A milestone in quantum entanglement the first programmable optical quantum memory

The 2022 Nobel Prize for Physics goes to quantum entanglement: tiny particles that are interconnected despite sometimes being separated by thousands of kilometers; Einstein called this entanglement phenomenon "ghostly action at a distance," and it is a fundamental component of quantum physics. Entangled systems containing multiple quantum particles have great benefits in implementing quantum algorithms that can potentially be used for communication, data security or quantum computing.

Recently, researchers at the University of Paderborn and the University of Ulm in Germany have collaborated to develop the first programmable optical quantum memory. The results were published as "Editor's Choice" in the journal Physical Review Letters under the title "Scalable multi-photon entangled states through active feedforward and multiplexing" [1].

01Entangled photons

The group "Integrated Quantum Optics", led by Professor Christine Silberhorn of the Department of Physics and the Institute for Photonic Quantum Systems (PhoQS) at the University of Paderborn, is using tiny photons as quantum systems. The researchers are seeking to entangle as many photons as possible; together with researchers at the Institute for Theoretical Physics at Ulm University, they have come up with a new approach.

Previously, trying to entangle more than two particles together only led to very inefficient entanglement. If researchers wanted to link two particles with other particles, this would require a long wait, because the interconnections that promote such entanglement operate with only a limited probability. This means that once the next suitable particle arrives, the photon is no longer part of the experiment: because storing quantum bit states is a major experimental challenge.

Researchers in the "Integrated Quantum Optics" working group entangle photons.

Traditional methods of obtaining multi-photon entanglement require a large number of photon sources. Each source simultaneously generates an entangled photon pair, and these photons then interfere with each other. The process is probabilistic because only pairs of entanglements are successfully produced at each step, for example, once every 20 attempts. As more and more photons are attempted to be entangled, this probability decreases exponentially.

Now, a new experiment shows how to improve one's odds in this quantum game of chance. The method works similarly to an entangled assembly line, in which entangled photon pairs are created sequentially and combined with stored photons [2].

02Quantum entanglement with higher probability

"We have now developed a programmable, optically buffered quantum memory that can dynamically switch back and forth between different modes; including a storage mode, an interference mode, and finally a release mode." Silberhorn explained [3]. In the experimental setup, a small quantum state can be stored until another state is created, and then the two can be entangled together. This allows a large, entangled quantum state to "grow" particle by particle.

With this programmable optical quantum memory, Silberhorn's team has created four- and six-photon entangled states with nine and 35 times the success rate of conventional methods, respectively. In comparison, the largest photon pair entanglement ever performed by German researchers consisted of 14 individual particles; however, it took much more time to create this state.

The left side of the image (green triangle) is a quantum light source that is pumped until two entangled photons are produced. Then one photon (yellow square) is measured and an electronic signal is generated. The other photon enters the memory. The heart of the experiment can be seen on the right side of the picture: a purely optical quantum polarization memory (right square) that can be dynamically programmed by a feed-forward signal (through the black cable). This means that if a photon is detected, a "partner photon" is stored until the next pair of photons is generated. At this point, the operating mode of the programmable memory is switched and the interference between the newly generated photon and the stored photon is activated. Thus, by repeating the process, the size of the multi-photon entangled state gradually increases.

(a) Working principle of the experimental method. Bell pairs are generated sequentially. The detection of a photon triggers a feedforward including a field-programmable gate array (FPGA), which in turn controls the mode of operation of an all-optical storage loop. (b) A sketch of the experimental setup.

Silberhorn explains, "Our system allows for the gradual build-up of increasingly larger entangled states, which is more reliable, faster and more efficient than any previous approach. For us, this represents a milestone that brings us closer to practical applications of large entangled states for useful quantum technologies. This new method can be combined with all common photon pair sources, which means that other scientists will be able to use it as well."

Reference links：

[1]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.150501

[2]https://physics.aps.org/articles/v15/s135

[3]https://www.hpcwire.com/off-the-wire/milestones-achieved-on-the-path-to-useful-quantum-tech/