arXiv subjects

Peter Lodahl

Publications and source records attributed to Peter Lodahl.

At least 91 records · Page 5Linked to original sources

arXiv2017

Numerical modelling of the coupling efficiency of single quantum emitters in photonic-crystal waveguides

Planar photonic nanostructures have recently attracted a great deal of attention for quantum optics applications. In this article, we carry out full 3D numerical simulations to fully account for all radiation channels and thereby quantify the coupling efficiency of a quantum emitter embedded in a photonic-crystal waveguide. We utilize mixed boundary conditions by combining active Dirichlet boundary conditions for the guided mode and perfectly-matched layers for the radiation modes. In this way, the leakage from the quantum emitter to the surrounding environment can be determined and the spectral and spatial dependence of the coupling to the radiation modes can be quantified. The spatial maps of the coupling efficiency, the $β$-factor, reveal that even for moderately slow light, near-unity $β$ is achievable that is remarkably robust to the position of the emitter in the waveguide. Our results show that photonic-crystal waveguides constitute a suitable platform to achieve deterministic interfacing of a single photon and a single quantum emitter, which has a range of applications for photonic quantum technology.

quant-ph↗

arXiv2017

Physics of quantum light emitters in disordered photonic nanostructures

Nanophotonics focuses on the control of light and the interaction with matter by the aid of intricate nanostructures. Typically, a photonic nanostructure is carefully designed for a specific application and any imperfections may reduce its performance, i.e., a thorough investigation of the role of unavoidable fabrication imperfections is essential for any application. However, another approach to nanophotonic applications exists where fabrication disorder is used to induce functionalities by enhancing light-matter interaction. Disorder leads to multiple scattering of light, which is the realm of statistical optics where light propagation requires a statistical description. We review here the recent progress on disordered photonic nanostructures and the potential implications for quantum photonics devices.

physics.optics↗

arXiv2016

Efficient fiber-coupled single-photon source based on quantum dots in a photonic-crystal waveguide

Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupler is recorded first on an independent device containing an on-chip reflector. Reflection measurements establish that the chip-to-fiber coupling efficiency exceeds 80 %. The detailed characterization of a high-efficiency photonic-crystal waveguide extended with a tapered out-coupling section is then performed. The corresponding overall single-photon source efficiency is 10.9 % $\pm$ 2.3 %, which quantifies the success probability to prepare an exciton in the quantum dot, couple it out as a photon in the waveguide, and subsequently transfer it to the fiber. The applied out-coupling method is robust, stable over time, and broadband over several tens of nanometers, which makes it a highly promising pathway to increase the efficiency and reliability of planar chip-based single-photon sources.

quant-ph↗

arXiv2016

Engineering chiral light--matter interaction in photonic crystal waveguides with slow light

We design photonic crystal waveguides with efficient chiral light--matter interfaces that can be integrated with solid-state quantum emitters. By using glide-plane-symmetric waveguides, we show that chiral light-matter interaction can exist even in the presence of slow light with slow-down factors of up to $100$ and therefore the light--matter interaction exhibits both strong Purcell enhancement and chirality. This allows for near-unity directional $β$-factors for a range of emitter positions and frequencies. Additionally, we design an efficient mode adapter to couple light from a standard nanobeam waveguide to the glide-plane symmetric photonic crystal waveguide. Our work sets the stage for performing future experiments on a solid-state platform.

physics.optics↗

arXiv2016

Chiral Quantum Optics

At the most fundamental level, the interaction between light and matter is manifested by the emission and absorption of single photons by single quantum emitters. Controlling light--matter interaction is the basis for diverse applications ranging from light technology to quantum--information processing. Many of these applications are nowadays based on photonic nanostructures strongly benefitting from their scalability and integrability. The confinement of light in such nanostructures imposes an inherent link between the local polarization and propagation direction of light. This leads to {\em chiral light--matter interaction}, i.e., the emission and absorption of photons depend on the propagation direction and local polarization of light as well as the polarization of the emitter transition. The burgeoning research field of {\em chiral quantum optics} offers fundamentally new functionalities and applications both for single emitters and ensembles thereof. For instance, a chiral light--matter interface enables the realization of integrated non--reciprocal single--photon devices and deterministic spin--photon interfaces. Moreover, engineering directional photonic reservoirs opens new avenues for constructing complex quantum circuits and networks, which may be applied to simulate a new class of quantum many--body systems.

