Atom patterning breaks new number record

24 May 2019 Belle Dumé

Defect-free pattern of 100 atomic qubits: a new platform for scalable quantum computing. Credit: Gerhard Birkl

Neutral atoms trapped by light in arrays of dipole traps could be used as quantum bits for quantum computing. For such applications, however, these atoms must be positioned individually within the traps to create defect-free arrays that can then be used in information processing. Researchers at the Technische Universität Darmstadt in Germany have now developed a new technique for patterning 111 atoms in this way, so breaking the previous record, set last year, of 72 atoms. The method should even be scalable to one million atoms or more, they say.

In their experiments, the researchers, led by Gerhard Birkl, began by creating a cloud of several million rubidium atoms in a room-temperature vacuum system using a magneto-optical trap. They then cooled the atoms down to around 100 microKelvin and transferred these atoms into a microtrap array, which consists of hundreds of laser traps arranged in a square lattice. They made this lattice by directing a laser beam through an array of commercially-available microlenses.

One atom per trap

At first, each trap contained a few atoms but Birkl and colleagues succeeded in generating patterns consisting of trap sites that contained either one or no atoms. They did this using a technique called collisional blockade to remove pairs of atoms from each site. Those initially containing an odd number of atoms were left with one, and those containing an even number with zero.

Next, the researchers took an image of the pattern, which allowed them to identify the occupied and empty sites. They then filled each empty site by picking up a single atom from a filled site outside the target pattern and transporting it to an empty site inside the pattern.

“We acheived this using a single focus laser beam that we can move in 2D throughout the whole trap array,” explains Birkl. “The process is like using tweezers made out of light, which is why they are called ‘optical tweezers’. These were invented by Arthur Ashkin in 1986, who received part of the 2018 Nobel Prize in Physics for his work.”

Once they had filled all the empty sites in this way, the team then took another image of the atoms’ distribution to determine how successful their process to generate defect-free atom patterns was. “In case our control program saw any empty sites left, we repeated the assembly process one more time,” says Birkl. “Indeed, we can repeat it up to 80 times in one experimental run, which is another reason for why we can successfully create large-defect patterns.”Multiple assembly processes

The technique can produce 10×10 atom squares, a checkerboard containing 105 atoms and two interconnected squares containing 111 atoms.

“The large number (361) of currently used traps, the corresponding large number of close to 200 single atoms as a resource and the large number of repetitions of the assembly process we can apply are key to breaking the previous number record of 72,” adds Birkl. This number could be further increased to one million thanks to the microfabricated microlens arrays used in this work. “In our lab, we already have systems containing close to 10 000 lenslets and the technology to generate lens patterns with up to 1 000 000 lenslets already exists. We could approach this number with enough available laser power and improved experimental apparatus.

Scalability will be key

The research is important for many subfields of quantum technology, including quantum simulation, quantum computing, quantum measurement or even atomic clocks, he tells Physics World.

“Pivotal to further progress in all of these fields is the scalability of the physical systems employed. In quantum computing, for example, the large number of atomic qubits we could create using our technique will allow us to realize large-scale quantum memories. And quantum error correction, which is a key element in any practical quantum computer, will be possible since we can store one bit of quantum information simultaneously in many physical quantum bits.”

The researchers, reporting their work in Physical Review Letters, say they would now like to scale up their system to 1000 atoms – to start with. “We will also work on initiating two-qubit quantum gates between the atoms to build a 2D quantum processor based on so-called Rydberg interactions,” reveals Birkl, “and implement large-scale quantum entanglement and quantum simulation.”

28/5/2019 FROM PHYSICSWORLD.COM