Superconducting detectors for semiconductor quantum photonics

In this thesis we present the first successful on-chip detection of quantum light, thereby demonstrating the monolithic integration of superconducting single photon detectors with semiconductor quantum dots in a prototypical quantum photonic circuit.
In order to fabricate highly efficient and ultra-fast detectors exhibiting dark count rates
≤ 1 cps, we start by optimizing 4 to 22 nm thick NbN films on GaAs substrates, as described in chapter 2. Whilst systematically analyzing the parameter space of film growth using reactive magnetron sputtering, we identify the nitrogen partial pressure to determine the crystal phase and structure. By combining low-temperature transport measurements with depth-dependent atomic concentration studies, we demonstrate that 13% (26%) nitrogen content during growth results primarily in the formation of face-centered cubic δ-phase (hexagonal E-pase) NbN thin films. Complementary investigations of the role of the growth temperature reveal an optimum of 475◦C representing a trade-off between enhanced surface diffusion improving crystal quality and arsenic desorption from the GaAs substrate promoting disorder at the interface.
Having obtained optimized 4 ± 0.5 nm thick NbN films on GaAs exhibiting a superconducting transition temperature of 10.2 ± 0.2 K, we continue by presenting the nano-lithographic fab- rication process of superconducting single photon detectors (SSPDs) throughout the second part of chapter 2. In detail we show how we optimized electron beam lithography and reac- tive ion etching to define defect-free 100 nm wide nanowires exhibiting a homogeneous width distribution < 5 nm. Employing low-temperature transport measurements, we study the crit- ical current IC of SSPDs in general and single nanowires in particular for different detector geometries and operation temperatures, thereby further optimizing the device design. While investigating the temporal evolution of photon induced normal conducting hotspots in both 4 ± 0.5 nm thin and 22 ± 0.5 nm thick devices, we identify the low detection efficiency and dark count rates of the thick film detectors as arising from rapid hotspot cooling via the heat reservoir provided by the NbN film. In strong contrast, we show that thin detectors exhibit maximum efficiencies of 21 ± 2% for top-illumination. By systematically characterizing the detector performance metrics as a function of the film thickness, we find 10 ± 0.5 nm thick SSPDs to represent the perfect compromise of a moderately high maximum top-illumination efficiency of ∼ 0.1 % combined with an ultra-low noise level of < 1 dark counts per second when biased at 0.95 IC . Finally, we characterize the temporal response of optimized detectors, thereby measuring an ultra-fast timing resolution of only 72 ± 2 ps.
Throughout chapter 3, we build up on the technology described above in order to develop waveguide coupled SSPDs for in-situ detection of quantum dot (QD) luminescence. There- fore, we start by optimizing both the emitter-waveguide and the waveguide-detector coupling. Here, we systematically simulate the mode profiles, propagation and absorption losses and the dipole emission fraction into the waveguide. Having optimized the basic device charac- teristics, the fabrication technology for low-loss active and passive GaAs/Al0.8Ga0.2As ridge waveguides is presented. By characterizing the different types of devices employing spatially and temporally resolved low-temperature opto-electrical measurements, we prove that the on-chip detected light almost exclusively stems from the emission of the embedded emitters. Employing the QDs as calibrated light sources, we determine the on-chip detection efficiency
to be ∼ 1%, thereby significantly exceeding the efficiency for top-illumination. We continue by utilizing the fast response of the integrated SSPDs to probe exciton capture and relaxation dynamics. Time-resolved luminescence measurements performed with on- and off-chip detection reveal a continuous decrease in the carrier relaxation time from τr = 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 as arising from a relaxation bottleneck at low excitation levels. A characteristic τr ∝ 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 circumventing this phonon relaxation bottleneck at higher excitation power levels.
Having established the technology for the monolithic integration of sources, waveguides and detectors on a hybrid semiconductor-superconductor chip, we continue by developing the first prototype of an integrated quantum optical circuit throughout chapter 4. Therefore, we iden- tify the source of the background signal in previous on-chip experiments to originate mostly from reflection and diffuse scattering of laser light at the sample backside. We efficiently suppress this stray light by employing > 2 mm long, low-loss waveguides featuring a propagation length of 664 ± 64 μm. Furthermore, we apply both a detector top-coverage and a sample backside treatment including polishing and coating with a silicon absorber layer to reduce the laser stray light by ∼ 40×. By additionally operating a time-resolved on-chip luminescence technique for both readout and signal filtering, we perform excitation spectroscopy on single QDs. Thereby, we are able to identify common excited state resonances in on- and off-chip detection. We continue to demonstrate and investigate resonant fluorescence (RF) of single dots with a line-width of 10 ± 1 μeV. By employing an optical gating laser to stabilize the
electro-static environment experienced by the dot, we show both an increase of the RF signal of ∼ 3× and a reduction of the line-width by ∼ 2×. We continue by performing measurements on the photon statistics of the dot, thereby showing the quantum nature of the detected light.
Finally, we present first indications of Rabi oscillations with on-chip detection, indicating the coherent interaction between the excitation laser and the quantum emitter.
The on-chip generation, distribution and detection of non-classical light within a prototypical quantum optical circuit fabricated from a semiconductor-superconductor hybrid nanosystem paves the way for further progress towards integrated solid-state quantum optics.