Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-25-26-33514
Timestamp: 2019-04-24 06:34:17+00:00

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Recent breakthroughs in solid-state photonic quantum technologies enable the generation and detection of single photons with near-unity efficiency as required for a range of photonic quantum technologies. The lack of methods to simultaneously generate and control photons within the same chip, however, is a main obstacle to achieving efficient multi-qubit gates and to harness the advantages of chip-scale quantum photonics. Here we propose and demonstrate an integrated voltage-controlled phase shifter based on the electro-optic effect in suspended photonic waveguides with embedded quantum emitters. The phase control allows the building of a compact Mach-Zehnder interferometer with two orthogonal arms, taking advantage of the anisotropic electro-optic response in gallium arsenide. Photons emitted by single self-assembled quantum dots can be actively routed into the two outputs of the interferometer. These results, together with the observed sub-microsecond response time, constitute a significant step towards chip-scale single-photon-source de-multiplexing, fiber-loop boson sampling, and linear optical quantum computing.
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Fig. 1 Integrated router based on suspended electro-optic waveguides. (a) Schematic outline of the device. In the presence of an electric field in the growth direction (z), the refractive index changes along the  and [11̄0] directions as indicated by the index ellipse. The phase change results in an anti-correlated output at the two output ports as shown in the inset for an ideal loss-less device. (b) Layer structure of the electro-optic waveguides. The white triangles indicate a layer of self-assembled quantum dots. (c) Plot of the absorption loss (right axis) and switching voltage Vπ (V) (left axis) as a function of length of the device. A device with 400 μm-long arms requires less than 2.5 V to achieve a full switching cycle, while losses are kept below 3 dB.
Fig. 2 Numerical analysis of the circuit components. (a) Finite element method (FEM) simulation of the fundamental transverse electric mode. The profile of the norm of the electric field is shown. (b) FEM simulation of the electric field (y-component) propagation in the Y-splitter at a wavelength of 904 nm. The mirror symmetry guarantees equal power and phase at the two outputs. (c) Simulated total (sum of both ports) transmission efficiency (black squares) and reflectivity (red circles) of the Y-splitter around the quantum-dot emission wavelength. The black dotted line indicates the wavelength of our experiment (λ = 904 nm). (d) Simulated electric-field propagation in the multi-mode interference (MMI) coupler when launching the same power on both ports with a relative phase difference of π/2. (e) Simulated total transmission (black squares) and reflection (red circles) in the MMI. The transmission level indicates the sum of both outputs T1 and T2 indicated in (d). (f) Transmission at the two output ports of the MMI as a function of the phase difference at the wavelength of our experiment.
Fig. 3 Scanning electron micrograph (SEM) of the on-chip electro-optical router. (a) The full MZI with the two orthogonal arms. The green and yellow boxes indicate the input power splitter (Y-splitter) and 3 dB multi-mode interference (MMI) combiner, respectively. These devices have been aligned to the  direction to make them insensitive to the electro-optic effect. The wavy pattern on the contact protection is an artifact due to charging effects. The two arms of the MZI are suspended with 100-nm-wide tethers. (b) The fabricated Y-splitter with trenches to electrically isolate the switch area from the emitter region. (c) SEM of the output 2×2 MMI beam splitter.
Fig. 4 Electro-optical switching of photons from a single quantum dot. (a,b) Single QD spectra collected at output port 1 (blue) and output port 2 (red) at −0.4V (a) and +1.2V (b). The intensity of the collected QD signal at the two ports varies by changing the voltage while its wavelength remains unchanged. (c) Experimental data showing the integrated intensity of the QD emission as a function of the bias. The vertical lines indicate the voltages at which the spectra in (a) and (b) are recorded. (d) Theoretical predictions of the experimental data in (c) for port 1 (blue) and port 2 (red) as a function of bias scaled to the experimental counts. (e) Comparison between the numerical model and the experimental transmission from a quantum dot. The normalized intensity is extracted as the fraction of power emitted in one port divided by the sum of both ports. The error bars are smaller than the size of the symbols and therefore not visible.
Fig. 5 Transmission measurements using a coherent light source. Intensity measured from (a) output port 2 and (b) output port 1 as a function of wavelength and voltage. (c) Comparison between the theoretical and the experimental normalized transmission intensity at λ = 918.7 nm. A clear drop in transmission is observed for both arms for V < −0.7 V due to electro-absorption. The oscillations originate from Fabry-Pérot modes caused by reflections in the circuit.
Fig. 6 Response time of the device. (a) A square wave voltage is applied to the sample, where Von and Voff are chosen so that the sample emission is maximized at Von = 1.55 V and is completely turned off at the average value Voff = 0.9 V. The purple dash-dotted (green dashed) line indicates the calculated system response below (above) the frequency response cut-off. (b) Integrated intensity from the wetting layer as a function of the applied bias. (c) The emission spectrum recorded at a modulation frequency of 10 kHz. (d) Same as (c) but at 10 MHz. (e) The integrated intensity of the wetting layer as a function of the modulation frequency of the square wave voltage (blue dots). The solid black line is the simulated response for a single-pole low-pass filter. The 3 dB cut-off is observed at around 2.8 MHz. The error bars are smaller than the size of the symbols and therefore not visible.

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