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Milestones achieved on the path to useful quantum technologies

Researchers from the ‘Integrated Quantum Optics’ working group entangle photons, the smallest possible light particles. @ Paderborn University, Besim Mazhiqi

Tiny particles that are interconnected despite sometimes being thousands of kilometres apart – Albert Einstein called this ‘spooky action at a distance’. Something that would be inexplicable by the laws of classical physics is a fundamental part of quantum physics. Entanglement like this can occur between multiple quantum particles, meaning that certain properties of the particles are intimately linked with each other. Entangled systems containing multiple quantum particles offer significant benefits in implementing quantum algorithms, which have the potential to be used in communications, data security or quantum computing. Researchers from Paderborn University have been working with colleagues from Ulm University to develop the first programmable optical quantum memory. The study was published as an 'editor’s suggestion’ in the Physical Review Letters journal.

Entangled light particles

The ‘Integrated Quantum Optics’ group ledby Prof. Christine Silberhorn from the Department of Physics and Institute for Photonic Quantum Systems (PhoQS) at Paderborn University is using minuscule light particles, or photons, as quantum systems. The researchers are seeking to entangle as many as possible in large states. Working together with researchers from the Institute of Theoretical Physics at Ulm University, they have now presented a new approach.

On the left of the picture (green triangle) is a quantum light source that is pumped until it produces two entangled photons. A photon is then measured (yellow square) and an electronic signal is generated. The other photon goes into the memory. The core of the exper-iment can be seen on the right of the picture: a purely optical quantum polarisation memory (right square) that can be dynamically programmed via a feed-forward signal (through the black cable). This means that if a photon is detected, the ‘partner photon’ is stored until the next pair is generated. At this point, the operating mode of the programmable memory is switched over and interference is activated between the newly generated photon and the stored photon. The size of the multi-photon entangled state is thus gradually increased by repeating this process. @ Silberhorn et al. 2022

Previously, attempts to entangle more than two particles only resulted in very inefficient entanglement generation. If researchers wanted to link two particles with others, in some cases this involved a long wait, as the interconnections that promote(?) this entanglement only operate with limited probability rather than at the touch of a button. This meant that the photons were no longer a part of the experiment once the next suitable particle arrived – as storing qubit states represents a major experimental challenge.

Gradually achieving greater entanglement

‘We have now developed a programmable, optical, buffer quantum memory that can switch dynamically back and forth between different modes – storage mode, interference mode and the final release’, Silberhorn explains. In the experimental setup, a small quantum state can be stored until another state is generated, and then the two can be entangled. This enables a large, entangled quantum state to ‘grow’ particle by particle. Silberhorn’s team has already used this method to entangle six particles, making it much more efficient than any previous experiments. By comparison, the largest ever entanglement of photon pairs, performed by Chinese researchers, consisted of twelve individual particles. However, creating this state took significantly more time, by orders of magnitude.

The quantum physicist explains: ‘Our system allows entangled states of increasing size to be gradually built up – which is much more reliable, faster, and more efficient than any previous method. For us, this represents a milestone that puts us in striking distance of practical applications of large, entangled states for useful quantum technologies.’ The new approach can be combined with all common photon-pair sources, meaning that other scientists will also be able to use the method. Reference Scalable Generation of Multiphoton Entangled States by Active Feed-Forward and Multiplexing

Evan Meyer-Scott, Nidhin Prasannan, Ish Dhand, Christof Eigner, Viktor Quiring, Sonja Barkhofen, Benjamin Brecht, Martin B. Plenio, and Christine Silberhorn


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