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Many-photon quantum entanglement

Periodic Reporting for period 2 - QLUSTER (Many-photon quantum entanglement)

Reporting period: 2020-12-01 to 2022-05-31

A second technological revolution is being prepared in the academic laboratories that proposes to exploit the most subtle concepts of quantum physics, such as coherence and entanglement, for quantum enhanced performances. These technologies promise applications in a wide range of area such as quantum communication and quantum computing. So far, while many proof-of-concept demonstrations have been achieved in the field of optical quantum technologies, scalability remains the major issue to develop full-fledged quantum computers or quantum networks. In the last few years, it has been proposed that large photonic quantum entangled states in the form of cluster and graph states, would be an enabling resource for quantum repeater networks and measurement-based quantum computing. These states of light consist of many photons, which are connected by multiple entanglement links. The advantage provided by such a state is the high degree of redundancy: if one photon is lost, for example in a quantum network, quantum entanglement remains.

While extremely powerful, many-entangled photon light sources are very difficult to produce. The currently available methods to produce such light states are mainly based on spontaneous parametric down-conversion sources that are probabilistic with a very small success probability already for two photons, making the scaling to large photon number exponentially hard. The current world record is 12 photons despite decades of efforts and without any scaling perspective.

Our goal in QLUSTER is to develop and fabricate efficient and scalable sources of many photons, a critical resource for optical quantum technologies. Different classes of cluster and graph states will be developed defined by the geometry (topology) of the photon entanglement links. The simplest form of cluster states are linear ones, forming a linear chain of photons that are entangled with their nearest neighbours only. Two-dimensional cluster states, with multiparticle entanglement links, are the holy grail for quantum technologies. In the present project we aim at:
* Producing of a linear chain of ≥20 entangled photons with an overall single photon efficiency >65% and detection rate for 20 photons significantly exceeding 1 Hz (for comparison, the detection rate in the best experiments to date would be less than 1 per day). In the near future, the same approach should allow the generation of hundreds of entangled photons.
* Synthesizing the first deterministic 2D cluster and graph states. We will investigate quasi-deterministic and deterministic production methods.
If successful, the many-photon entangled states will not only be directly useful for photon-based quantum technologies, but also enable well-controlled exploration of multi-particle quantum entanglement in yet unexplored parameter ranges.
In QLUSTER, we are developing beyond state of the art quantum dot systems with optical micro cavities for single photon sources, entangled photon sources, and develop and implement protocols for generation of multi-photon entanglement. In particular:

* We have improved single photon sources based on a semiconductor quantum dot in optical microcavities, in fully monolithic integrated systems and using the open cavity approach. With this, we have achieved a single photon source that simultaneously has very high brightness (57% in fiber), a photon purity of 98% and 97.5% indistinguishability. We also developed novel two-photon excitation schemes and demonstrated purities beyond 99.9%, while improving photon indistinguishability for this scheme from 60% to over 90%.

* We have done the most comprehensive investigation of the dynamics of the nuclear spin bath and their interaction with single quantum dot spins to date. We have developed protocols for controlling the nuclear spin bath, which resulted in experimental demonstration of extending the electron spin coherence time by nearly two orders of magnitude to more than 0.1 ms.

* We investigate alternative spin systems and developed hole-spin systems in single quantum dot and quantum dot molecule devices. This allowed us to probe hole spin dynamics for timescales up to 40µs, which is promising as alternative spin systems for the production of cluster and graph states.

* We have developed singlet-triplet qubits in quantum dot molecules, as we have shown before these might enable the direct production of 2D cluster states. We have conceived and optimized new device designs, including annular Bragg reflectors for enhanced out-coupling of light, and achieved reproducible production of such devices.

* We introduced an ultra-fast single-shot readout scheme of the quantum dot spin - and demonstrated experimentally spin read-out within 3 ns with a fidelity of 97%. This is a crucial ingredient for spin-based production of cluster and graph states.

* Based on our single photon sources and using novel linear-optical setups, we have achieved the quasi-deterministic productioon of linear (1D) cluster states of up to 4 photons at a detection rate of 10 Hz.

* We have developed cavity-compatible spin control techniques that are compatible with polarization encoding of the cluster states. With this, we were able to show spin-photon and spin-photon-photon entanglement at a rate which is 3 and 2 orders of magnitude higher than the previous state of the art, respectively.

* With linear optics, Bell-state measurements are inefficient, we have developed a novel protocol based on encoding of Bell states in graph states that strongly improves efficiency.

* We have designed and optimized protocols for the generation of cluster and graph states with simple quantum emitters.

* We have theoretically analyzed the performance of all-photonic quantum repeater schemes based on graph states, and identified parameter regions where these schemes show a clear advantage over quantum memory based approaches.
As described above, we have developed beyond state of the art single photon sources, have shown the highest spin coherence time to date in quantum dots, and produced linear cluster states with high rate. We have shown fastest spin-photon and photon-photon entanglement generation. Novel protocols have been developed which we expect to test experimentally.
We aim to demonstrate quasi-deterministic production of linear cluster states for >=6 photons soon, and hope to increase the spin-based deterministic production of cluster states to a handfull of photons - and hope to also be able to produce graph states, also possibly by advanced spin systems. In principle, if we can implement successfully nuclear spin control techniques in cavity systems, larger cluster and graph states are within reach.
Members of our consortium have already found new challenges in academia and industry also based on the excellent results they obtained in QLUSTER. Physical methods and protocols are investigated for exploitation in companies of the partners. Several companies are actively watching our progress since the availability of deterministic many-photon entanglement would change the field of optical quantum information processing (the key motivation of QLUSTER). All partners are actively disseminating results to a broader public, from single-photon production to multi-photon entanglement.
Achievements QLUSTER RP2