Periodic Reporting for period 4 - QD-NOMS (Elementary quantum dot networks enabled by on-chip nano-optomechanical systems)
Période du rapport: 2021-07-01 au 2022-12-31
We are at the dawn of quantum information processing, and its future appears bright in spite of the many remaining challenges. A most outstanding one is scalability, which ensures the interconnection between individual microscopic quantum systems and then the construction of a macroscopic quantum system for practical applications. The project QD-NOMS will demonstrate the downscaling of single quantum dot (QD) systems with full tunabilities, and as well as the upscaling of the complexity of an elementary QD network. The longstanding challenge in scalability of QDs is solved with an innovative on-chip NOMS and several other important techniques. The ambition of this project is to bring the research on semiconductor QD systems to a level comparable to state-of-the-art researches with atoms and spontaneous parametric down conversion sources. With the disruptive technologies developed here, scalable, deterministic, on-chip integrated and semiconductor-compatible single and entangled photon sources will be realized. They can offer a fresh opportunity in future quantum networks and bring optical quantum information processing into a new regime. The success of QD-NOMS will also bring the semiconductor QD based platforms, after a decade of development, to the attention of practical applications.
With great efforts in the past years, QD-NOMS has been able to generate many important results. For the very first time, we were able to demonstrate the entanglement swapping between entangled photon pairs emitted by a semiconductor QDs. This is a strong proof that semiconductor QDs can be used to build an elementary quantum network. Of course, this milestone was achieved due to the many other important breakthroughs in QD-NOMS. For example, we were able to demonstrate a polarisation entangled photon source with a record-high brightness. Also, we were able to integrate multiple sources on a single chip, which was envisioned at the beginning of QD-NOMS. The strain tuning technique allowed us to tune remote QDs to the same wavelength, with the help of a rubidium-based atomic vapor cell. All these works together show that we can finally achieve the scalability of QD-based single photon and entangled photons sources, opening the route to building a large-scale quantum network in real world.
With great efforts in the past years, QD-NOMS has been able to generate many important results. For the very first time, we were able to demonstrate the entanglement swapping between entangled photon pairs emitted by a semiconductor QDs. This is a strong proof that semiconductor QDs can be used to build an elementary quantum network. Of course, this milestone was achieved due to the many other important breakthroughs in QD-NOMS. For example, we were able to demonstrate a polarisation entangled photon source with a record-high brightness. Also, we were able to integrate multiple sources on a single chip, which was envisioned at the beginning of QD-NOMS. The strain tuning technique allowed us to tune remote QDs to the same wavelength, with the help of a rubidium-based atomic vapor cell. All these works together show that we can finally achieve the scalability of QD-based single photon and entangled photons sources, opening the route to building a large-scale quantum network in real world.
The primary goal is to build an elementary quantum dot network via entanglement swapping, which has been successfully demonstrated. Three workpackges (WP) were designed to achieve the objectives.
(1) In WP1, our task is to grow high quality QD samples with high homogeneity and low initial fine structures splitting (FSS). In Nature Communications 8, 15501 (2017), we show that a large ensemble of as-grown polarization-entangled photon emitters can be obtained, using an emerging family of GaAs/AlGaAs QDs grown by droplet etching and nanohole infilling. These QDs exhibit very small FSS, very high inhomogeneity (wavelength distribution<2nm, one of the best values so far) and short radiative lifetime. All measured QDs can emit single pairs of entangled photons with ultra-high purity and high degree of entanglement. In Appl. Phys. Lett. 110, 151102 (2017), we further showed that an independent tuning of emission energy and decay time of neutral excitons confined in single QDs can be achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. This offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. In AIP Advances 9, 085112 (2019), Optics Express 28, 19457 (2020) and AIP Advances 12, 055302 (2022), we studied QDs that emit in the telecom wavelengths. We studied the QDs that are close to surface, see publication Applied Physics Letters 118, 221107 (2021).
