Fig. 1. (Color online) (a) Scheme of the cascade emission of the XX. The solid (dashed) line represents the XX decay path by emitting H(V) polarized photons. By applying tensile or compressive strain, the E_B(XX) energy of XX relative to X can be robustly adjusted. (b) Schematic of the device used for strain tuning of InGaAs/GaAs QDs, as located on the cold figure of a liquid He cryostat. Both the laser excitation and photon collection are performed by a photoluminescence setup above the device. The inset shows a 200 nm-thick QD containing nanomembrane. (c) Low-temperature PL spectrum with different E_B(XX). The emission line lying on the higher energy side of X and XX in both spectra is attributed to the positive trion X⁺ emission. Reprinted figure with permission from Ref. [60]. Copyright 2019, the American Physical Society.

Fig. 2. (Color online) PL spectra of (a) QD1 and (c) QD2 as a function of applied voltage on the PMN-PT actuator. The linear increase of E_B(XX) and E_B(X⁺) with E_X for both QD1 and QD2 are shown in (b) and (d). Reprinted figure with permission from Ref. [60]. Copyright 2019, the American Physical Society.

Fig. 3. (Color online) Sketch and characterization of a nanomembrane-based strain tunable single- photon-emitting diode. (a) n–i–p diode nanomembrane containing InGaAs/GaAs QDs bonded on a PMN-PT crystal via gold-to-gold thermo-compression bonding. The p-contact of the nanomembrane and the PMN-PT share a common ground. A diode voltage V_d and piezo voltage V_pp can be applied independently. (b) Microscope image of the strain tunable single-photon LED device. (c) The electric pulse applied on the nanomembrane is composed of two parts, the DC bias V_d and ultra-short pulse V_pp. (d) EL spectra from LED under different DC bias V_d. (e) Fluorescence lifetime histograms of the neutral X photon emission for different DC bias. Reprinted with permission from Ref. [61]. Copyright 2013, American Chemical Society.

Fig. 4. (Color online) (a) Tunable X photon emission line under in-plane biaxial strain ε_||. (a) ε_|| < 0 (compressive) and ε_|| > 0 (tensile) strain is obtained for positive and negative electric fields applied to the PMN-PT substrate, respectively. (b)–(d) Autocorrelation measurements are implemented with the X emission, and the suppression of coincidence counts at zero-time confirm the single-photon emission when varying the strain is applied to the QDs. Reprinted with permission from Ref. [61]. Copyright 2013, American Chemical Society.

Fig. 5. (Color online) (a) Schematic of the GaAs QDs heterostructure in the GaAs substrate, the length of the arrows are indicating the magnitude of in-plane strain. (b) Atomic force microscopy (AFM) image of a droplet-etched nanohole on the AlGaAs surface before GaAs filling. (c) Schematic of the GaAs QDs heterostructure after the etching of the AlAs sacrificial layer. (d) The energy level of the dipole transition in the QDs. X represents the light hole exciton, which is composed of a dark state (D) and three bright states (B). x and y denote the in-plane crystal direction and z denotes the out-plane direction. Reprinted with permission from Ref. [66]. Copyright 2013, Springer Nature.

Fig. 6. (Color online) PL spectra of a heavy hole and light hole exciton in GaAs/AlGaAs QDs. In (a) and (c), the open and filled circles represent the two perpendicular in-plane polarized components of light along the B₁ line has the polarization close to the x-direction [110]. In (b) and (d) the light is collected from the cleaved edge of the sample, the open diamond line has the polarization close to the y-direction [1 0]. Besides, there is an additional emission at higher energy in (d) indicting the dominated light hole exciton emission. Reprinted with permission from Ref. [66]. Copyright 2013, Springer Nature.

Fig. 7. (Color online) Polarization-resolved PL spectra of the light hole exciton emission independence of a magnetic field applied in the z-direction. Emission spectra are collected along z-direction (a) and x-direction (b). The dashed lines in (b) represent fits of the peak positions for the in-plane polarized B₁ and B₂ lines (white) and z-polarized B_z and D_z lines (green). Reprinted with permission from Ref. [66]. Copyright 2013, Springer Nature.

Fig. 8. (Color online) (a) Autocorrelation measurement of light hole exciton emission without any strain applied by PMN-PT. (b) Light hole exciton emission wavelength as a function of applied electric field on PMN-PT. Autocorrelation measurements (c) and (d) of the light hole emission, conducted at different electric field F_p. Reprinted with permission from Ref. [67]. Copyright 2015, American Chemical Society.

Fig. 9. (Color online) Schematic and characterization of a strain tunable entangled-light-emitting diode. (a) Sketch of the device. (b) EL emission spectra from a single QD as a function of the electric field applied to the piezoelectric actuator. (c) FSS of the exciton (X) state as a function of F_p for different QDs. The inset shows the biexciton cascade and FSS between X states. (d) X photon polarization angle relative to the nanomembrane crystal orientation [110] as a function of the electric field at the piezo. (e)–(i) represent the FSS and polarization angle of different QDs at zero electric fields F_p. Figure is taken from Ref. [65] without changes are made. CC BY^[119].

Fig. 10. (Color online) Entanglement characterization of the XX and X photons. (a) Cross-correlation measurements for co-polarized photons (blue) and cross-polarized photon (red) in different polarization projection bases H, V, D, A, R or L. (b) Real and (c) imaginary part of the two-photon density matrix, from which the degree of entanglement can is obtained. Figure is taken from Ref. [65] without changes are made. CC BY^[119].

