Showing papers by "Sven Höfling published in 2012"
TL;DR: The present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres.
Abstract: Future quantum networks will combine ideally stationary quantum bits (qubits), such as single electron spins, with 'flying' qubits, which are photons that transfer quantum states between distant qubits. It has therefore been a long-standing challenge in the field of quantum computation and communication to couple a single electron spin to a single photon in a solid-state platform. Two groups working independently have now achieved that goal, by demonstrating entanglement between a photon and a single electron spin trapped in a semiconductor 'quantum dot' structure. The quantum dot acts as the stationary node. This achievement is a small step towards eventual implementation of quantum networks that can support long-distance quantum communication.
503 citations
TL;DR: The results indicate that Berezinskii–Kosterlitz–Thouless-like phase order survives in open-dissipative systems and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions.
Abstract: We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short- and long-distance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the two-dimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a power-law decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9–1.2, larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii–Kosterlitz–Thouless-like phase order survives in open-dissipative systems.
114 citations
TL;DR: In this article, the photoluminescence properties of 2D kagome lattice potentials have been investigated and a single-particle band structure calculation was performed to compare measured band structures.
Abstract: Microcavity exciton–polariton condensates, as coherent matter waves, have provided a great opportunity to investigate hydrodynamic vortex properties, superfluidity and low-energy quantum state dynamics. Recently, exciton condensates were trapped in various artificial periodic potential geometries: one-dimensional (1D), 2D square, triangular and hexagonal lattices. The 2D kagome lattice, which has been of interest for many decades, exhibits spin frustration, giving rise to magnetic phase order in real materials. In particular, flat bands in the 2D kagome lattice are physically interesting in that localized states in the real space are formed. Here, we realize exciton–polariton condensates in a 2D kagome lattice potential and examine their photoluminescence properties. Above quantum degeneracy threshold values, we observe meta-stable condensation in high-energy bands; the third band exhibits a signature of weaker dispersive band structures, a flat band. We perform a single-particle band structure calculation to compare measured band structures.
102 citations
01 Nov 2012
TL;DR: In this article, the Hong-Ou-Mandel interference visibility of 0.970 was achieved for pulsed resonance fluorescence single photons on demand from a single, microcavity-embedded quantum dot with less than 0.3% background contributions.
Abstract: We generate pulsed resonance fluorescence single photons on demand from a single, microcavity-embedded quantum dot with less than 0.3% background contributions and a Hong-Ou-Mandel interference visibility of 0.970(19). Two single photons are further used to implement a high-fidelity quantum controlled-NOT gate.
96 citations
TL;DR: In this paper, in-lab free space quantum key distribution (QKD) experiments over 40cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range.
Abstract: We report on in-lab free space quantum key distribution (QKD) experiments over 40cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2kbits 1 (35.4kbits 1 ) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g (2) (0) value of 0.35 (0.49) at moderate (high) excitation. The
95 citations
TL;DR: In this paper, the authors used femtosecond laser pulses to create acoustic pulses that strain quantum dots and modulate their transition energies, and they found that tuning the quantum dots to the optical resonance of the cavity causes the emission output to be enhanced by more than two orders of magnitude.
Abstract: Researchers use femtosecond laser pulses to create acoustic pulses that strain quantum dots and modulate their transition energies. When the quantum dots are housed in a microcavity, tuning the quantum dots to the optical resonance of the cavity causes the emission output to be enhanced by more than two orders of magnitude.
84 citations
TL;DR: Bloch-wave engineering is employed to realize submicron diameter high quality factor GaAs/AlAs micropillars (MPs) that lead to record-high vacuum Rabi splitting of the strong coupling in MPs with modest oscillator strength quantum dots.
Abstract: We have employed Bloch-wave engineering to realize submicron diameter high quality factor GaAs/AlAs micropillars (MPs). The design features a tapered cavity in which the fundamental Bloch mode is subject to an adiabatic transition to match the Bragg mirror Bloch mode. The resulting reduced scattering loss leads to record-high vacuum Rabi splitting of the strong coupling in MPs with modest oscillator strength quantum dots. A quality factor of 13, 600 and a splitting of 85 μeV with an estimated visibility v of 0.41 are observed for a small mode volume MP with a diameter d{c} of 850 nm.
68 citations
TL;DR: Sensing of formaldehyde (H(2)CO) using a room-temperature distributed feedback interband cascade laser (ICL) emitting around 3493 nm is demonstrated using a characterized and proved to be very suitable for tunable laser spectroscopy (TLS).
