The German Federal Ministry of Education and Research‘s (BMBF) project ‘PhotonQ’ investigates photonic quantum computing, and has created a collaboration between seven universities, research institutions, and industrial partners.
Why are companies investing in quantum technology?
Quantum computers have the potential to eventually be able to solve problems at a high speed that cannot be handled by classical computer systems. In order for these computers to become practical, they must process a significantly higher number of qubits, and do so with minimal margins for error.
The BMBF funded research project, led by Professor Stefanie Barz from the University of Stuttgart, is now developing a photonic quantum computing processor for this purpose of practicality. The heart of the quantum processor will be an integrated photonic chip, and it will ideally allow the realisation of quantum algorithms, with only a few qubits and, in the future, enable rapid scaling to qubit numbers that are relevant for practical applications.
What are the current approaches to researching quantum processors?
There are a variety of different approaches that are being utilised to research new, scalable quantum processors, such as, atom and ion traps, superconductors, semiconductors, or entangled photons. The typical starting point for a measurement-based quantum processor is a highly entangled quantum state.
Entanglement means that a measurement of one particle can change the state of another particle regardless of distance. In order to perform universal quantum computations, adaptive measurements are made on a large, entangled state and adapted to the computational problem at hand.
How will this be considered in project PhotonQ?
Professor Stefanie Barz, project coordinator from the Institute for Functional Matter and Quantum Technologies at the University of Stuttgart, explained: “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.”
Therefore, deterministic photon sources, scalable silicon photonic circuits, better interconnection technology, and novel single photon detectors will be considered in the PhotonQ project.
This photonic quantum computing approach is photon-based, and is associated with several advantages, that are not shared by other platforms when it comes to an experimental generation of a corresponding system, such as, the higher procession rates for photonic qubits at room temperature.
However, scientists have noted that there is a potential complication in that it is difficult to couple two photons together to perform two-qubit gate, as this normally requires a high level of non-linearity.
Professor Peter van Loock, of the Institute of Physics at Johannes Gutenberg University Mainz (JGU) said: “It is true that in principle, gates like this are also needed to create the entangled resource required for measurement-based quantum computing.
“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 realise entangling gates that function only occasionally using simple linear-optics elements, such as, beam splitters and phase shifters.”
What are the teams aims and timeline regarding PhotonQ?
The research tea working on this project aim to optimise the linear gates for the experimental photonic quantum processors, investigate new methods for the efficient implementation of the gates, as well as codes for quantum error correction. It is expected that suitable coding will improve both the functional efficiency and the loss and error tolerance of the photonic processors.
The 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 from 2022 to 2025, four generations of processors will be developed, which will increase in complexity. The research team are currently developing special hardware components, or theory and software concepts, for optimising and characterising the processor.