(2) In WP2, we have done preliminary experiments for realizing the multiphoton interference between QDs. In the publication Nature Communications 9, 2994 (2018), we fulfilled the task of achieving a high source brightness of 10 million counts/s. We build a broadband optical antenna with an extraction efficiency of 65% (i.e. more than 40 million counts/s on the detector and 4 times better than our best estimation previously) and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. This is to date among the brightest entangled photon sources. In the publication Physical Review B 98, 161302 (2018), we successfully fulfilled the task of building a FPGA-based active feedback system. It is used to stabilize the frequency of single photons emitted by two separate QDs to an atomic standard. These results, together with the achievements in WP1, removes a major road blocker for multiphoton quantum interference experiments based on QDs. In the publication Phys. Rev. Lett. 123, 160502 (2019), we finally demonstrated the entanglement swapping of entangled photons from quantum dots. The related PhD thesis won the first place in the BMBF Quantum Future competition. In Physical Review B 103, 075413 (2021), we have successfully increased the operating speed of QD-based entangled photon sources to 1 GHz. The related spin properties in these QDs were studied in Physical Review B 104, 075301 (2021).
(3) In WP3, we have invented a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN–PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. Currently we are designing new MEMS structures, and investigating the possibility of coupling semiconductor quantum light sources with waveguides. The related work has resulted in a prestigious BMBF quantum future grant for a team member.
(1) In WP1, our task is to grow high quality QD samples with high homogeneity and low initial fine structures splitting (FSS). In Nature Communications 8, 15501 (2017), we show that a large ensemble of as-grown polarization-entangled photon emitters can be obtained, using an emerging family of GaAs/AlGaAs QDs grown by droplet etching and nanohole infilling. These QDs exhibit very small FSS, very high inhomogeneity (wavelength distribution<2nm, one of the best values so far) and short radiative lifetime. All measured QDs can emit single pairs of entangled photons with ultra-high purity and high degree of entanglement. In Appl. Phys. Lett. 110, 151102 (2017), we further showed that an independent tuning of emission energy and decay time of neutral excitons confined in single QDs can be achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. This offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. In AIP Advances 9, 085112 (2019), Optics Express 28, 19457 (2020) and AIP Advances 12, 055302 (2022), we studied QDs that emit in the telecom wavelengths. We studied the QDs that are close to surface, see publication Applied Physics Letters 118, 221107 (2021).
(2) In WP2, we have done preliminary experiments for realizing the multiphoton interference between QDs. In the publication Nature Communications 9, 2994 (2018), we fulfilled the task of achieving a high source brightness of 10 million counts/s. We build a broadband optical antenna with an extraction efficiency of 65% (i.e. more than 40 million counts/s on the detector and 4 times better than our best estimation previously) and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. This is to date among the brightest entangled photon sources. In the publication Physical Review B 98, 161302 (2018), we successfully fulfilled the task of building a FPGA-based active feedback system. It is used to stabilize the frequency of single photons emitted by two separate QDs to an atomic standard. These results, together with the achievements in WP1, removes a major road blocker for multiphoton quantum interference experiments based on QDs. In the publication Phys. Rev. Lett. 123, 160502 (2019), we finally demonstrated the entanglement swapping of entangled photons from quantum dots. The related PhD thesis won the first place in the BMBF Quantum Future competition. In Physical Review B 103, 075413 (2021), we have successfully increased the operating speed of QD-based entangled photon sources to 1 GHz. The related spin properties in these QDs were studied in Physical Review B 104, 075301 (2021).
(3) In WP3, we have invented a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN–PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. Currently we are designing new MEMS structures, and investigating the possibility of coupling semiconductor quantum light sources with waveguides. The related work has resulted in a prestigious BMBF quantum future grant for a team member.
Thanks to the rapid development in our group (and, as well as in the whole community in recent years), we are able to achieve several exceptional results which are beyond the state of the art. For example, our goal in WP1 is to grow high quality QD samples with high homogeneity and low initial fine structures splitting (FSS). In the publication Nature Communications 8, 15501 (2017), we show that all measured QDs in our newly developed samples can emit single pairs of entangled photons with ultra-high purity and high degree of entanglement. One goal in WP2 is to achieve a bright QD sample that can emit 10 million counts per second into the first collection lens. This was a good estimation by considering the developments in the community in year 2016 (when I applied for the ERC grant). In 2018 we built a broadband optical antenna with an extraction efficiency of 65% (i.e. more than 40 million counts/s on the detector and 4 times better than our best estimation previously) and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. This is to date among the brightest entangled photon sources.
We have successfully demonstrated the scalability of semiconductor based quantum light sources. Together with our initial experiments on QDs emitting at telecom wavelengths, these results finally allow the semiconductor QD-based quantum technology to leap out of the lab.
We have successfully demonstrated the scalability of semiconductor based quantum light sources. Together with our initial experiments on QDs emitting at telecom wavelengths, these results finally allow the semiconductor QD-based quantum technology to leap out of the lab.