quant-ph↗

arXiv2016

Quantum networks with chiral light--matter interaction in waveguides

We propose a scalable architecture for a quantum network based on a simple on-chip photonic circuit that performs loss-tolerant two-qubit measurements. The circuit consists of two quantum emitters positioned in the arms of an on-chip Mach-Zehnder interferometer composed of waveguides with chiral light--matter interfaces. The efficient chiral light--matter interaction allows the emitters to perform high-fidelity intranode two-qubit parity measurements within a single chip, and to emit photons to generate internode entanglement, without any need for reconfiguration. We show that by connecting multiple circuits of this kind into a quantum network, it is possible to perform universal quantum computation with heralded two-qubit gate fidelities ${\cal F} \sim 0.998$ achievable in state-of-the-art quantum dot systems.

quant-ph↗

arXiv2016

Observation of the exciton Mott transition in the photoluminescence of coupled quantum wells

Indirect excitons in coupled quantum wells have long radiative lifetimes and form a cold quasi-two-dimensional population suitable for studying collective quantum effects. Here we report the observation of the exciton Mott transition from an insulating (excitons) to a conducting (ionized electron-hole pairs) phase, which occurs gradually as a function of carrier density and temperature. The transition is inferred from spectral and time-resolved photoluminescence measurements around a carrier density of $2\times10^{10} \mathrm{cm}^{-2}$ and temperatures of $12-16 \mathrm{K}$. An externally applied electric field is employed to tune the dynamics of the transition via the quantum-confined Stark effect. Our results provide evidence of a gradual nature of the exciton Mott transition.

cond-mat.mes-hall↗

arXiv2016

Role of multi-level states on quantum-dot emission in photonic-crystal cavities

Semiconductor quantum dots embedded in photonic-crystal nanostructures have been the subject of intense study. In this context, quantum dots are often considered to be simple two-level emitters, i.e., the complexities arising from the internal finestructure are neglected. We show that due to the intricate spatial variations of the electric field polarization found in photonic crystal, the two orthogonal finestructure states of quantum dots in general both couple significantly to a cavity mode, implying that the two-level description is not sufficient. As a consequence the emission dynamics and spectra, which are often recorded in experiments, are modified both in the weak- and strong-coupling regimes. The proposed effects are found to be significant for system parameters of current state-of-the-art photonic-crystal cavities.

cond-mat.mes-hall↗

arXiv2016

Deterministic Single-Phonon Source Triggered by a Single Photon

We propose a scheme that enables the deterministic generation of single phonons at GHz frequencies triggered by single photons in the near infrared. This process is mediated by a quantum dot embedded on-chip in an opto-mechanical circuit, which allows for the simultaneous control of the relevant photonic and phononic frequencies. We devise new opto-mechanical circuit elements that constitute the necessary building blocks for the proposed scheme and are readily implementable within the current state-of-the-art of nano-fabrication. This will open new avenues for implementing quantum functionalities based on phonons as an on-chip quantum bus.

quant-ph↗

arXiv2015

Theory and experiments of disorder-induced resonance shifts and mode edge broadening in deliberately disordered photonic crystal waveguides

We study both theoretically and experimentally the effects of introducing deliberate disorder in a slow-light photonic crystal waveguide on the photon density of states. We first introduce a theoretical model that includes both deliberate disorder through statistically moving the hole centres in the photonic crystal lattice and intrinsic disorder caused by manufacturing imperfections. We demonstrate a disorder-induced mean blueshift and an overall broadening of the photonic density of states for various amounts of deliberate disorder. By comparing with measurements from a GaAs photonic crystal waveguide, we find good qualitative agreement between theory and experiment which highlights the importance of carefully including local field effects for modelling high-index contrast perturbations. Our work also demonstrates the importance of using asymmetric dielectric polarizabilities for modelling positive and negative dielectric perturbations when modelling a perturbed dielectric interface in photonic crystal platforms.

