Photonic wire bonds have been developed as an interface for the collection of single photon emission from quantum dots within a Bragg waveguide. When resonantly excited from the top of the waveguide via free space excitation a low multiphoton contribution in the quantum dot emission with g(2)(0) = (9.5 ± 1.4) × 10−2 is shown. Our measurements demonstrate the ability to collect single-photon emission from a ridge waveguide into an optical fiber via photonic wire bonds at cryogenic temperatures. This allows for a seamless plug-and-play operation of the fiber-coupled single-photon source. Furthermore, the demonstrated approach allows for resonance fluorescence excitation without the need for any additional cross-polarization filtering.
In the quest to realize a scalable quantum network, semiconductor quantum dots (QDs) offer distinct advantages, including high single-photon efficiency and indistinguishability, high repetition rate (tens of gigahertz with Purcell enhancement), interconnectivity with spin qubits, and a scalable on-chip platform. However, in the past two decades, the visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50%, and the distances were limited from a few meters to kilometers. Here, we report quantum interference between two single photons from independent QDs separated by a 302 km optical fiber. The single photons are generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions are used to eliminate the QD inhomogeneity and shift the emission wavelength to the telecommunication band. The observed interference visibility is 0.67 ± 0.02 (0.93 ± 0.04) without (with) temporal filtering. Feasible improvements can further extend the distance to ∼600 km. Our work represents a key step to long-distance solid-state quantum networks.
Following the ultrafast optical excitation of an inhomogeneously broadened ensemble, the macroscopic optical polarization decays rapidly due to dephasing. This destructive interference is, however, reversible in photon echo experiments. Here, we propose a concept in which a control pulse slows down either the dephasing or the rephasing of the exciton ensemble during its presence. We analyze and visualize this optical freezing process by showing and discussing results for different single and multiple sequences of control pulses using a simple model of inhomogeneously broadened two-level systems. This idea has been realized in experiments performed on self-assembled (In,Ga)As quantum dots where it was possible to retard or advance the photon echo emission time by several picoseconds. The measurements are in very good agreement with numerical simulations for a more realistic model which, in particular, takes the spatial shape of the laser pulses into account.
Epitaxially grown self-assembled quantum dots (QDs) are promising candidates for an efficient single-photon generation. In order to maximize the number of photons extracted from the device, QDs are frequently embedded into photonic structures such as micro-cavities. Once the QD is positioned inside an optical cavity, the Purcell effect ensures that light is emitted predominantly into the cavity mode.
In this contribution, we demonstrate results of emission tuning of QDs inserted in micro-cavities. A sample containing an InAs/GaAs QDs embedded in a planar cavity based on Bragg reflectors has been integrated onto the PMN-PT piezo crystal. Subsequently, micro-cavities have been fabricated by electron-beam lithography and reactive ion etching.
The application of external stress produces linear shifts of QDs emission which could be tuned into the resonance with fundamental cavity mode and allow enhancement of QD emission due to the Purcell effect.
Light sources for applications in quantum information, quantum-enhanced sensing and quantum metrology are attracting increasing scientific interest. To gain inside into the underlying physical processes of quantum light generation, efficient photon detectors and experimental techniques are required to access the photon statistics. In this work, we employ photon-number-resolving (PNR) detectors based on superconducting transition-edge sensors (TESs) for the metrology of photonic microstructures with semiconductor quantum dots (QDs) as emitters. For the PNR analysis, we developed a state of the art PNR detection system based on fiber-coupled superconducting TESs. Our stand-alone system comprises six tungsten TESs, read out by six 2-stage-SQUID current sensors, and operated in a compact detector unit integrated into an adiabatic demagnetization refrigerator. This PNR detection system enables us to directly access the photon statistics of the light field emitted by our photonic microstructures. In this contribution, we focus on the PNR study of deterministically fabricated quantum light sources emitting single indistinguishable photons as well as twin-photon states. Additionally, we present a PNR-analysis of electrically pumped QD micropillar lasers exhibiting a peculiar bimodal behavior. Employing TESs our work provides direct insight into the complex emission characteristics of QD- based light sources. We anticipate, that TES-based PNR detectors, will be a viable tool for implementations of photonic quantum information processing relying on multi-photon states.
