properties of photons. These technologies may use single photons, quantum states of bright laser beams or both, and will undoubtedly apply and drive state-of-the-art developments in photonics.

Photonic quantum technologies

(b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>

The Quantum Theory of Light

Coherent photonic coupling of semiconductor quantum dots: erratum

Bose-Einstein condensates exhibit fascinating behavior such as superfluidity and vortex formation, effects owing their presence to self-interactions between the underlying bosons [1]. In exciton-polariton condensates another peculiar effect has been theoretically predicted, where the negative energy Bogoliubov dispersion becomes observable in the photoluminescence (PL) spectrum [2–5]. The effect originates from the anomalous mixing of creation and destruction operators of polaritons, which results in the creation of a Bogoliubov particle when the PL is observed. Here we directly observe the PL of the negative Bogoliubov dispersion branch for the first time in a high density regime far above the exciton-polariton condensation threshold. The observation is made possible by strong coupling being maintained at densities corresponding to ∼ 100 times the threshold density for condensation. The presence of strong coupling at such densities is evidenced by second-order coherence and PL measurements showing clear differences to photon lasing, a weak coupling phenomenon. Comparison of the negative dispersion PL spectrum shows good agreement to theoretical predictions.

Direct photoluminescence observation of the negative Bogoliubov branch in an exciton-polariton condensate

A high-Q vertical emitting AlAs/GaAs Fabry-Perot micropillar cavities are fabricated, with a new type of GaInAs quantum dots within a one lambda-cavity using high reflectivity distributed Bragg reflectors. In this paper, micropillars with a maximum quality factor of up to 34500 for a 4 mum wide microcavity are presented

Semiconductor quantum dot micropillar cavities for quantum electrodynamic experiments

Coherence properties of high-beta micropillar lasers have been investigated by microphotoluminescence, first- and second-order correlation measurements. (In,Ga)As/GaAs quantum dots (QDs) were used as a gain materials. A smooth transition from spontaneous into stimulated emission is traced from mu-PL measurements, accompanied by a strong increase of the coherence time of the lasing mode.

Coherence Properties of High-Beta Semiconductor Micropillar Lasers

Detailed experimental and theoretical investigations on coherence properties of high-β (In,Ga)As/GaAs quantum dot-based microcavity lasers are presented. Power dependent micro-photoluminescence measurements on the fundamental mode of the studied micropillar cavities exhibited a smooth transition from spontaneous into stimulated emission with a strong linewidth narrowing around the onset of lasing. This is accompanied by a steep increase in the coherence time of the fundamental mode emission, as derived from first-order field correlation measurements which revealed a gradual change in the g(1)(τ) lineshapes from Gaussian-like to more exponential. In addition, systematic coherence time measurements on various micropillars demonstrated that devices with smaller spontaneous emission coupling factor β reveal longer coherence time of the lasing emission. All experimental results are fully verified by an enhanced microscopic theory which describes the lasing properties of quantum dot (QD) micropillars. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Andreas Löffler

Papers

Coherent photonic coupling of semiconductor quantum dots: erratum

Direct photoluminescence observation of the negative Bogoliubov branch in an exciton-polariton condensate

Semiconductor quantum dot micropillar cavities for quantum electrodynamic experiments

Coherence Properties of High-Beta Semiconductor Micropillar Lasers

Coherence length of high‐β semiconductor microcavity lasers