Entangling more than a dozen photons efficiently in a defined way

Creating a basis for a new type of quantum computer.


Entanglement plays a crucial role in quantum information science. It can be used in a quantum computer that can perform numerous mathematical operations simultaneously. To use a quantum computer efficiently, many entangled particles must work together. They are the essential elements for calculations, so-called qubits.

A team of physicists at the Max Planck Institute of Quantum Optics in Garching has now, for the very first time, demonstrated this task with photons emitted by a single atom. They could generate up to 14 entangled photons in an optical resonator, which can be prepared into specific quantum physical states in a targeted and very efficient manner. The new method could allow the construction of powerful and robust quantum computers and serve secure data transmission in the future.

This is the first time the team has generated up to 14 entangled photons in a defined way and with high efficiency.

Philip Thomas, a doctoral student at the Max Planck Institute of Quantum Optics (MPQ) in Garching near Munich, said, “The trick to this experiment was that we used a single atom to emit the photons and interweave them in a very specific way. To do this, we placed a rubidium atom at the center of an optical cavity – an echo chamber for electromagnetic waves. The atom’s state could be precisely addressed with laser light of a certain frequency. Using an additional control pulse, the researchers also specifically triggered the emission of a photon entangled with the atom’s quantum state.”

“We repeated this process several times and in a previously determined manner. In between, the atom was manipulated in a certain way – in technical jargon: rotated. In this way, it was possible to create a chain of up to 14 light particles entangled by the atomic rotations and brought into the desired state.”

“To the best of our knowledge, the 14 interconnected light particles are the largest number of entangled photons generated in the laboratory so far.”

“Because the chain of photons emerged from a single atom, it could be produced deterministically. This means: that, in principle, each control pulse delivers a photon with the desired properties. Until now, the entanglement of photons usually took place in special, non-linear crystals. The shortcoming: the light particles are created randomly and in a way that cannot be controlled. This also limits the number of particles bundled into a collective state.”

The method that scientists used allows any number of entangled photons to be generated. It is also efficient: We proved the efficiency of almost 50 percent by measuring the photon chain produced.

Thomas said“This means: almost every second “push of a button” on the rubidium atom delivered a usable light particle – far more than has been achieved in previous experiments.”

Director Gerhard Rempe said, “All in all, our work removes a long-standing obstacle on the path to scalable, measurement-based quantum computing.”

The researchers at the MPQ want to get rid of one more obstacle. For instance, two atoms would be required as photon sources in the resonator for complex computer operations. There is a two-dimensional cluster state, according to quantum physicists.

Philip Thomas said, “We are already working on tackling this task.”

Journal Reference:

  1. Thomas, P., Ruscio, L., Morin, O. et al. Efficient generation of entangled multiphoton graph states from a single atom. Nature 608, 677–681 (2022). DOI: 10.1038/s41586-022-04987-5
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