Integrated quantum photonic circuits are promising for an on-chip realization of a quantum advantage. It is desirable to develop a platform which allows dense integration of functionalities, which includes sources, photon processing units and detectors on the single photon level. Among the different material platforms currently being investigated, direct-bandgap semiconductors and particularly gallium arsenide offer the widest range of functionalities, including single and entangled-photon generation by radiative recombination, low-loss routing, electro-optic modulation, and single-photon detection. We summarize the potential and current status in the field of quantum integrated photonic components and circuits based on the GaAs technology platform.
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.
The development and physical understanding of high-beta nanolasers operating in regime of cavity-quantum-electrodynamics (cQED) is a highly interdisciplinary field of research, involving important aspects of nanotechnology, quantum optics, and semiconductor physics. Of particular interest is the quantum limit of operation, in which a few or even a single emitter act as gain material.
The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cQED systems, strong coupling may persevere even at elevated excitation levels sufficient to cross the threshold to lasing. In the presence of stimulated emission, the vacuum-Rabi doublet in the emission spectrum is modified and the established criterion for strong coupling no longer applies.
Based on an analytic approach, we provide a generalized criterion for strong coupling and the corresponding emission spectrum that includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a state-of-the-art few-emitter quantum-dot (QD)–micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model [1]. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved. By using a master-equation approach for up to 8 QDs coupled to the mode, we provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions by identifying signatures in the mean-photon number, the photon-autocorrelation function, and the emission linewidth.
[1] C. Gies et al., accepted for publication in PRA, arxiv:1606.05591
Despite their name polariton lasers do not rely on stimulated emission of cavity photons. The less stringent threshold conditions are the cause that bosonic polariton lasers can outperform standard lasers in terms of their threshold currents. The part-light and part-matter quasiparticles called polaritons, can undergo a condensation process into a common energy state. The radiated light from such a system shares many similarities with the light emitted from a conventional photon laser, even though the decay of the polaritons out of the finite lifetime cavity is a spontaneous process. We discuss properties of polariton condensates in GaAs based microcavities. The system’s response to an external magnetic field is used as a reliable tool to distinguish between polariton laser and conventional photon laser. In particular, we will discuss the realization of an electrically pumped polariton laser, which manifests a major step towards the exploitation of polaritonic devices in the real world.
Polariton Lasers do not rely on stimulated emission of photons, a criterion that sets stringent conditions on the threshold current in a conventional laser. Therefore, they have the capability to outperform photon lasers in the weak coupling regime in terms of the threshold power consumption. We present the first successful realization of an electrically pumped polariton laser based on a GaAs/AlAs distributed Bragg reflector cavity. We have furthermore identified the system’s response to an applied magnetic field as a sensitive tool to distinguish a polariton laser from a standard VCSEL device in the weak light-matter coupling regime
Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μs-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λ-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.
Individual electron and hole quantum dot spin qubits can be coherently manipulated using picosecond modelocked laser pulses; an all-optical spin-echo was implemented that decouples slow environmental changes. While dephasing and decoherence mechanisms for electrons and holes are intrinsically different, similar qualitative results are obtained,
except for dynamic nuclear polarization effects that affect the controllability of electrons. In addition, we demonstrate
spin-photon entanglement in a charged InAs quantum dot, using an ultrafast downconversion technique that converts a single, spontaneously emitted photon at 900 nm into a 1560 nm photon with picosecond timing resolution. This ultrafast conversion technique allows quantum erasure of which-path frequency information in the spontaneous emission process.