Fig. 11. (Color online) Sketch and characterization of the device allowing for strain-tuning of QD containing nanowires. (a) Sketch of InP nanowire containing InAsP QD placed on a PMN-PT substrate, and the PL setup featuring QD excitation from the top and photon collection from the side. Inset shows the scanning electron microscopy (SEM) image of the nanowires with different taper. (b) QD exciton emission energy as a function of the voltage applied to PMN-PT. (c) PL spectra of two QDs in separate nanowires on different piezoelectric substrates. (d) and (e) show the fluorescence lifetime measurements on the excitons for nanowire 1 and 2, respectively. Red lines are the fit functions consisting of an exponential decay convoluted with the detector response function. Reprinted from Ref. [70], with the permission of AIP Publishing.

Fig. 12. (Color online) Silicon-based wavelength-tunable entangled photon source. (a) The vision of large scale integration of wavelength-tunable sources on silicon chips. (b) Sketch of the device. Focused ion beam (FIB) is used to create trenches on the PMN-PT surface and chemical etching is performed to suspend the different parts. A nanomembrane is then transferred to the top center of the four “legs”. (c) microscope image of the device, showing the electric contacts on the legs A, B, C, and D, obtained by wire bonding. (d) Tuning of the QD emission wavelength with the applied voltage V_BD on PMN-PT. (e) Sketch of the biexciton cascade and the possibility of reducing the FSS in the QDs by strain. Figure is taken from Ref. [39] without changes are made. CC BY^[119].

Fig. 13. (Color online) (a) and (b) show the strain-induced changes in FSS and polarization angle of X photons relative with the QD principal axis. The voltage is applied on legs A and C and the voltages on legs B and D are fixed at 0 and –25 V respectively. The ellipse in the inset indicates the asymmetric quantum dot confinement potential and the red solid line indicates the X photon polarization direction. (c) The X photon emission wavelength changes as a function of the voltage V_AC with two different fixed voltages V_BD at 0 V and –25 V. (d) The change of FSS while altering the strain in the QD along with two orthogonal directions (A–C and B–D). The dashed line indicates the track of minimal FSS with different strain combinations. Figure is taken from Ref. [39] without changes are made. CC BY^[119].

Fig. 14. (Color online) Simultaneous tuning of wavelength and fine structure splitting. (a) FSS of a single QD varies as a function of the tunable emission wavelength. A series of V_BD with a step size of 25 V from 25 to –100 V are investigated, while the voltage V_AC is changed at the same time to adjust the FSS. The red dashed line at FSS = 1 μeV implies the FSS limit for generating entangled photon pairs from the QD. (b) Polarization-dependent cross-correlation measurements on XX and X photons are performed and an entangle ement fidelity f⁺ = 0.733 ± 0.075 is obtained. Figure is taken from Ref. [39] without changes are made. CC BY^[119].

Fig. 15. (Color online) Experimental setup for Hong-Ou-Mandel interference with photons from separated and frequency stabilized quantum dots. A grating-based spectrum filter (SF) is used to filter the Ti: Sapphire excitation laser for resonant excitation of the biexciton state. The QDs containing membranes are placed on piezoelectric actuators in the 4 K environment and covered with a solid immersion lens (SIL) to enhance the brightness. The emitted photons from QD1 and QD2 are sent to a frequency discrimination setup. Parts of the photon streams from both QDs are sent to the Faraday filter setup via the half-wave plate (HWP) and the polarizing beam splitter. The faraday filter setup consists of a heated rubidium vapor cell in a longitudinal magnetic field and two crossed polarizers in front and behind the cell. Photons are then detected by single-photon counting modules 1 and 2 (SPCM1 and SPCM2). The estimated photon rates are used by a digital PI controller to apply feedback on the voltage applied to the piezoelectric actuators. Thereby the interference visibility is stabilized in the subsequent Hong-Ou-Mandel experiment. Reprinted figure with permission from Ref. [78]. Copyright (2019) by the American Physical Society.

Fig. 16. (Color online) (a) Emission spectrum of QD1 and QD2. Two-photon resonant π pulse excitation is used and the two XX photon emissions are tuned into resonance with the rubidium D1 transitions. (b) Faraday filter transmission for a weak and narrow-band laser (black solid line) and frequency-tuned QD2 (red line). The convolution of the laser transmission and QD2 emission profile is used to model the QD transmission (black dash line). The setpoint for frequency feedback is highlighted. Reprinted figure with permission from Ref. [78]. Copyright 2019, the American Physical Society.

Fig. 17. (Color online) Frequency feedback for stable Hong-Ou-Mandel interference. (a) Emission wavelength drifts over time for the frequency stabilized (red line) and free-running mode (blue line). (b) Estimated Hong-Ou-Mandel interference visibility over time, imagining frequency drifts as determined in (a). The red line (blue dash line) shows the estimated visibility with (without) frequency feedback. The blue solid line represents the free-running mode of the QD emission with an integration time as used in the experiment. (c) Second-order correlation measurement after Hong-Ou-Mandel interference at a beam splitter. The red (black) coincidences are obtained for parallel (orthogonal) XX photon polarizations at the beam splitter. (d) Hong-Ou-Mandel interference visibility over time with (red) and without (blue) frequency feedback. The visibility for stabilized emission is always higher than for the free-running mode. The shaded areas represent the uncertainty of visibility based on Poissonian counting statistics. Reprinted figure with permission from Ref. [78]. Copyright 2019, the American Physical Society.