16 Jun 2016

Scaling up Quantum Technologies: The Rare Case When 1+1 Is Just 2

In quantum mechanics, physicists study tiny single particles –photons, electrons – and usually only a small number of them at a time. Sometimes, these particles behave like waves which gives them very unique properties that can be manipulated or controlled. In fact, it can even allow a particle to be moved through walls in a way that is not possible in our everyday world. A team of researchers at Okinawa Institute of Science and Technology Graduate University (OIST) has shown theoretically that it is possible to reliably transport not one, but two particles completely through barriers. They have recently published their results in Physical Review A.

“Only very recently people have been able to manipulate single atoms, molecules, and electrons with high reliability,” Thomas Busch, author and leader of OIST’s Quantum Systems Unit said. “Now, we want to be able to do this with two particles. It sounds trivial, but it is not.”

The wave-like particles considered by the Quantum Systems Unit are contained in traps that can be created with light or magnetic fields. In order to move a particle from one trap to another, the traps need to be brought close together to make the walls between them thinner. When this happens a phenomenon called tunnelling occurs, allowing the particle to move through the walls and from one trap to the other. However, this does not happen all at once and the particle can begin to tunnel back and forth, making it very difficult to get the entire particle through the wall and into a different trap. Physicists have found that by using two additional traps to the right of the trap containing the particle and moving the farthest right trap closer to the middle trap, then the farthest left trap also closer to the middle trap, a single particle can be fully transferred to the rightmost trap with very high reliability.

Representation of the Tunnelling Process

The particle(s) begin in the leftmost trap and when the traps are moved in a “counterintuitive” manner (first move the right trap, and then the left one, towards the middle trap), there is a 100% probability that the particle moves into the right trap because of the quantum nature of the atom. This process is called "spatial adiabatic passage.”

While this process works perfectly for one particle, it becomes exponentially more difficult trying to do it for two particles because the particles interact and repel each other.

“The more particles you have the more difficult it becomes to manipulate them,” Busch said. “1 + 1 is not simply 2, you get a lot more. You need to control the interaction as well.”

Despite the increased complexity, the researchers found that for a large range of interactions both particles were simultaneously transported to another trap, using the same method as described above.

“It is surprising that this counterintuitive method works for many particles at the same time,” Albert Benseny, first author and OIST post-doctoral scholar said. “You would think that because the particles are repelling each other they would not want to stay together, but what we see here is that they are able to move through the traps because at some point they begin to act as if they are one molecule instead of two particles, and tunnel together. From this point on 1+1 is just 2.”

It is important to find the interaction regions at which this process works for the same arrangement, as it guarantees that the process will work perfectly every single time. This kind of robustness is a pre-requisite for advances in quantum technologies.

“In quantum computing, you need processes to work with 99.99 percent certainty,” Busch said. “And these kind of techniques do work with 100 percent reliability each and every time.”

The team is currently working on speeding this process up, using it to create interesting quantum states, and finding ways to manipulate even more particles at a time.

“This is one step towards exploring the new physics that appears when quantum states have more than single particle physics,” Busch said. “This technique allows us to build quantum machines from the bottom up, while knowing we are getting exactly what we want.”

By Rebecca Hofland

For press inquiries, please contact media@oist.jp.

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