physics.optics↗

arXiv2015

Deterministic photon-emitter coupling in chiral photonic circuits

The ability to engineer photon emission and photon scattering is at the heart of modern photonics applications ranging from light harvesting, through novel compact light sources, to quantum-information processing based on single photons. Nanophotonic waveguides are particularly well suited for such applications since they confine photon propagation to a 1D geometry thereby increasing the interaction between light and matter. Adding chiral functionalities to nanophotonic waveguides lead to new opportunities enabling integrated and robust quantum-photonic devices or the observation of novel topological photonic states. In a regular waveguide, a quantum emitter radiates photons in either of two directions, and photon emission and absorption are reverse processes. This symmetry is violated in nanophotonic structures where a non-transversal local electric field implies that both photon emission and scattering may become directional. Here we experimentally demonstrate that the internal state of a quantum emitter determines the chirality of single-photon emission in a specially engineered photonic-crystal waveguide. Single-photon emission into the waveguide with a directionality of more than 90\% is observed under conditions where practically all emitted photons are coupled to the waveguide. Such deterministic and highly directional photon emission enables on-chip optical diodes, circulators operating at the single-photon level, and deterministic quantum gates. Based on our experimental demonstration, we propose an experimentally achievable and fully scalable deterministic photon-photon CNOT gate, which so far has been missing in photonic quantum-information processing where most gates are probabilistic.

quant-ph↗

arXiv2014

Interfacing single photons and single quantum dots with photonic nanostructures

Photonic nanostructures provide means of tailoring the interaction between light and matter and the past decade has witnessed a tremendous experimental and theoretical progress in this subject. In particular, the combination with semiconductor quantum dots has proven successful. This manuscript reviews quantum optics with excitons in single quantum dots embedded in photonic nanostructures. The ability to engineer the light-matter interaction strength in integrated photonic nanostructures enables a range of fundamental quantum-electrodynamics experiments on, e.g., spontaneous-emission control, modified Lamb shifts, and enhanced dipole-dipole interaction. Furthermore, highly efficient single-photon sources and giant photon nonlinearities may be implemented with immediate applications for photonic quantum-information processing. The review summarizes the general theoretical framework of photon emission including the role of dephasing processes, and applies it to photonic nanostructures of current interest, such as photonic-crystal cavities and waveguides, dielectric nanowires, and plasmonic waveguides. The introduced concepts are generally applicable in quantum nanophotonics and apply to a large extent also to other quantum emitters, such as molecules, nitrogen vacancy ceters, or atoms. Finally, the progress and future prospects of applications in quantum-information processing are considered.

quant-ph↗

arXiv2014

Unraveling the mesoscopic character of quantum dots in nanophotonics

We provide a microscopic theory for semiconductor quantum dots that explains the pronounced deviations from the prevalent point-dipole description that were recently observed in spectroscopic experiments on quantum dots in photonic nanostructures. At the microscopic level the deviations originate from structural inhomogeneities generating a large circular quantum current density that flows inside the quantum dot over mesoscopic length scales. The model is supported by the experimental data, where a strong variation of the multipolar moments across the emission spectrum of quantum dots is observed. Our work enriches the physical understanding of quantum dots and is of significance for the fields of nanophotonics, quantum photonics, and quantum-information science, where quantum dots are actively employed.

