We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5  ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8  meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

We report the routing of quantum light emitted by self-assembled InGaAs quantum dots (QDs) into the optical modes of a GaAs ridge waveguide and its efficient detection on-chip via evanescent coupling to NbN superconducting nanowire single photon detectors (SSPDs). The waveguide coupled SSPDs primarily detect QD luminescence, with scattered photons from the excitation laser onto the proximal detector being negligible by comparison. The SSPD detection efficiency from the evanescently coupled waveguide modes is shown to be two orders of magnitude larger when compared with operation under normal incidence illumination, due to the much longer optical interaction length. Furthermore, in-situ time resolved measurements performed using the integrated detector show an average QD spontaneous emission lifetime of 0.95 ns, measured with a timing jitter of only 72 ps. The performance metrics of the SSPD integrated directly onto GaAs nano-photonic hardware confirms the strong potential for on-chip few-photon quantum optics using such semiconductor-superconductor hybrid systems.

Quantum optical circuits can be used to generate, manipulate, and exploit nonclassical states of light to push semiconductor based photonic information technologies to the quantum limit. Here, we report the on-chip generation of quantum light from individual, resonantly excited self-assembled InGaAs quantum dots, efficient routing over length scales ≥1 mm via GaAs ridge waveguides, and in situ detection using evanescently coupled integrated NbN superconducting single photon detectors fabricated on the same chip. By temporally filtering the time-resolved luminescence signal stemming from single quantum dots we use the quantum optical circuit to perform time-resolved excitation spectroscopy on single dots and demonstrate resonance fluorescence with a line-width of 10 ± 1 μeV; key elements needed for the use of single photons in prototypical quantum photonic circuits.

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ∼ 2 meV. Excited state transitions ∼ 8 − 16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with two laser fields we demonstrate a strong (88% on-off contrast) laser induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.Quantum dot (QD) nanostructures formed by strain driven self-assembly are ideal for solid state quantum optics experiments due to their discrete optical spectrum, strong interaction with light and robust quantum coherence for both interband polarization 1,2 and spin 3 . The ease with which such nanostructures can be embedded into electrically active devices allows for tuning of the transition frequency and control of charge occupancy 4 . Self-assembly provides a natural way to realize few dot systems via vertical stacking to produce more sophisticated nanostructures with coherent inter-dot coupling due to carrier tunneling 5-12 . When combined with the potential to coherently manipulate excitons over ultrafast timescales using precisely timed laser and electrical control pulses 13-15 such systems raise exciting prospects for the operation of small scale few qubit systems in a solid-state device. Very recently, conditional quantum dynamics for a single resonantly driven QD-molecule (QDM) 16 and spin dependent quantum jumps have been observed 8,17 .In this paper we employ photocurrent (PC) absorption, photoluminescence (PL) emission and PLexcitation (PLE) spectroscopy to trace the spectrum of ground and excited state transitions of an individual selfassembled QD-molecule as their character is electrically tuned from spatially direct to indirect. PC absorption allows us to identify the spatially direct neutral exciton transitions in both the upper (X ud ) and lower (X ld ) dots in the molecule. A number of excited state transitions are identified in PLE ∼ 8 − 16 meV above X ud . These excited states exhibit pronounced anticrossings (energy splitting ∆E ∼ 3.2 − 3.5 meV) as the electric field F is tuned. Excited state transitions are identified from voltage dependent PLE measurements to correspond to transitions between these hybridized electronic states and different hole orbitals in the upper dot. By performing a multi-color experiment where the QDM is simultaneously excited with different frequency lasers, we demonstrate how the resonant excitation of indirect excitons or exci-X ld X ud X ind FIG. 1. (Color onli...