Studying cavity quantum electrodynamical effects is an emerging and important field of research for the understanding
of the many body quantum theory as well as for the generation of a new type of efficient lasers. Here we report a
dramatic change in the photon statistics of quantum dot based micropillar lasers where a finite fraction of the emission is
reflected back into the microcavity after a roundtrip time τ in an external cavity, where τ greatly exceeds the coherence
time. Photon bunching was observed above the threshold current where the second order autocorrelation function g(2)(τ)
at zero-lag can reach values up to 3.51±0.06. The change in the photon statistics of the two non-degenerated fundamental
modes were found to be correlated, indicating non-trivial interactions between both cavity modes. Furthermore the
optical feedback led to revivals of the bunching signal in integer multiples of the round trip time of the external cavity
and to a decrease in the coherence time of the laser. These phenomena compare well with milliwatt chaotic lasers
induced by an external feedback, indicating that chaos might occur in the nanowatt lasing regime where fluctuations in
the photon statistics are in the leading order.
Efficient sources of indistinguishable single photons are a key resource for various applications in fields like quantum
sensing, quantum metrology and quantum information processing. In this contribution we report on single photon
generation based on III-V semiconductor quantum dots (QDs). To increase the emission efficiency of single photons, it
is essential to tailor the radiative properties of the quantum dot emitters by engineering their photonic environment. We
present optimized single photon emitters being based on both micropillar and photonics crystal cavities, for applications
in a vertical platform and on-chip in-plane platform, respectively.
Electrically driven single photon sources with self assembled semiconductor QDs embedded into GaAs/AlAs
micropillar cavities emit on demand net rates of ~35 MHz single photons, thus being well exploitable in quantum key
distribution systems. In order to establish also a spatially deterministic fabrication platform, position controlled quantum
dots are integrated into p-i-n micropillar cavities and single photon emission of a coupled QD-micropillar diode system
is observed. Efficient broadband coupling of single photons into photonic crystal waveguides provides the basis for all
on-chip quantum information processing, and an according approach is reported.
Novel nanostructured III-V semiconductor devices are investigated for light detection in the near infrared
spectral region. Single-electron memories based on site-controlled InAs quantum dots embedded in a GaAs/AlGaAs
quantum-wire transistor were fabricated and studied. By using a nanohole structure template on a modulation-doped
GaAs/AlGaAs heterostructure, two single InAs quantum dots were centrally positioned in a quantum-wire transistor so
that pronounced shifts of the transistor threshold occur by charging of the QDs with single electrons. Single-electron
read and write functionalities up to room temperature were observed and the memory function can be also controlled by
light with a wavelength in the telecommunication range. Furthermore, AlGaAs/GaAs/AlGaAs double barrier resonant
tunneling diodes (RTD) with an embedded GaInNAs absorption layer have been fabricated for telecom wavelength light
detection at room temperature. The absorption layer was lattice matched grown within the GaAs system of the RTD.
We demonstrate that the devices exhibit typical RTDs characteristic and they are light sensitive at the telecom
wavelength 1.3 μm in the order of just a few nW. Routes to further reduce the detection limit are discussed whereas the
envisaged devices have prospects to deliver sensitivities approaching the quantum limit.
Semiconductor quantum dots (QDs) are fascinating nanoscopic structures for photonics and future quantum information
technology. However, the random position of self-organized QDs inhibits a deterministic coupling in devices relying on
cavity quantum electrodynamics (cQED) effects which complicates, e.g., the large scale fabrication of quantum light
sources. As a result, large efforts focus on the growth and the device integration of site-controlled QDs. We present the
growth of low density arrays of site-controlled In(Ga)As QDs where shallow etched nanoholes act as nucleation sites.
The nanoholes are located relative to cross markers which allows for a precise spatial alignment of the site-controlled
QDs (SCQDs) and the photonic modes of high quality microcavites with an accuracy better than 50 nm. We also
address the optical quality of the SCQDs in terms of the single SCQD emission mode linewidth, the oscillator strength
and the quantum efficiency. A stacked growth of strain coupled SCQDs forming on wet chemically etched nanoholes
provide the smallest linewidth with an average value of 210 μeV. Using time resolved photoluminescence studies on
samples with a varying thickness of the capping layer we determine a quantum efficiency of the SCQD close to 50 % and an oscillator strength of about 10. Finally, single photon emission with associated with g(2)(0) = 0.12 of a weakly
coupled SCQD - micropillar system will be presented.
