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Quantum computers will one day be able to solve problems at high speed that cannot be handled by classical computer systems. However, in order for these computers to become practical, they must process a significantly higher number of qubits and have lower error rates. A research project led by Professor Stefanie Barz from the University of Stuttgart is now developing a photonic quantum processor for this purpose. This processor will allow the realization of quantum algorithms with only a few qubits and, in the future, should enable rapid scaling to qubit numbers that are relevant for practical applications.

There are many different approaches to researching new, scalable quantum processors: atom and ion traps, superconductors, semiconductors, or entangled photons. In PhotonQ, funded by the German Federal Ministry of Education and Research (BMBF) with a total of EUR 16 million, the universities of Stuttgart, Würzburg, Mainz, and Ulm, the Technical University of Munich, the Institute for Microelectronics Stuttgart, and Vanguard Automation GmbH will develop a photonic quantum processor. The heart of the quantum processor is an integrated photonic chip.

**Measurement-based quantum computing approach**

The starting point for a measurement-based quantum processor is a highly entangled quantum state. Entanglement means that a measurement on one particle can change the state of another particle regardless of distance. To perform universal quantum computations, adaptive measurements are made on a large entangled state adapted to the computational problem at hand. "The challenge here is to produce and process such a state in a photonic system with high efficiency and quality. The development of integrated optical components and circuits plays a central role here. Very importantly, optical losses in the system must be kept as low as possible. At the same time, there must be a high level of efficiency in the generation and detection of photons. This requires the development of new or significantly improved components in all subsystems," explained project coordinator Professor Stefanie Barz from the Institute for Functional Matter and Quantum Technologies at the University of Stuttgart. Accordingly, deterministic photon sources, scalable silicon photonic circuits, better interconnection technology, and novel single photon detectors will be realized in the PhotonQ project.

**Subproject at JGU aims at the theoretical optimization of the quantum processor**

A purely photon-based approach to quantum computing is associated with several attractive benefits not shared by other platforms when it comes to experimental generation of a corresponding system. One in particular is the fact that photonic qubits can be processed at high rates at room temperature. Unfortunately, there is a complication in that it is difficult to couple two photons together to perform a two-qubit gate as normally this requires a high level of non-linearity. "It is true that in principle, gates like this are also needed to create the entangled resource required for measurement-based quantum computing," Professor Peter van Loock of the Institute of Physics at Johannes Gutenberg University Mainz (JGU) pointed out. "But as this state can be generated before calculation of the actual quantum algorithm – "offline" so to say – and as no quantum information is involved at this stage, it is not necessary for the gates to function at all times. It is indeed possible to realize entangling gates that function only occasionally using simple linear-optics elements, i.e., beam splitters and phase shifters." The aim of Professor Peter van Loock’s team working on the theoretical JGU subproject is to optimize these linear gates for the experimental photonic quantum processors. They will be investigating new methods for the efficient implementation of the gates as well as codes for quantum error correction. The researchers expect that suitable coding will improve both the functional efficiency and the loss and error tolerance of the photonic processors.

The overall system of the quantum processor will be built at the University of Stuttgart. It will demonstrate quantum information processing with eight qubits and prove the fundamental suitability of the measurement-based principle for photonic quantum computing. Over the project period of four years from 2022 to 2025, four generations of processors will be developed, which will increase in complexity. The partners are developing special hardware components or theory and software concepts for optimizing and characterizing the processor.