Entangled photons tailored

Physicists effectively entangle a dozen photons

The optical resonator is set up in a vacuum. A single rubidium atom is trapped between the conical mirrors inside the holder. Credit: MPQ

Physicists at the Max Planck Institute for Quantum Optics have managed to efficiently entangle a dozen photons in a well-defined way. So they are laying the groundwork for a new type of quantum computer.Their research was published in nature.


The phenomena of the quantum world, which often seem strange from the perspective of the ordinary everyday world, have long found their way into technology. For example, entanglement: the quantum physical connection between particles, connecting them over arbitrarily long distances in strange ways. For example, it could be used in quantum computers—computers that, unlike conventional computers, can perform a lot of math at the same time. However, in order to use quantum computers profitably, a large number of entangled particles must work together. They are the fundamental elements of computing, the so-called qubits.

“Photons, particles of light, are particularly suitable for this because they are inherently robust and easy to manipulate,” says Philip Thomas, a doctoral student at the Max Planck Institute for Quantum Optics (MPQ) in Garching near Munich. Now, together with colleagues from the Quantum Dynamics Department headed by Prof. Gerhard Rempe, he has successfully taken an important step towards making photons available for technical applications such as quantum computing: for the first time, the team has made a clear and efficient approach.

An atom as a photon source

“The trick with this experiment is that we use individual atoms to emit photons and interweave them in very specific ways,” Thomas said. To do this, the Max Planck researchers placed a rubidium atom at the center of an optical cavity — a type of echo chamber for electromagnetic waves. Using a laser of a certain frequency, the state of the atoms can be precisely manipulated. Using additional control pulses, the researchers also specifically triggered the emission of photons entangled with the atom’s quantum state.

Entangled photons tailored

An experimental setup with a vacuum chamber on an optical table. Credit: MPQ

“We repeated this process multiple times in a previously identified manner,” Thomas reports. In between, atoms are manipulated in some way—in technical terms: spin. In this way, it is possible to create a chain of up to 14 light particles that are entangled with each other through atomic rotation and enter the desired state. “To our knowledge, 14 interconnected particles of light are the largest number of entangled photons ever produced in the laboratory,” Thomas said.

Deterministic build process

But it’s not just the number of entangled photons that marks an important step toward the development of powerful quantum computers — they’re also produced in a way that’s very different from conventional methods. “Because the photon chain is created from a single atom, it can be created in a deterministic way,” Thomas explained. This means: In principle, each control pulse actually delivers a photon with the desired properties. So far, entanglement of photons has usually occurred in special nonlinear crystals. Cons: There, light particles are essentially randomly created and cannot be controlled. This also limits the number of particles that can be bundled into a collective state.

Entangled photons tailored

The optical resonator is set up in a vacuum. A single rubidium atom is trapped between the conical mirrors inside the holder. Credit: MPQ

On the other hand, the method used by Garching’s team basically allows the generation of any number of entangled photons. In addition, the method is particularly efficient—another important measure for possible future technological applications: “By measuring the resulting photon chains, we were able to demonstrate efficiencies approaching 50 percent,” says Philip Thomas. This means that nearly every second “button press” on the rubidium atom delivers a usable particle of light — far more than in previous experiments. “All in all, our work removes a long-standing hurdle on the road to scalable, measurement-based quantum computing,” said department head Gerhard Rempe.

More room for quantum communication

The scientists at MPQ wanted to remove another hurdle. For example, complex computational operations will require at least two atoms as the source of photons in the resonator. Quantum physicists speak of two-dimensional cluster states. “We’re already working on that task,” Philip Thomas said.

The Max Planck researchers also emphasize that possible technological applications go well beyond quantum computing: “Another application example is quantum communication” – the eavesdropping-proof transmission of information, for example via light in an optical fiber. There, light suffers unavoidable losses as it travels due to optical effects such as scattering and absorption — limiting how far data can travel. Using the method developed by Garching, quantum information can be encapsulated in entangled photons and also withstand a certain amount of light loss and enable secure communication over longer distances.


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More information:
Philip Thomas et al., Efficient Generation of Entangled Multiphoton Graphs from Single Atoms, nature (2022). DOI: 10.1038/s41586-022-04987-5

Courtesy of the Max Planck Society

Citation: Physicists effectively entangle a dozen photons (25 Aug 2022), 26 Aug 2022 from https://phys.org/news/2022-08-physicists-entangle-dozen-photons- efficiently.html retrieved

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