Electron spins in quantum dots under coherent control exhibit a number of novel feedback processes. Here, we
present experimental and theoretical evidence of a feedback process between nuclear spins and a single electron
spin in a single charged InAs quantum dot, controlled by the coherently modified probability of exciting a trion
state. We present a mathematical model describing competition between optical nuclear pumping and nuclear
spin-diffusion inside the quantum dot. The model correctly postdicts the observation of a hysteretic sawtooth
pattern in the free-induction-decay of the single electron spin, hysteresis while scanning a narrow-band laser
through the quantum dot's optical resonance frequency, and non-sinusoidal fringes in the spin echo. Both the
coherent electron-spin rotations, implemented with off-resonant ultrafast laser pulses, and the resonant narrowband
optical pumping for spin initialization interspersed between ultrafast pulses, play a role in the observed
behavior. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse
timing, potentially allowing more complex coherent control operations.
The quest for efficient light sources and light detectors is a driving force in the development of semiconductor
quantum dot (QD) devices. Self assembled QDs in bulk material are characterized by high quantum efficiency
and can act as single photon emitters. However, they suffer from a poor light in- and outcoupling efficiency. We
demonstrate highly efficient QD-micropillar based light detectors and single photon emitters exploiting cavity
quantum electrodynamics (cQED) effects. An advanced fabrication technique allows us to realize ultra sensitive
and wavelength selective light detectors as well as triggered, electrically driven single photon sources with photon
outcoupling efficiencies exceeding 60 %.
Light and matter can be unified under the strong coupling regime, creating superpositions of both, called dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems ,strong coupling phenomena have been realized in solid state optical microcavities. They form an essential ingredient in the exciting physics spanning from many-body quantum coherence phenomena, like Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics (cQED). A widely used approach within cQED is the
Jaynes-Cummings (JC) model that describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling (QSC), a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently published. In our latest report we applied advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity
system. Specifically, we measured and simulated its four-wave mixing (FWM) response, granting direct access to the first two rungs of the JC ladder. This paper summarizes the main results of Ref. 15 and adds
FWM experiments obtained on a micropillar cavity in which a doublet of quantum dot (QD) excitons interacts with the cavity mode in the limit of weak to strong coupling.
The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control
methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much
faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we
demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a
semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from
nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate
time exceeds 105, suggesting strong promise for future quantum information processors.
Quantum light emitters have great application potential in quantum key distribution, precision metrology and
quantum imaging. We present triggered electrically driven single photon sources based on semiconductor quantum dots
in GaAs/AlAs micropillar cavities with on demand single photon rates of 35 MHz while a record outcoupling efficiency
up to 34 % is obtained. Photon autocorrelation measurements reveal g(2)(0) down to 0.13. The high efficiency is
achieved due to an optimized contact scheme which allows for the injection of electrical current into micropillar cavities
which are characterized by low absorption losses and diameters down to 1 μm. By exploiting the established
fabrication procedure, micropillar cavities exhibiting pronounced cavity quantum electrodynamics effects have been
realized. Furthermore, by applying a reverse bias to the micropillar cavities, photocurrent measurements allow for
wavelength selective sensing of light at powers down to 20 nW and further design changes promise photon detection
sensitivities approaching the quantum limit.
We report on laser emission from high quality quantum dot micropillar cavities. In these structures cavity quantum
electrodynamics (cQED) effects are exploited to realize high efficient, low threshold lasing. We demonstrate that cQED
effects allow for the observation of low threshold laser emission from a low number of InGaAs quantum dots embedded
in optically and electrically pumped high-Q micropillar laser structures. For instance, lasing with threshold currents as
low as 4 μA (160 A/cm2) is observed for electrically pumped microlasers at cryogenic temperatures. Moreover, single
quantum dot controlled lasing effects are achieved in optically pumped micropillar lasers with particular high quality
factors.
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