Abstract: We have demonstrated sensing of formaldehyde (H2CO) using a room-temperature distributed feedback interband cascade laser (ICL) emitting around 3493 nm. The ICL has been characterized and proved to be very suitable for tunable laser spectroscopy (TLS). The H2CO TLS spectra were recorded in direct absorption mode and showed excellent agreement with the Pacific Northwest National Laboratory database. The measurements reported here were taken from a series of measurements of a mixture of H2CO in air obtained by vaporizing a solution also containing methanol and formic acid. We obtained a resolution limit better than 1 ppm×m assuming a relative absorption of 10−3.
63 citations
TL;DR: In this paper, a downconversion quantum interface between a semiconductor quantum dot at 910 nm and a 2.2-μm pulsed pump laser was proposed to achieve coherent optical control of the quantum dot electron spin.
Abstract: Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. We implement a downconversion quantum interface, where the single photons emitted from a semiconductor quantum dot at 910 nm are downconverted to 1560 nm using a fiber-coupled periodically poled lithium niobate waveguide and a 2.2-μm pulsed pump laser. The single-photon character of the quantum dot emission is preserved during the downconversion process: we measure a cross-correlation g(2)(τ = 0) = 0.17 using resonant excitation of the quantum dot. We show that the downconversion interface is fully compatible with coherent optical control of the quantum dot electron spin through the observation of Rabi oscillations in the downconverted photon counts. These results represent a critical step towards a long-distance hybrid quantum network in which subsystems operating at different wavelengths are connected through quantum frequency conversion devices and 1.5-μm quantum channels.
63 citations
TL;DR: In this paper, a study of the quantum dot (QD) emission in short photonic crystal waveguides is presented, where the authors show that the combination of slow group velocity and Fabry-Perot cavity resonance provides an avenue to efficiently channel photons from quantum dots into waveguide for integrated quantum photonic applications.
Abstract: We report a study of the quantum dot (QD) emission in short photonic crystal waveguides. We observe that the quantum dot photoluminescence intensity and decay rate are strongly enhanced when the emission energy is in resonance with Fabry-Perot (FP) cavity modes in the slow-light regime of the dispersion curve. The experimental results are in agreement with previous theoretical predictions and are further supported by three-dimensional finite element simulations. Our results show that the combination of slow group velocity and Fabry-Perot cavity resonance provide an avenue to efficiently channel photons from quantum dots into waveguides for integrated quantum photonic applications.
63 citations
TL;DR: In this article, the authors compare two regimes indicative of polariton lasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton.
Abstract: We compare two regimes indicative of polariton lasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton. In particular, we investigate the cavity emission subsequent to nonresonant pulsed excitation. For the ground state emission from the lower energy-momentum dispersion branch we find a two-threshold behavior in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. We demonstrate that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration. Our findings show that a comprehensive study of spectral and temporal characteristics of the emission from a semiconductor microcavity can be used to characterize the different emission regimes.
TL;DR: In this paper, the integration of single site-controlled quantum dots (SCQDs) into electrically driven micropillar cavities was reported, and the enhancement of electroluminescence by quantum dot-cavity coupling was demonstrated by temperature dependent investigations.
Abstract: In this work we report on the integration of single site-controlled quantum dots (SCQDs) into electrically driven micropillar cavities. The electroluminescence of these devices features emission of single SCQDs with inhomogeneous broadenings down to 170 µeV. The enhancement of electroluminescence by quantum dot-cavity coupling is demonstrated by temperature dependent investigations. Single photon emission from a spatially and spectrally coupled SCQD-resonator system is confirmed by photon autocorrelation measurements under electrical excitation yielding a g(2)(0) value of 0.42.
TL;DR: In this article, the authors demonstrate room temperature, continuous wave lasing of laser diodes based on AlGaAs whispering gallery mode (WGM) resonators embedding a quantum dot (QD) active layer.
Abstract: We demonstrate room temperature, continuous wave lasing of laser diodes based on AlGaAs whispering gallery mode (WGM) resonators (microcylinder and microring) embedding a quantum dot (QD) active layer. Using InGaAlAs QDs, high-Q (>60 000) lasing modes are observed around 910 nm, up to 50 °C. Lasing with similar performance is obtained around 1230 nm, using InAs QDs. Furthermore, we show that the current injection in the active part of the device is improved in ring resonators, leading to threshold currents of approximately 4 mA for a device with 80 μm diameter. This geometry also suppresses WGMs with a high radial order, thus simplifying the lasing spectra. In these conditions, stable single-mode and two-color lasing can be obtained.
