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ERC

SEQUOIA Report Summary

Project ID: 307392
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Final Report Summary - SEQUOIA (A scalable quantum architecture)

The work conducted in this project centered around the goal of creating a scalable quantum architecture. What does this mean? By “quantum”, we mean the ability to generate and manipulate single particles of light (photons) and matter (electrons) in a coherent fashion. And, by “scalable architecture” we mean being able to make multiple copies of these single quantum states in a semiconductor platform that can be joined together or used simultaneously. A significant challenge is making the architecture ‘coherent’, which means the delicate quantum information of these quantum states is not lost via weak interactions with the environment.

Our approach is to use semiconductor quantum dots, often referred to as artificial atoms, which enables one to trap a single charge on demand and interface it with light – either to control it by a laser pulse or to generate single light particles. During the project, our ERC StG team has contributed to the following aspects of this challenging research field.
1) Understanding and taming the decoherence mechanisms impacting streams of single photons and single spins. A significant goal is to generate so-called indistinguishable photons, which means the light particles exhibit interference when they meet each other. This property drives many applications and protocols in quantum technologies. We have gained new understanding in how noise in the environment of the quantum dot, in particular fluctuating charges nearby or fluctuating nuclear spins within the quantum dot, affects the ability to generate coherent photons. We then showed how using quantum optical techniques (in particular resonance fluorescence), improved engineering of the device design, and proper selection of the quantum dot state can help mitigate these sources of noise which are ubiquitous in semiconductors. We further demonstrated that these approaches also can help improve the coherence of a single spin in a quantum dot.
2) Developing and implementing nanophotonic strategies and designs to increase the strength of the light-matter interaction. Example structures include nanowire and planar cavity antennas. A particular focus was to achieve tunability of the quantum dots in these structures using either in-situ strain or electric fields - an essential feature for scalability.
3) Developing protocols for scalability, with a particular focus on a) strategies to generate indistinguishable single photons at a precise time that can be used as a resource in a larger quantum network; and b) generating strings of photons that are entangled (quantum mechanically linked) with each other and a spin in a quantum dot.

Finally, during the course of the project, a new quantum photonic platform based on two-dimensional semiconductors emerged. Atomically thin quantum dots provide many unique opportunities for a scalable quantum architecture. First, we discovered the correlation between local pockets of strain and the formation of the quantum dots, which we later exploited to construct a deterministically positioned array of quantum dots. Further, we characterized the effect of magnetic fields, charge noise, and electric fields on the quantum dots. Recently, we successfully characterized a pristine quantum device which enable deterministic charging of electrons or holes into the quantum dot (Coulomb blockade). Further, we showed that the spins are unique and promising for coherent spin-photon interfaces. Finally, we show that we can coherently couple the localized spin in the quantum dot with a ‘sea’ of electrons in a layer of graphene. Taken in their entirety, these results open the door to construct a scalable quantum architecture using this new quantum photonic platform.

Reported by

HERIOT-WATT UNIVERSITY
United Kingdom
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