In order to effectively use a quantum computer, a larger number of entangled basic building blocks are needed to carry out computational operations. 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. Following a novel technique, the researchers generated 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 faci­litate the construction of powerful and robust quantum computers, and serve the secure transmission of data in the future.

“Photons are particularly well suited for this because they are robust by nature and easy to manipulate," says doctoral student Philip Thomas. Together with colleagues from the quantum dynamics division led by Gerhard Rempe, he has now succeeded in taking an important step towards making photons usable for techno­logical appli­cations such as quantum computing: For the first time, the team generated up to fourteen entangled photons in a defined way and with high efficiency. “The trick to this experiment was that we used a single atom to emit the photons and interweave them in a very specific way,” says Thomas. To do this, the researchers placed a rubidium atom at the center of an optical cavity. With laser light of a certain frequency, the state of the atom could be precisely addressed. Using an additional control pulse, the researchers also speci­fically triggered the emission of a photon that is entangled with the quantum state of the atom. 

“We repeated this process several times and in a previously determined manner,” Thomas reports. In between, the atom was mani­pulated in a certain way. In this way, it was possible to create a chain of up to fourteen light particles that were entangled with each other by the atomic rotations and brought into a desired state. “To the best of our knowledge, the fourteen inter­connected light particles are the largest number of entangled photons that have been generated in the laboratory so far,” Thomas says. But it is not only the quantity of entangled photons that marks a major step towards the develop­ment of powerful quantum computers – the way they are generated is also very different from conventional methods.

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

The method used by the Garching team, on the other hand, allows basically any number of entangled photons to be generated. In addition, the method is parti­cularly efficient – another important measure for possible future technical appli­cations: “By measuring the photon chain produced, we were able to prove an efficiency of almost fifty percent,” says Thomas. 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. “All in all, our work removes a long-standing obstacle on the path to scalable, measurement-based quantum computing,” summarizes Gerhard Rempe the results.

The scientists want to remove yet another hurdle. Complex computing operations for instance would require at least two atoms as photon sources in the resonator. The quantum physicists speak of a two-dimensional cluster state. “We are already working on tackling this task,” reveals Thomas. The researcher also emphasizes that possible technical appli­cations extend far beyond quantum computing: “Another application example is quantum communi­cation” – the tap-proof trans­mission of information, for example by light in an optical fiber. There, the light experiences unavoidable losses during its propagation due to optical effects such as scattering and absorption – which limits the distance over which data can be trans­ported. Using the new method developed, quantum information could be packaged in entangled photons and could also survive a certain amount of light loss and enable secure communi­cation over greater distances. (Source: MPQ)

Reference: P. Thomas et al.: Efficient generation of entangled multiphoton graph states from a single atom, Nature 608, 677 (2022); DOI: 10.1038/s41586-022-04987-5

Link: Quantum Dynamics Group, Max-Planck-Institute of Quantum Optics, Garching, Germany