TL;DR: T tunable on-chip single photon sources are demonstrated using the Stark tuning of single quantum dot (QD) excitonic transitions in short photonic crystal waveguides (PhC WGs) and a strong enhancement of spontaneous emission is observed.
Abstract: We demonstrate tunable on-chip single photon sources using the Stark tuning of single quantum dot (QD) excitonic transitions in short photonic crystal waveguides (PhC WGs). The emission of single QDs can be tuned in real-time by 9 nm with an applied bias voltage less than 2V. Due to a reshaped density of optical modes in the PhC WG, a large coupling efficiency β ≥ 65%to the waveguide mode is maintained across a wavelength range of 5 nm. When the QD is resonant with the Fabry-Perot mode of the PhC WG, a strong enhancement of spontaneous emission is observed leading to a maximum coupling efficiency β = 88%. These results represent an important step towards the scalable integration of single photon sources in quantum photonic integrated circuits.
TL;DR: In this paper, the authors demonstrate directional far field emission from whispering gallery modes (WGMs) in electrically driven quantum dot micropillar lasers with an upper air-bridge contact for current injection.
Abstract: We experimentally demonstrate directional far field emission from whispering gallery modes (WGMs) in electrically driven quantum dot micropillar lasers. In-plane directionality of whispering gallery mode emission is obtained by patterning micropillars with Limacon-shaped cross-section and an upper air-bridge contact for current injection. The micropillar lasers with radii R0 down to 4.5 μm show Q-factors of 40 000 and threshold currents of 40 μA at low temperature. We achieved a far field divergence of about 30° and a directionality of 1.67 ± 0.15 for an optimal Limacon deformation factor ɛ ≈ 0.5. Parameter dependent studies of the directional emission as a function of ɛ reveal good qualitative agreement with theoretical predictions.
TL;DR: In this paper, the optical properties of single InGaN quantum dots (QDs) were investigated by means of microphotoluminescence (μPL) spectroscopy. The QDs were grown on sapphire substrate using metal organic vapor phase epitaxy.
Abstract: We have investigated the optical properties of single InGaN quantum dots (QDs) by means of microphotoluminescence (μPL) spectroscopy. The QDs were grown on sapphire substrate using metal organic vapor phase epitaxy. Sharp and isolated single exciton emission lines in the blue spectral range were observed. The QD luminescence shows a strong degree of linear polarization up to 96% perpendicular to the growth axis (c-axis) with no preferential alignment in the xy plane. Second order autocorrelation measurements were performed under pulsed excitation and single photon emission up to 50 K is demonstrated.
TL;DR: In this paper, the authors show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a nontrivial way, depending on excitation conditions.
Abstract: We study experimentally and theoretically polarization-dependent luminescence from an ensemble of quantum-dot-like nanostructures with a very large in-plane shape anisotropy (quantum dashes). We show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a nontrivial way, depending on the excitation conditions. Using an approximate model based on the $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory, we are able to relate this degree of polarization to the amount of light-hole admixture in the exciton states which, in turn, depends on the symmetry of the envelope wave function. Agreement between the measured properties and theory is reached under assumption that the ground exciton state in a quantum dash is trapped in a confinement fluctuation within the structure and thus localized in a much smaller volume of lower asymmetry than the entire nanostructure.
TL;DR: A large reversible and controllable electromechanical wavelength tuning (> 15 nm) of the cavity modes is demonstrated and a design to increase the flexibility of the structure is discussed.
Abstract: We present the design, the fabrication and the characterization of a tunable one-dimensional (1D) photonic crystal cavity (PCC) etched on two vertically-coupled GaAs nanobeams. A novel fabrication method which prevents their adhesion under capillary forces is introduced. We discuss a design to increase the flexibility of the structure and we demonstrate a large reversible and controllable electromechanical wavelength tuning (> 15 nm) of the cavity modes.
TL;DR: In this article, the spontaneous emission rate of single InAs quantum dots embedded in a double-membrane photonic crystal cavity was demonstrated by the electromechanical tuning of the cavity resonance.
Abstract: We demonstrate the control of the spontaneous emission rate of single InAs quantum dots embedded in a double-membrane photonic crystal cavity by the electromechanical tuning of the cavity resonance. Controlling the separation between the two membranes with an electrostatic field, we obtain the real-time spectral alignment of the cavity mode to the excitonic line and we observe an enhancement of the spontaneous emission rate at resonance. The cavity has been tuned over 13 nm without shifting the exciton energies. A spontaneous emission enhancement of ≈4.5 has been achieved with a coupling efficiency of the dot to the mode β≈92%.
