Fourteen Entangled Photons Eliminate a Quantum Computing Bottleneck | Research & Technology | August 2022

GARCHING BEI MÜNCHEN, Germany, August 26, 2022 — Physicists at the Max Planck Institute for Quantum Optics have developed a method that could facilitate the construction of powerful and robust quantum computers, as well as the secure transmission of data. Physicists generated up to 14 entangled photons, in an optical resonator, which could be prepared into specific quantum physical states in a targeted and efficient manner.

To use a quantum computer profitably, a large number of entangled particles must work together, like qubits in the quantum system. Until now, photon entanglement usually took place in special nonlinear crystals. The downside of this method is that the photons are essentially created randomly and in a way that cannot be controlled. It also limits the number of particles that can be grouped together in a collective state.

The newly developed method allowed virtually any number of entangled photons to be generated, the researchers said.

According to Philip Thomas, a doctoral student at the Max Planck Institute for Quantum Optics, as far as scientists know, the 14 interconnected light particles constitute the largest number of entangled photons generated in the laboratory.

Photons are well suited to entanglement because they are sturdy in nature and easy to handle, he said.

The research team used a single atom to emit the photons and intertwine them in a specific way. To do this, the researchers placed a rubidium atom in the center of an optical cavity.

With laser light of a specific frequency, the state of the atom could be processed precisely. Using an additional control pulse, the researchers also specifically triggered the emission of a photon entangled with the quantum state of the atom.


Installation of an optical resonator in vacuum. A single rubidium atom is trapped between the conically shaped mirrors inside the holder. Courtesy of MPQ.


“We repeated this process several times and in a previously determined way,” Thomas said. Between the two, the atom was rotated. In this way it was possible to create a chain of up to 14 photons which were entangled with each other by the atomic rotations and brought into a desired state.

More than the quantity of entangled photons marking a major step towards the development of powerful quantum computers, it was the importance of how they were generated, which was also very different from conventional methods. “Because the string of photons emerged from a single atom, it could be produced deterministically,” Thomas said. This means that in principle each drive pulse actually delivered a photon with the desired properties.

Experimental setup with vacuum chamber on optical table.  Courtesy of MPQ.


Experimental setup with vacuum chamber on optical table. Courtesy of MPQ.


Additionally, the method is efficient – another important metric for potential future technical applications. “By measuring the photon string produced, we were able to prove almost 50% efficiency,” Thomas said. This means that almost every second “press of a button” on the rubidium atom produced a usable particle of light – far more than was achieved in previous experiments.

“Overall, our work removes a long-standing obstacle in the way of scalable, measurement-based quantum computing,” said Gerhard Rempe, director of the Max Planck Institute for Quantum Optics.

Installation of an optical resonator in vacuum.  A single rubidium atom is trapped between the conically shaped mirrors inside the holder.  Courtesy of MPQ.


Installation of an optical resonator in vacuum. A single rubidium atom is trapped between the conically shaped mirrors inside the holder. Courtesy of MPQ.


Scientists want to remove another obstacle. Complex computational operations, for example, would require at least two atoms as photon sources in the resonator. Quantum physicists speak of a two-dimensional cluster state. “We are already working on this task,” Thomas said.

He said the possible technical applications extend beyond quantum computing. “Another application example is quantum communication,” he said.

For example, using the method, quantum information could be packed into entangled photons and could also survive a certain amount of light loss and enable secure communication over greater distances.

The research has been published in Nature (www.doi.org/10.1038/s41586-022-04987-5).

Sherry J. Basler