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

Researchers have found a way to use light and a single electron to communicate with a cloud of quantum pieces and sense their behavior, making it possible to detect a single quantum bit in a dense cloud.

The researchers, from the University of Cambridge, were able to inject a ‘needle’ with very fragile quantum information into a ‘straw’ of 100,000 nuclei. Using lasers to control an electron, researchers can use that electron to control the behavior of the straw, making it easier to find the needle. They were able to detect the ‘needle’ with an accuracy of 1.9 parts per million: high enough to detect a single quantum bit in this large ensemble.

The technique makes it possible to send very fragile quantum information optically to a nuclear storage system and to verify its embedding with minimal disturbance, an important step in developing a quantum Internet based on quantum light sources. The results are reported in the journal Physics of nature.

The first quantum computers – which will exploit the strange behavior of subatomic particles to surpass even the most powerful supercomputers – are on the horizon. However, fully exploiting their potential will require a way to network them: a quantum internet. Light channels that transmit quantum information are promising candidates for a quantum Internet, and there is currently no better source of quantum light than semiconductor quantum dots: tiny crystals that are essentially artificial atoms.

However, one thing stands in the way of quantum dots and a quantum internet: the ability to store quantum information temporarily in posting across the network.

“The solution to this problem is to store fragile quantum information by hiding it in the cloud of 100,000 atomic nuclei contained in each quantum dot, like a needle in a haystack,” said Professor Mete Atatüre of Cambridge Cavendish Laboratory, who led the research. . “But if we try to communicate with these nuclei as if we were communicating with pieces, they tend to ‘slide’ randomly, creating a noisy system.”

The cloud of quantum bits contained in a quantum dot does not normally operate in a collective state, making it a challenge to obtain information inside or outside of them. However, Atatüre and his colleagues showed in 2019 that when cooled to ultra low temperatures also using light, these nuclei can be made to do ‘quantum dance’ in unison, significantly reducing the amount of noise in the system.

Now, they have shown another fundamental step towards storing and retrieving quantum information in nuclei. By checking the collective state of 100,000 nuclei, they were able to detect the existence of quantum information as a ‘rolled quantum bit’ with an ultra-high accuracy of 1.9 parts per million: enough to see a single bit of flip in the cloud. nuclei.

“Technically this is extremely demanding,” said Atatüre, who is also a member of St. John’s College. “We do not have a way to ‘talk’ to the reindeer and the cloud does not have a way to talk to us. “But what we can talk about is an electron: we can communicate with it like a sheepdog.”

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

Talking to the electron, the chaotic ensemble of rotations begins to calm down and gather around the grass electron; from this more regulated state, the electron can create rotational waves in the nuclei.

“If we imagine our reindeer reindeer as a herd of 100,000 sheep moving randomly, a sheep suddenly changing direction is difficult to see,” Atatüre said. “But if the whole herd is moving like a well-defined wave, then a single sheep changing direction becomes very visible.”

In other words, injecting a spin wave made from a single nuclear spin spin into the ensemble makes it easier to detect a single nuclear spin spin between 100,000 nuclear spins.

Using this technique, researchers are able to send information to the quantum particle and ‘listen’ to what the spins with minimal disturbances say, down to the basic limit set by quantum mechanics.

“Once we have utilized this control and sensory ability over this large ensemble of nuclei, our next step will be to demonstrate the preservation and retrieval of an arbitrary quantum bit from the nuclear rotation register,” said first co-author Daniel Jackson , a PhD student at Cavendish Laboratory.

“This step will complete a light-connected quantum memory – a key building block on the road to quantum Internet realization,” said first co-author Dorian Gangloff, a research fellow at St John’s College.

In addition to its potential use for a quantum internet in the future, the technique may also be useful in developing solid state quantum computing.

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

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

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