TL;DR: In this paper, double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.04As0.96 absorption layer.
Abstract: Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the AlGaAs tunnel barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 μm wavelength leads to a pronounced photo-effect with a sensitivities of around 103 A/W.
TL;DR: The fabrication of single mode quantum cascade lasers using a shallow-etched distributed Bragg reflector as frequency selective element and spectral resolution of a gas absorption experiment is determined to be better than 0.0078 cm-1.
Abstract: We report the fabrication of single mode quantum cascade lasers using a shallow-etched distributed Bragg reflector as frequency selective element. Quasi-continuous single mode tuning over 15 cm-1 at room temperature and 25 cm-1 via temperature tuning at Peltier temperatures is demonstrated. The behavior of both electro-optic and spectral characteristics under variation of the segment currents is analyzed, showing a maximum peak output power at room temperature of 600 mW. Thermal crosstalk between the laser segments is investigated. The spectral resolution of a gas absorption experiment is determined to be better than 0.0078 cm-1.
TL;DR: In this article, the authors describe epitaxial growth and investigations of optical properties of In(Ga)As/GaAs site-controlled quantum dots (QDs) fabricated on (001)-oriented GaAs substrates.
Abstract: In this article, we describe epitaxial growth and investigations of optical properties of In(Ga)As/GaAs site-controlled quantum dots (QDs) fabricated on (001)-oriented GaAs substrates. The QD nucleation is directed by pre-patterning planar GaAs surfaces with shallow nanoholes. The focus of this work lies on the realization of arrays of site-controlled QDs (SCQDs) with a tailored morphology and optical properties comparable to QDs fabricated on planar substrates. By maximizing the migration length during QD deposition, we were able to increase the QD pitches to values exceeding device dimensions of typical semiconductor microresonators. The introduction of a seeding layer in our growth scheme allows us to extend the vertical distance between the QDs and the etched nucleation centres to about 20 nm without suffering from nanoholes being occupied by multiple QDs. Furthermore, the extended distance between the QD layer and the re-growth interface allows us to preserve excellent optical properties of the single QDs as probed in photoluminescence with an average single QD related linewidth of 133 µeV and minimum values as low as 25 µeV for non-resonant excitation. The high yield of optically active QDs on the pre-defined nucleation positions is studied by cathodoluminescence (CL) with high spatial resolution. We find emission from single SCQDs on more than 90% of the nucleation centres, which is a pre-requisite for any scalable QD-device fabrication scheme.
TL;DR: In this paper, the fine structure splitting (FSS) from self-organized In(Ga)As quantum dots (QDs) grown on GaAs substrates with different lattice orientations is analyzed and compared to small QDs on commonly used (001) substrates.
Abstract: We present a comparative investigation of the fine structure splitting (FSS) from self-organized In(Ga)As quantum dots (QDs) grown on GaAs substrates with different lattice orientations. QDs grown on (111)B- and (112) oriented substrates are analyzed and compared to small QDs on commonly used (001) substrates. Mean values for the FSS as low as (5.6 ± 0.6) μeV are obtained for QDs on (111)B-GaAs, comparing favorably to the other two approaches ((11.8 ± 1.7) μeV for (112)-surfaces and (14.0 ± 2.2) μeV for (001)-surfaces). Single photon emission from (111)B QDs grown by droplet epitaxy is demonstrated via photon autocorrelation studies with a g(2)(0) value of 0.07.
TL;DR: The second-order correlation function g (2) ( = 0), input-output curves and pulse duration of the emission from a microcavity exciton-polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures as mentioned in this paper.
Abstract: The second-order correlation function g (2) ( = 0), input-output curves and pulse duration of the emission from a microcavity exciton-polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures. At low temperatures a two-threshold behaviour emerges, which has been attributed to the onset of polariton lasing and conventional lasing at the first and the second threshold, respectively. We observe that polariton lasing is stable up to temperatures comparable with the exciton binding energy. At higher temperatures a single threshold displays the direct transition from thermal emission to photon lasing.
TL;DR: A method to controllably reduce the density of self-assembled InP quantum dots (QDs) by cyclic deposition with growth interruptions with great promise for the exploitation of InP QDs as quantum emitters is demonstrated.
