Tailor-made entangled photons – News Physics and Quantum Computing

In order to effectively use a quantum computer, more specially prepared basic building blocks – in technical terms: entangled – are needed to perform computational operations. A team of physicists from the Max Planck Institute for Quantum Optics in Garching has for the first time demonstrated this task with photons emitted by a single atom. Following a new 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 highly 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.

The phenomena of the quantum world, which often seem bizarre from the perspective of the common everyday world, have long since found their way into technology. For example, entanglement: a quantum physical connection between particles that connects them in a strange way over arbitrarily long distances. It can be used, for example, in a quantum computer – a computing machine that, unlike a conventional computer, can perform many mathematical operations simultaneously. However, to use a quantum computer profitably, a large number of entangled particles must work together. These are the basic elements of calculations, called qubits.

“Photons, particles of light, are particularly well suited for this because they are inherently robust and easy to manipulate,” says Philip Thomas, PhD student at the Max Planck Institute for Quantum Optics (MPQ) in Garching near Munich. . Together with colleagues from the Quantum Dynamics Division led by Professor 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 14 entangled photons in a defined manner and with high efficiency.

An atom as a source of photons

“The trick to this experiment was that we used a single atom to emit the photons and intertwine them in a very specific way,” says Thomas. To do this, the Max Planck researchers placed a rubidium atom in the center of an optical cavity, a sort of echo chamber for electromagnetic waves. 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 entangled with the quantum state of the atom.

“We repeated this process several times and in a previously determined way,” reports Thomas. In between, the atom has been manipulated in some way—in technical jargon: twisted. In this way it was possible to create a chain of up to 14 light particles which were entangled with each other by the atomic rotations and brought into a desired state. “To our knowledge, the 14 interconnected light particles represent the largest number of entangled photons that have been generated in the laboratory so far,” Thomas points out.

Deterministic generation process

But it’s not just the amount 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 string of photons emerged from a single atom, it could be produced deterministically,” says Thomas. This means: in principle, each control pulse actually delivers a photon with the desired properties. Until now, the entanglement of photons generally took place in special nonlinear crystals. The flaw: there, the light particles are essentially created randomly and out of control. It also limits the number of particles that can be grouped together in a collective state.

The method used by Garching’s team, on the other hand, can generate almost any number of entangled photons. In addition, the method is particularly efficient – ​​another important measurement for possible future technical applications: “By measuring the photon chain produced, we were able to prove an efficiency of almost 50%,” says Philip Thomas. 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,” summarizes department head Gerhard Rempe.

More space for quantum communication

he 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”, reveals Philip Thomas. The Max Planck researcher also points out that the possible technical applications extend well beyond quantum computing: “Another example of application is quantum communication”, the transmission of information in the test of tapping, for example by light in an optical fiber. There, light suffers unavoidable losses as it propagates due to optical effects such as scattering and absorption, which limits the distance over which data can be transported. Using the method developed in Garching, 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.

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Tailor-made entangled photons – News Physics and Quantum Computing

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