We prepare NbN thin films by DC magnetron sputtering on [100] GaAs substrates, optimise their quality and demonstrate their use for efficient single photon detection in the near-infrared. The interrelation between the Nb:N content, growth temperature and crystal quality is established for 4−22nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6±0.2K for a film thickness of 22 ± 0.5nm and 10.2 ± 0.2K for 4 ± 0.5nm thick films that are suitable for single photon detection. The optimum growth temperature is shown to be ∼ 475 • C reflecting a trade-off between enhanced surface diffusion, which improves the crystal quality, and arsenic evaporation from the GaAs substrate. Analysis of the elemental composition of the films provides strong evidence that the δ-phase of NbN is formed in optimised samples, controlled primarily via the nitrogen partial pressure during growth. By patterning optimum 4nm and 22nm thick films into a 100nm wide, 369µm long nanowire meander using electron beam lithography and reactive ion etching, we fabricated single photon detectors on GaAs substrates. Time-resolved studies of the photo-response, absolute detection efficiency and dark count rates of these detectors as a function of the bias current reveal maximum single photon detection efficiencies as high as 21 ± 2% at 4.3 ± 0.1K with ∼ 50k dark counts per second for bias currents of 98%I C at a wavelength of 950nm. As expected, similar detectors fabricated from 22nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤ 3cps. The maximum lateral extension of a photo-generated hotspot is estimated to be 30±8nm, clearly identifying the low detection efficiency and dark count rate of the thick film detectors as arising from hotspot cooling via the heat reservoir provided by the NbN film. PACS numbers: 74.78.-w 74.25.Gz 78.67.Uh 85.25.Oj 85.25.-j 42.50.-p

Using integrated superconducting single photon detectors we probe ultra-slow exciton capture and relaxation dynamics in single self-assembled InGaAs quantum dots embedded in a GaAs ridge waveguide. Time-resolved luminescence measurements performed with on-and off-chip detection reveal a continuous decrease in the carrier relaxation time from 1.22 ± 0.07 ns to 0.10 ± 0.07 ns upon increasing the number of non-resonantly injected carriers. By comparing off-chip time-resolved spectroscopy with spectrally integrated on-chip measurements we identify the observed dynamics in the rise time (τ r ) as arising from a relaxation bottleneck at low excitation levels. From the comparison with the temporal dynamics of the single exciton transition with the on-chip emission signal, we conclude that the relaxation bottleneck is circumvented by the presence of charge carriers occupying states in the bulk material and the two-dimensional wetting layer continuum. A characteristic τ r ∝ P −2/3 power law dependence is observed suggesting Auger-type scattering between carriers trapped in the quantum dot and the two-dimensional wetting layer continuum which circumvents the phonon relaxation bottleneck.Semiconductor based photonic information technology is rapidly being pushed to the quantum limit where single photon states can be generated and manipulated in nanoscale optical circuits 1 . Over recent years quantum dots (QDs) embedded in such semiconductor systems have been shown to be excellent sources of quantum light 2-4 and have shown their suitability for use as a gain medium in QD lasers 5-7 . However, for short response times and fast operation of such devices, injected charge carriers must relax rapidly from continuum wetting layer electronic states into the lasing state. For a fully discrete electronic structure, efficient relaxation is expected to be hindered by phonon bottleneck phenomena 8 , caused by the large energetic spacing of QD energy levels that inhibits single-phonon mediated scattering processes 9 . To directly observe such relaxation bottleneck effects, superconducting single photon detectors (SSPDs) are suitable due to their near unity quantum efficiency and picosecond timing resolution 10,11 . Building up on recent progress in this field 12-14 , we developed highly efficient 15,16 NbNSSPDs on GaAs 17 and demonstrated the monolithic integration of InGaAs QDs as single photon emitters together with waveguides and detectors on a single chip 18 . In this letter, we compare photoluminescence (PL) dynamics recorded from a single dot with confocal off-chip detectors with on-chip PL using integrated SSPDs that provide temporal resolution better than 70 ps. By probing the carrier capture and energy relaxation dynamics

Abstract:Semiconductor quantum photonic circuits can be used to efficiently generate, manipulate, route and exploit non-classical states of light for distributed photon based quantum information technologies. In this article, we review our recent achievements on the growth, nanofabrication and integration of highquality, superconducting Niobium nitride thin films on optically active, semiconducting GaAs substrates and their patterning to realise highly efficient and ultrafast superconducting detectors on semiconductor nanomaterials containing quantum dots. Our state-of-the-art detectors reach external detection quantum efficiencies up to 20 % for ∼ 4 thin films and single photon timing resolutions < 72 . We discuss the integration of such detectors into quantum dot loaded, semiconductor ridge waveguides, resulting in the on-chip, time-resolved detection of quantum dot luminescence. Furthermore, a prototype quantum optical circuit is demonstrated that enabled the on-chip generation of resonance fluorescence from an individual InGaAs quantum dot, with a linewidth < 15 displaced by 1 from the superconducting detector on the very same semiconductor chip. Thus, all key components required for prototype quantum photonic circuits with sources, optical components and detectors on the same chip are reported. Main text:Semiconductors are ubiquitous in modern opto-electronics and quantum photonic devices and are also expected to play a major role in photonic quantum

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