Abstract: We demonstrate a method to controllably reduce the density of self-assembled InP quantum dots (QDs) by cyclic deposition with growth interruptions. Varying the number of cycles enabled a reduction of the QD density from 7.4 × 10 10 cm −2 to 1.8 × 10 9 cm −2 for the same total amount of deposited InP. Simultaneously, a systematic increase of the QD size could be observed. Emission characteristics of different-sized InP QDs were analyzed. Excitation power dependent and time-resolved measurements confirm a transition from type I to type II band alignment for large InP quantum dots. Photon autocorrelation measurements of type I QDs performed under pulsed excitation reveal pronounced antibunching (g (2) (τ = 0) = 0.06 ± 0.03) as expected for a single-photon emitter. The described growth routine has great promise for the exploitation of InP QDs as quantum emitters. (Some figures may appear in colour only in the online journal)
TL;DR: In this paper, the authors demonstrate a simple and reconfigurable way to create a polariton condensate in well defined discrete momentum states, allowing them to manipulate the local polariton flow.
Abstract: Here we demonstrate a simple and reconfigurable way to create a polariton condensate in well defined discrete momentum states, allowing us to manipulate the local polariton flow. To this end, we created a spatially varying potential formed in the presence of noncondensed carriers by subjecting a microcavity to spatially modulated nonresonant optical excitation. The choice of the spatial shape of this potential allows us to tailor the properties of the polariton condensate in momentum space. Our results demonstrate a way to prepare a polariton condensate in an adjustable momentum state and provide a first step toward the creation of functional all-optical elements for polaritonic logic circuits on demand by projecting circuits onto an unprocessed planar sample.
TL;DR: In this article, site-controlled growth of InP/GaInP quantum dots (QDs) on GaAs substrates is reported, where the QD nucleation sites are defined by shallow nanoholes etched into a GaInP layer.
Abstract: We report on site-controlled growth of InP/GaInP quantum dots (QDs) on GaAs substrates. The QD nucleation sites are defined by shallow nanoholes etched into a GaInP layer. Optimized growth conditions allow us to realize QD arrays with excellent long range ordering on nanohole periods as large as 1.25 µm. Single QD lines with an average linewidth of 553 µeV and best values below 200 µeV are observed. Photoluminescence spectroscopy reveals excitonic and biexcitonic emission in the wavelength range of about 670 nm (1.85 eV) with an exciton-biexciton splitting of 1.8 meV. Second-order photon-autocorrelation measurements show clear single photon emission with g(2)(0) = 0.13 ± 0.01.
TL;DR: In this article, pump-power-dependent emission features of a nonthermalized and interacting dynamical condensate of exciton polaritons were investigated by measuring the energy-momentum dispersion characteristics, the spatial coherence, and the photon statistics under resonant, fs-pulsed optical excitation at high momentum.
Abstract: We report on pump-power-dependent emission features of a nonthermalized and interacting dynamical condensate of exciton polaritons. The system based on a planar AlAs/AlGaAs microcavity sample with twelve GaAs quantum wells in the active region is investigated comprehensively by measuring the energy-momentum dispersion characteristics, the spatial coherence, and the photon statistics under resonant, fs-pulsed optical excitation at high momentum. We observe a significant polariton-population-dependent modification of the emission signatures above the quantum degeneracy. The nonequilibrium polariton condensate is confirmed by its polaritonic energy-momentum dispersion as well as excitation-power-dependent coherence properties and its photon statistics, being different from that of an ideal coherent state. The polaritonic condensate is characterized by a spatial coherence length of up to 4 \ensuremath{\mu}m and a super-Poissonian photon statistic of the emitted light well above threshold. Results obtained in second-order photon autocorrelation measurements in momentum-space resolved spectroscopy indicate polariton repulsive interaction throughout the condensate and a spatial coherence length being shorter than the condensate extension of 20--30 \ensuremath{\mu}m.
TL;DR: In this paper, the Hanbury Brown-Twiss correlation measurements for the excitonic emission under cw excitation show anti-bunching behavior with an autocorrelation value of g(2)(0) = 0.2.
Abstract: Quantum dots as single-photon sources have several advantages, such as emitting light over a broad spectral range and being photostable. Quantum dots with densities as low as 1 dot/μm2 have been achieved using ultra-low-rate epitaxy and single-dot emission measured without apertures or post-growth processing. Both excitionic and biexcitonic emissions are observed from single dots created in this way, appearing as doublets with a fine-structure splitting of 320 μeV. The polarization of the split states is also investigated. Hanbury Brown-Twiss correlation measurements for the excitonic emission under cw excitation show anti-bunching behavior with an autocorrelation value of g(2)(0) = 0.2.