Light used to locate 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 bits 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’ of very fragile quantum information into a ‘100-core’ haystack. Using lasers to control an electron, the researchers were then able to use the electron to control the behavior of the haystack, making it easier to find the needle. They were able to locate the ‘needle’ with a precision 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 highly brittle quantum information optically to a core system for storage, and to verify its imprint with minimal disturbance, an important step in the development of a quantum internet based on quantum light sources. The results are reported in the diary Natural Physics.

The first quantum computers – which will exploit the strange behavior of subatomic particles to outperform even the most powerful supercomputers – are on the horizon. To reach their full potential, one way will be needed to network them: a quantum internet. Light channels that send out quantum information are promising candidates for a quantum internet, and currently there is no better quantum light source than the semiconductor quantum dot: small crystals that are essentially artificial atoms.

However, one thing stands in the way of quantum dots and a quantum internet: the ability to temporarily compile quantum information at posts along the network.

“The solution to this problem is to store the fragile quantum information by hiding it in the 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, said. “But when we try to communicate with these cores the way we communicate with bits and pieces, it tends to ‘turn’ randomly and create a noisy system.”

The cloud of quantum bits in a quantum dot does not normally work in a collective state, making it a challenge to get information in or out of it. However, Atatüre and his colleagues showed in 2019 that when cooled to ultra-low temperatures, also using light, these nuclei can be ‘assembled’ to perform quantum dances, significantly reducing the amount of noise in the system.

Now they have shown another fundamental step to store and retrieve quantum information in the core. By controlling the collective state of the 100,000 nuclei, they were able to detect the existence of quantum information as an ‘inverted quantum bit’ at an ultra-high precision of 1.9 parts per million: enough for a single bit in to see the cloud turn. of nuclei.

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

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

By speaking to the electron, the chaotic ensemble of turn begins to cool and converge around the shepherd electron; from this more ordered state, the electron can create spin waves in the nuclei.

“If we imagine our turn from the cloud as a flock of 100,000 sheep at random, one sheep that suddenly changes direction is hard to see,” Atatüre said. “But when the whole flock moves like a well-defined wave, a single sheep changing direction becomes very noticeable.”

In other words, injecting a rotating wave from a single core-rotating flip into the ensemble makes it easier to detect a single core-rotating-flip under 100,000 core-rotating.

Using this technique, the researchers are able to send information to the quantum bit and, with a minimal disturbance, ‘listen in’ to what the turn says, up to the fundamental limit set by the quantum mechanics.

“Once we have harnessed this control and observation capability over this large ensemble of nuclei, our next step will be to demonstrate the storage and retrieval of an arbitrary quantum bit from the nuclear spin register,” said co-first author Daniel Jackson, a Ph.D. .D. . student at the Cavendish Laboratory.

“This step will connect a quantum memory connected to light – an important building block on the way to the quantum internet,” said co-author Dorian Gangloff, a research fellow at St John’s College.

In addition to its potential use for a future quantum Internet, the technique can also be useful for the development of solid-state quantum computers.