Light and one electron used to detect quantum information stored in 100,000 nuclear quantum bits

The researchers found a way to use light and one electron to communicate with a cloud of quantum bits and sense their behavior, allowing the detection of a single quantum bit in a dense cloud.

Researchers at the University of Cambridge have managed to inject a “needle” of very fragile quantum information into a “haystack” of 100,000 cores. Using lasers to control the electron, researchers could then use that electron to control the behavior of the haystack, making it easier to find the needle. They were able to detect a “needle” with an accuracy of 1.9 parts per million: high enough to detect a single quantum bit in this large whole.

The technique enables the sending of very fragile quantum information optically into a nuclear system for storage and verification of their footprint with minimal interference, which is an important step in the development of a quantum internet based on quantum light sources. The results were published in the journal Physics of nature.

The first quantum computers are on the horizon – which will use the unusual behavior of subatomic particles to surpass even the most powerful supercomputers. However, exploiting their full potential will require a way of networking: quantum internet. The light channels that transmit quantum information are promising candidates for quantum internet, and there is currently no better quantum light source than a semiconductor quantum dot: tiny crystals that are basically artificial atoms.

However, one thing stands in the way of quantum dots and the quantum internet: the possibility of temporarily storing quantum information on posting along the network.

“The solution to this problem is to store fragile quantum information by hiding in a cloud of 100,000 atomic nuclei that each quantum dot contains, like a needle in a haystack,” said Professor Mete Atatüre of the Cavendish Laboratory in Cambridge, who led the research. “But if we try to communicate with these cores as we communicate with bits, they accidentally ‘rotate’, creating a noisy system.”

The cloud of quantum bits contained in a quantum dot usually does not operate in a collective state, which poses a challenge for obtaining information from or from them. However, Atatüre and his colleagues showed in 2019 that when cooled to ultra low temperatures, also using light, these cores can be made to “play quantum” at an angle, significantly reducing the amount of noise in the system.

They have now shown another fundamental step towards storing and finding quantum information in nuclei. By controlling the collective state of 100,000 nuclei, they were able to detect the existence of quantum information as an “inverted quantum bit” with extremely high precision of 1.9 parts per million: enough to see how one bit rotates in a cloud of nuclei.

“Technically, this is extremely demanding,” said Atatüre, who is also an associate at St John’s College. “We have no way to” talk “to the cloud, and the cloud has no way to talk to us. But what we can talk to is an electron: we can communicate with it in some way like a dog chasing sheep. “

Using light from a laser, researchers are able to communicate with an electron, which then communicates with the spins or inherent angular momentum of the nucleus.

Talking to the electron, the chaotic whole of the spins begins to cool and gather around the pastoral electron; from this more ordered state, the electron can create spin waves in the nucleus.

“If we imagine our cloud spinning like a flock of 100,000 sheep moving randomly, it’s hard to see one sheep abruptly changing direction,” Atatüre said. “But if the whole flock is moving like a well-defined wave, then one sheep changing direction becomes very noticeable.”

In other words, injecting a spin wave made from one turn of nuclear spin into an ensemble makes it easier to detect one turn of nuclear spin between 100,000 nuclear spins.

Using this technique, researchers are able to send information to the quantum bit and “listen” to what the spins are saying with minimal interference, all the way to the basic limit set by quantum mechanics.

“Once we have taken advantage of this control and sensory capability over this large group of nuclei, our next step will be to demonstrate storing and finding an arbitrary quantum bit from the nuclear spin registry,” said co-author Daniel Jackson, PhD at Cavendish Laboratory.

“This step will complete the quantum memory associated with light – the main building block on the road to quantum internet,” said co-author Dorian Gangloff, a researcher at St John’s College.

In addition to being potentially used for future quantum internet, the technique could also be useful in developing a solid state quantum computation.

Reference: February 15, 2021, Physics of nature.
DOI: 10.1038 / s41567-020-01161-4

The research was partly supported by the European Research Council (ERC), the Engineering and Physical Research Council (EPSRC) and the Royal Society.