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

The researchers 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 highly fragile quantum information into a ‘haystack’ of 100,000 cores. Using lasers to control an 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 the ‘needle’ with an accuracy of 1.9 parts per million: high enough to detect a single quantum bit in this large array.

The technique allows optically sending highly fragile quantum information to a nuclear system for storage and verifying its impression with the least amount of disturbance, an important step in the development of a quantum internet based on quantum light sources. The results are reported in the newspaper Nature Physics.

The first quantum computers – which will take advantage of the strange behavior of subatomic particles to overcome even the most powerful supercomputers – are on the horizon. However, to take full advantage of their potential, you will need a way to network them: a quantum internet. Channels of light that transmit quantum information are promising candidates for a quantum internet, and today there is no better source of quantum light than the semiconductor quantum dot: tiny crystals that are essentially artificial atoms.

However, one thing gets in the way of quantum dots and the quantum internet: the ability to temporarily store quantum information on test stations across the network.

“The solution to this problem is to store fragile quantum information, 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 Cambridge Cavendish Laboratory, who led the search . “But if we try to communicate with these cores as we communicate with bits, they tend to ‘spin’ randomly, creating a noisy system.”

The cloud of quantum bits contained in a quantum dot does not normally act in a collective state, making it challenging to get information into or out of 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 do ‘quantum dances’ in unison, significantly reducing the amount of noise in the system.

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

“Technically, this is extremely demanding,” said Atatüre, who is also a member of St John’s College. “We don’t have a way to ‘talk’ to the cloud and the cloud doesn’t have a way to talk to us. But we can talk to an electron: we can communicate with it like a dog like flocks of sheep.”

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

When talking to the electron, the chaotic set of spins begins to cool and form around the shepherd electron; outside this more orderly state, the electron can create spin waves in the nuclei.

“If we imagine our spin cloud as a flock of 100,000 sheep moving at random, it is difficult to see a sheep suddenly changing direction,” said Atatüre. “But if the whole herd is moving like a well-defined wave, then a single sheep changing direction becomes highly noticeable.”

In other words, injecting a spin wave composed of a single nuclear spin spin into the pool makes it easier to detect a single nuclear spin spin among 100,000 nuclear spins.

Using this technique, researchers are able to send information to the quantum bit and ‘hear’ what the spins are saying with the least amount of disturbance, up to the fundamental limit set by quantum mechanics.

“Having taken advantage of this control and detection capability over this large set of cores, our next step will be to demonstrate the storage and retrieval of an arbitrary quantum bit from the nuclear spin record,” said co-first author Daniel Jackson, a Ph.D . student at the Cavendish Laboratory.

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

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