The optical effect takes quantum computing with atomic qubits to a new dimension

Darmstadt physicists have developed a technique that could overcome one of the biggest hurdles in building a practically relevant quantum computer. They make use of an optical effect discovered here by British photography pioneer William Talbot in 1836. The team led by Malte Schlosser and Gerhard Birkl from the Institute of Applied Physics at the Technische Universitt Darmstadt present this success in the magazine Physical Review Letters.

Quantum computers are capable of solving certain tasks much faster than even supercomputers. However, so far there have only been prototypes with up to a few hundred “qubits”. These are the basic units of information in quantum computing, corresponding to “bits” in classical computing. However, unlike bits, qubits can process the two values ​​”0″ or “1” simultaneously rather than one after the other, which allows quantum computers to perform a great many calculations in parallel.

Quantum computers with many thousands, if not several million, of qubits would be needed for practical applications, such as optimizing complex traffic flows. However, adding qubits consumes resources, such as laser output, which have so far hindered the development of quantum computers. The Darmstadt team has now demonstrated how the optical Talbot effect can be used to increase the number of qubits from several hundred to over ten thousand without requiring proportionally additional resources.

Qubits can be made in several ways. Tech giants like Google, for example, use artificially manufactured superconducting circuit elements. However, individual atoms are also excellent for this purpose. To control them in a targeted way, single-atom qubits must be kept in a regular lattice, similar to a chessboard.

Physicists usually use an “optical lattice” of regularly arranged points of light for this, which is formed when laser beams cross each other. “If you want to increase the number of qubits by a certain factor, you also need to increase the laser output accordingly,” explains Birkl.

His team manufactures the optical reticle in an innovative way. They aim a laser at a glass element the size of a fingernail, on which tiny optical lenses similar to a chessboard are arranged. Each microlens bundles a small portion of the laser beam, thus creating a plane of focal points, which may contain atoms.

Now, the Talbot effect is happening above, which until now has been considered a nuisance: the layer of focal points repeats several times at equal intervals; what are known as “self-images” are created. Therefore, a 2D optical lattice becomes a 3D one with many times the light points. “We get it for free,” says Malte Schlosser, the lead author of the work. It means that no additional laser output is needed for this.

The high-precision manufacturing of microlenses leads to very regularly arranged self-images, which can be used for qubits. The researchers were able to actually charge the extra layers with individual atoms. With the provided laser output, 16 such free layers were created, potentially allowing for more than 10,000 qubits. According to Schlosser, conventional lasers can be used to quadruple the power in the future.

“The field of the microlenses can also be optimized further,” explains Birkl, for example by creating more focal points with smaller lenses. In the near future, 100,000 qubits and more will therefore be possible. The scalability in the number of qubits shown by the team represents an important step towards developing viable quantum computers.

Schlosser points out that the technology isn’t limited to quantum computers. “Our platform could also potentially be applicable to high-precision optical atomic clocks.” The Darmstadt team plans to further develop its new qubit platform and foresees a variety of possible applications in the field of quantum technologies.