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Deterministic Generation of Polarization Entangled single Photons Cluster States

Periodic Reporting for period 3 - DG-PESP-CS (Deterministic Generation of Polarization Entangled single Photons Cluster States)

Reporting period: 2019-06-01 to 2020-11-30

The vision of quantum engineering relies on the availability of devices that can prepare and control quantum states. The possibility to store single and multiple qubits, to reset, read, and write their quantum states, to engineer quantum gates, to generate and to detect entangled states, to interface qubits realized in different physical media and to generate cluster states of entangled qubits.

Despite the fact that quantum optics is a mature field with an impressive toolkit of devices for handling in particular photonic qubits, photons are unlikely to be the medium of choice for tasks that rely on localization and interactions between them. For example, 2-qubit gates rely on interactions between the qubits and therefore cannot be realized in a deterministic manner with photons alone. As a consequence, entangled states with only a limited number of photons have been demonstrated so far and only in a probabilistic manner.

We propose to overcome this problem by using confined electronic spin in a single semiconductor quantum dot. The electronic spin strongly interacts with light. Due to this interaction the spin will act as a needle in a knitting machine and it will entangle the polarization states of sequentially emitted single photons resulting from periodic optical excitation of the quantum dot confined electronic spin.

The overall objective of our proposal is therefore to demonstrate prototype devices, capable of deterministic generation of cluster states of polarization entangled photons.
Such cluster states are invaluable resources for quantum information processing.
The first 30 months of the project have been a success, well exceeding our expectations. Very close to the beginning of the project, we made a major breakthrough, which gained worldwide recognition, resulting in many invitations for conferences and workshops. The breakthrough was the first demonstration of a prototype device capable of generating a one dimensional cluster state of polarization entangled photons, in which the entanglement lasted up to about five sequential qubits. Following this breakthrough, we proceeded as planned, devoting a major effort into constructing and building a state-of-the-art experimental system, which is required in order to capitalize on our initial success and further achieve the ambitious goals of our proposal.

Our research efforts resulted so far in 4 publications and about 25 invited conferences and workshops presentations.
The demonstration of a prototype device (“quantum knitting machine”) for deterministic generation of a cluster state of entangled photons is clearly a breakthrough and a progress far beyond the state of the art.

We continue to develop our experimental setups and studies in order to achieve the following improvements until the end of the project:
a) Increasing the robustness of the entanglement beyond 5 qubits by enhancing the radiative rate of the biexciton decay, using the Purcell effect.
b) Demonstrating cluster states of higher dimensionality (entanglement connectivity) using coupled quantum dots.
c) Using the confined heavy-hole as entangler, thereby increasing the indistinguishability of the cluster state photons.