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Entanglement distribution via Semiconductor-Piezoelectric Quantum-Dot Relays

Periodic Reporting for period 2 - SPQRel (Entanglement distribution via Semiconductor-Piezoelectric Quantum-Dot Relays)

Reporting period: 2017-11-01 to 2019-04-30

Scalable sources of single and entangled photons are fundamental building blocks of quantum information science. They can be used for quantum communication, quantum teleportation, optical quantum computing, quantum networks, and, more in general, are needed to develop future quantum technologies. It is widely accepted that these technologies have the potential to change our society significantly. In order to unlock their full potential, it is essential to go beyond proof of principle experiments performed in research laboratories. This, however, turns out to be an extremely challenging task. The main reason is that any prototype of quantum device has to meet a set of rigorous criteria to be considered for the envisioned application. The ideal source of quantum light, for example, should deliver single and entangled photons on-demand, with high purity, efficiency, indistinguishability and degree of entanglement. Moreover, it should be compatible with current photonic integration technologies and, at the same time, with other quantum systems. While a large number of sources have been developed over the years, there is at present no source that can fulfill all these requirements simultaneously. As a consequence, optical quantum technologies have kick-start difficulties.
This project aims at the fabrication and study of near-ideal sources of non-classical light, which enable the construction of a quantum network for the distribution of quantum entanglement among distant parties. The sources are based on epitaxial quantum dots integrated onto innovative semiconductor-piezoelectric devices which allows for full control over the quantum-dot in-plane strain tensor. These devices, which shall be developed during the project, can deterministically generate highly indistinguishable and strongly entangled photons with high efficiency. Moreover, their emission wavelength can be finely adjusted via the application of voltages to the piezo-actuators without degrading the quality of the emitted photons. This unique feature enables the fabrication of solid-state-based quantum relays (devices that can teleport entanglement between two distant quantum dots) and allows building up artificial-natural atomic interfaces where entangled photons from quantum dots are frequency-locked to absorption resonances in atomic vapors. The vision of the project is to build up a quantum network in which remote quantum relays interfaced with warm atomic vapors are used to distributed quantum entanglement over distant nodes - a breakthrough in the field of large distance quantum communication.
During the first period we have closely followed the plans foreseen for the project and we have already achieved important scientific results:
- We have developed several cleanroom protocols for the integration of semiconductor nano- and micro-membranes onto micro-machined piezoelectric actuators: we can now routinely fabricate wavelength-tunable sources of entangled photons.
- We have extensively designed, grown, fabricated, and characterized several types of quantum dot devices and we have clearly identified the system which gives the best results in terms of single-photon purity, degree of entanglement, and indistinguishability of the emitted photons. In particular, we have demonstrated that GaAs quantum dots integrated onto micro-machined piezo-actuators and driven under two-photon resonant excitation can deterministically generate indistinguishable entangled photons with tunable wavelength and with near-unity degree of entanglement.
- We have recently demonstrated that single and entangled photons generated on-demand by a single quantum dot can be used to implement successfully a quantum teleportation protocol – an important step towards the construction of a quantum network with quantum-dot relays. As quantum networks require quantum information to be transferred among distant parties, we have also demonstrated quantum interference between single photons from two remote quantum dots.
- We have fabricated quantum light emitting diodes integrated onto piezo-actuators and interfaced single photons from quantum dots with a cloud of natural atoms, i.e. we have built an all electrically-controlled artificial-natural atomic interface for single photons. Moreover, we have experimentally and theoretically investigated the possibility to interface single photons with a rubidium-based quantum memory, another fundamental step towards the construction of a hybrid quantum network.
At the core of the project lies the fabrication of near-ideal sources of entangled photons based on strain-tunable quantum dots. To achieve this goal we developed innovative methodologies that led to a series of fundamental and technological advances:
From the fundamental point of view, we have developed an unprecedented understanding of the effect of in-plane-strain on the optical and electronic properties of semiconductor nanostructures. These studies have reached such an advanced level that we can now use strain to flip the natural quantization axis of quantum dot – a result that paves the way for the exploitation of quantum emitters in integrated quantum photonics.
The strain actuator that we have developed is the only (to the best of our knowledge) existing device that allows for full control over the in-plane strain tensor in semiconductors. This result has a technological interest which goes well beyond the field of quantum communication. In particular, we envisage that our strain-actuator is going to play a key role in the emerging field of strain-engineering of two-dimensional materials.
Our results related to the fabrication of a nearly-dephasing free source of entangled photons on-demand that can be used for quantum teleportation represents the most important breakthrough of the first period of the project, as it proves the potential of our approach for the construction of a quantum network.
During the first period, we have thus posed the basis for the successful realization of a quantum-dot based quantum network and we have already started interfacing remote quantum dots and building up atomic-natural interfaces to interconnect them. At the beginning of the second period we expect to demonstrate entanglement swapping with photons from a quantum dot, the fundamental prerequisite for the fabrication of semiconductor-piezoelectric quantum-dot relays. Before the end of the project, we envisage that we will be able to teleport entanglement between photons stemming from distant quantum relays interfaced with clouds of natural atoms and, therefore, realize our vision to construct a quantum-dot based quantum network.