Independent Electrical Control of Two Quantum Dots Coupled through a Photonic-Crystal Waveguide

Xiao-Liu Chu, Camille Papon, Nikolai Bart, Andreas D. Wieck, Arne Ludwig, Leonardo Midolo, Nir Rotenberg, and Peter Lodahl

Phys. Rev. Lett. 131, 033606 – Published 20 July 2023

Abstract

Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned $16 μ m$ away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveal signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multiemitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters.

Received 2 March 2023
Accepted 12 June 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.033606

Physics Subject Headings (PhySH)

Collective effects in quantum optics Light-matter interaction Nanophotonics Optical coherence Photonic crystals Photonics Quantum optics Semiconductor quantum optics Superradiance & subradiance

Green's function methods Liquid helium cooling

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied PhysicsQuantum Information, Science & Technology

Authors & Affiliations

Xiao-Liu Chu ^1,*, Camille Papon ¹, Nikolai Bart², Andreas D. Wieck², Arne Ludwig ², Leonardo Midolo¹, Nir Rotenberg ^1,†, and Peter Lodahl ¹

¹Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
²Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany

^*Corresponding author. xchu@ic.ac.uk Present address: MRC London Institute of Medical Sciences, Du Cane Road, London, W12 0NN, United Kingdom.
^†Present address: Centre for Nanophotonics, Department of Physics, Engineering Physics and Astronomy, Queen’s University, 64 Bader Lane, K7L 3N6 Kingston, Ontario, Canada.

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Vol. 131, Iss. 3 — 21 July 2023

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Images

Figure 1
Independent electrical operation of two QDs in a PhCW. (a) Epitaxially grown QDs are embedded within a PhCW on a p-i-n diode. The trench separates the waveguide, such that an independent gate voltage can be applied to each half, here illustrated as the metal contacts $V_{1}$ and $V_{2}$ . Inset: An SEM image of the PhCW and corresponding 100 nm trench, with the rough positions of the two studied QDs marked ( $15.7 \pm 2 μ m$ separation; see Ref. [22]). (b) Independent control of the two QDs as demonstrated by keeping $V_{1}$ constant and measuring the transmission through the PhCW as $V_{2}$ is scanned. The corresponding resonant extinction dips corresponding to the two orthogonal dipoles (separated by about 2.4 GHz) of QD1 are independent of $V_{2}$ , appearing as two horizontal lines on this map, while those of QD2 vary linearly with the gate voltage. From this map, an overlap of two QD resonances near 326.614 THz is identified (green dashed region). (c),(d) Measured power-dependent resonant extinction for QD1 and QD2, respectively, when the other QD is far-detuned. The corresponding theoretical fits (see Ref. [22]) are used to determine the individual QD parameters. The insets show exemplary RT scans, with $I_{R T}$ defined as the extinction dip depth and the transitions that are tuned into resonance are highlighted by the shaded regions.
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Figure 2
Independent optical excitation of each QD in a RF configuration, where the photons emitted from one QD travel through the waveguide to interact with the second emitter. (a) In RF1, the continuous wave laser is scanned through the QD1 resonance, while voltage tuning ( $V_{1}$ ) across the QD2 frequency and simultaneously keeping the QD2 resonance constant at $ν_{2} = 326.614 THz$ . The emitted single photons from QD1 scatter off QD2 and the resultant transmission spectrum reveals the coherent extinction of the QD2 resonance. (b) Cross sections taken along the blue and green dashed curves in (a), showing the RF spectrum of QD1 when QD2 is far-detuned and the response of the two-QD system when the QDs are on resonance, respectively. (c) In RF2, the detected spectrum results from the interference of photons that are initially emitted from QD2 toward the right interfered with photons emitted to the left and subsequently reflected from QD1; see illustration in the inset. This signal depends on the relative phase $Δ φ$ gained by the scattered photons (see Ref. [22]) (d) Same as in (a), except that the green curve shows the RF spectrum of QD2.
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Figure 3
Simultaneous optical excitation of both QDs through a single photonic waveguide. (a) Power-dependent transmission through the waveguide when the two QDs are both on resonance. Overlayed on the measurements (circular markers, errors due to detector dark counts) are models of $I_{RT}$ for QD1 and QD2 alone (dashed red and blue curves, respectively) as well as their joint response assuming they either are independent or coherently coupled (solid purple and teal curves, respectively). Note that all parameters used in the models are determined from previous experiments. (b) An exemplary RT spectra of the two QDs on resonance overlayed with theoretical curves for the coupled and uncoupled response. The individual QD frequency scans are also added for comparison.
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