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Many-photon quantum entanglement

Periodic Reporting for period 1 - QLUSTER (Many-photon quantum entanglement)

Reporting period: 2019-12-01 to 2020-11-30

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 day18). 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 (Fig. 4) 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.
We have designed, fabricated and characterized two world-class single photon sources. We also used them to produce for the first time a 4-photon cluster state with high rate, using a delay-loop entangling apparatus. A similar apparatus was used to create complex entangled quantum states of light, but also for the first time producing coherent (laser) like light from single photons. Quantum dot molecules were developed and characterized that promise improved coherence times. Nuclear spins were gotten under control, potentially lifting one long-standing disadvantage of the GaAs platform. Theoretically, quantum repeater performance based on graph states was analyzed and regions of clear advantage identified.
We are on the path towards producing tens of entangled photons, best evidence are many tens of coherent operations done on a single quantum dot spin showing sufficient coherence time to entangle many tens of photons.
Achievements QLUSTER 1st year