A Single-Photon Source Featuring Unity Efficiency And Unity Indistinguishability For Scalable Optical Quantum Information Processing

Project Information

UNITY

Grant agreement ID: 865230

DOI

10.3030/865230

EC signature date 20 December 2019

Start date 1 September 2020

End date 28 February 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 119 637,00

EU contribution

€ 2 119 637,00

Coordinated by

DANMARKS TEKNISKE UNIVERSITET
Denmark

Periodic Reporting for period 3 - UNITY (A Single-Photon Source Featuring Unity Efficiency And Unity Indistinguishability For Scalable Optical Quantum Information Processing)

Reporting period: 2023-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

Addressing global challenges such as good health and well-being, affordable and clean energy and climate action requires new solutions brought about by new technologies. As example, the development of new drugs in the health sector currently relies on expensive time-consuming trial-and-error studies. The introduction of new drugs can be greatly accelerated using computer simulations of their effects, however this requires solutions of complex quantum chemical problems, which are intractable for the supercomputers existing today. Here, quantum technology offers a solution in the form of the quantum computer, offering unprecedented computational power, as required for drug design in the health sector.

In an optical quantum computer, the quantum information (the quantum bit) is encoded on a photon, which is the smallest particle of light according to quantum mechanics. A key component in an optical quantum computer is thus a single-photon source, which can emit single quanta of light carrying the information. However, producing a single photon in a controlled manner represents a huge scientific challenge.

In this project, we aim at producing single photons in a highly controlled deterministic manner by employing semiconductor quantum dots placed in carefully engineered nanostructures. The quality of the emitted photons is characterized by the purity of the photons, the overall efficiency of the emission and collection processes as well as the indistinguishability (“identicalness”) of the emitted photons. Using advanced numerical simulations, we will design and predict the performance of nanostructures based on a vertical “hourglass” geometry shown in the image. We will then fabricate the devices and characterize them in our quantum optics laboratory. The objective is to produce photons of far better quality than what can be achieved today, as needed for the construction of the optical quantum computer.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In the first half of the project, we have prepared designs of the light sources to be fabricated in the later stages of the project. The light emission is governed by well-known physical equations, the Maxwell equations, however predicting the behavior of a quantum dot in a semiconductor nanostructure is generally impossible with assistance from computer simulations. The design work thus requires advanced numerical simulations. While commercial software packages for this purpose exist, they do not provide direct access to the underlying physics. A first step was thus to construct a new simulation tool suitable for analyzing the performance of the structures under study.

Equipped with this new simulation tool, we proceeded to investigate and optimize an existing known single-photon source design, known as the “micropillar”. In particular, we have identified a trade-off in this design limiting the quality of the emitted photons. The quality can be improved by changing the “micropillar” design, in particular by introducing a narrow cross section at the position of the quantum dot, as shown in the image. This leads to the characteristic “hourglass” shape of the device. Using advanced numerical simulations, we have shown that the quality of the photons emitted by the “hourglass” geometry is superior.

In addition, we have installed a brand new optical laboratory allowing us to investigate the performance of real devices. The new “UNITY-LAB” includes a closed-cycle cryostat, which can cool down the semiconductor nanostructure to -269 degrees Celcius. This cooling is needed as lattice vibrations occurring at room temperature can lower the quality of the photons. The lab also includes a titanium sapphire-based pump laser to prepare the quantum dots for light emission, as well as a spectrometer and detectors needed to measure the quality of the light emission.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In the first period, we have identified specific single-photon source designs based on the “micropillar” and the “hourglass” geometries with predicted performance beyond state-of-the-art.

In the second period, we will start the nanofabrication of semiconductor devices based on the designs. We will receive semiconductor material from the University of Würzburg which is equipped with the highly specialized “molecular beam epitaxy” machines, which can grow the material according to our specifications. The wafers will be shipped to DTU, where we will perform the nanofabrication of devices in the cleanroom at DTU Nanolab. The fabrication procedure is complex and will include lithography, reactive ion etching, wafer bonding and metal contact deposition.

Subsequently, we will use state-of-the-art excitation techniques including resonant and phonon-assisted excitation to optically characterize the fabricated devices. The expectation is that we will measure performance in terms of improved efficiency and indistinguishability far beyond state-of-the-art.

If we are successful, we will have constructed one of the key components needed for building an optical quantum computer. After the project, our ambition is to continue exploring and overcoming any further remaining barriers for its realization. Ultimately, we will realize quantum computing with full benefits for society, within quantum chemistry, drug design, energy optimization, code breaking, etc.

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