cond-mat.mes-hall↗

arXiv2014

Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

cond-mat.mes-hall↗

arXiv2014

Probing electric and magnetic vacuum fluctuations with quantum dots

The electromagnetic-vacuum-field fluctuations are intimately linked to the process of spontaneous emission of light. Atomic emitters cannot probe electric- and magnetic-field fluctuations simultaneously because electric and magnetic transitions correspond to different selection rules. In this paper we show that semiconductor quantum dots are fundamentally different and are capable of mediating electric-dipole, magnetic-dipole, and electric-quadrupole transitions on a single electronic resonance. As a consequence, quantum dots can probe electric and magnetic fields simultaneously and can thus be applied for sensing the electromagnetic environment of complex photonic nanostructures. Our study opens the prospect of interfacing quantum dots with optical metamaterials for tailoring the electric and magnetic light-matter interaction at the single-emitter level.

quant-ph↗

arXiv2013

Statistical measurements of quantum emitters coupled to Anderson-localized modes in disordered photonic-crystal waveguides

Optical nanostructures have proven to be meritorious for tailoring the emission properties of quantum emitters. However, unavoidable fabrication imperfections may represent a nuisance. Quite remarkably, disorder offers new opportunities since light can be efficiently confined by random multiple scattering leading to Anderson localization. Here we investigate the effect of such disorder-induced cavities on the emission dynamics of single quantum dots embedded in disordered photonic-crystal waveguides. We present time-resolved measurements of both the total emission from Anderson-localized cavities and from single emitters that are coupled to the cavities. We observe both strongly inhibited and enhanced decay rates relative to the rate of spontaneous emission in a homogeneous medium. From a statistical analysis, we report an average Purcell factor of 2 in without any control on the quantum dot - cavity detuning. By spectrally tuning individual quantum dots into resonance with Anderson-localized modes, a maximum Purcell factor of 23.8 is recorded, which lies at the onset of the strong coupling regime. The presented data quantify the potential of naturally occurring Anderson-localized cavities for controlling and enhancing the light-matter interaction strength, which is of relevance not only for cavity quantum-electrodynamics experiments but potentially also for efficient energy harvesting and controllable random lasing.

physics.optics↗

arXiv2012

Solid-state quantum optics with quantum dots in photonic nanostructures

Quantum nanophotonics has become a new research frontier where quantum optics is combined with nanophotonics in order to enhance and control the interaction between strongly confined light and quantum emitters. Such progress provides a promising pathway towards quantum-information processing on an all-solid-state platform. Here we review recent progress on experiments with single quantum dots in nanophotonic structures. Embedding the quantum dots in photonic band-gap structures offers a way of controlling spontaneous emission of single photons to a degree that is determined by the local light-matter coupling strength. Introducing defects in photonic crystals implies new functionalities. For instance, efficient and strongly confined cavities can be constructed enabling cavity-quantum-electrodynamics experiments. Furthermore, the speed of light can be tailored in a photonic-crystal waveguide forming the basis for highly efficient single-photon sources where the photons are channeled into the slowly propagating mode of the waveguide. Finally, we will discuss some of the surprises that arise in solid-state implementations of quantum-optics experiments in comparison to their atomic counterparts. In particular, it will be shown that the celebrated point-dipole description of light-matter interaction can break down when quantum dots are coupled to plasmon nanostructures.

cond-mat.mes-hall↗

arXiv2012

An Anderson-localized random nanolaser

Precision is a virtue throughout science in general and in optics in particular where carefully fabricated nanometer-scale devices hold great promise for both classical and quantum photonics [1-6]. In such nanostructures, unavoidable imperfections often impose severe performance limits but, in certain cases, disorder may enable new functionalities [7]. Here we demonstrate on-chip random nanolasers where the cavity feedback is provided by the intrinsic disorder in a semiconductor photonic-crystal waveguide, leading to Anderson localization of light [8]. This enables highly efficient and broadband tunable lasers with very small mode volumes. We observe an intriguing interplay between gain, dispersion-controlled slow light, and disorder, which determines the cross-over from ballistic transport to Anderson localization. Such a behavior is a unique feature of non-conservative random media that enables the demonstration of all-optical control of random lasing. Our statistical analysis shows a way towards ultimate thresholdless random nanolasers.

physics.optics↗