Light 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 bits and detect their behavior, which can detect a single quantum bit in a dense cloud.

The researchers at Cambridge University were able to inject a “needle” of very fragile quantum information into a “haystack” of 100,000 cores. By 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 assembly.

The technique makes it possible to optically send very fragile quantum information to a nuclear system for storage, and to verify its footprint with minimal disturbance, an important step in the development of 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 bizarre behavior of subatomic particles to far surpass the most powerful supercomputers – are on the horizon. However, harnessing their full potential will require a way to network them: a quantum internet. Channels of light that transmit quantum information are promising candidates for a quantum internet, and currently there is no better source of quantum light than the semiconductor quantum dot: tiny crystals that are essentially man-made atoms. .

However, one thing stands in the way of quantum dots and a quantum internet: the ability to temporarily store quantum information at transit stations 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. . “But if we try to communicate with these cores like we communicate with bits, they tend to ‘flip’ randomly, creating a noisy system.”

The quantum bit cloud contained in a quantum dot does not normally act in a collective state, making it difficult to get information inside or outside of them. However, Atatüre and colleagues showed in 2019 that when cooled to ultra-low temperatures using light as well, these nuclei can be made to do “ quantum dances ” in unison, dramatically reducing the amount of noise in the system.

Now they have shown another fundamental step towards the storage and retrieval of quantum information in nuclei. By monitoring the collective state of 100,000 nuclei, they were able to detect the existence of quantum information in the form of an “ inverted quantum bit ” with ultra-high precision of 1.9 parts per million: enough to see a single bit switch in the cloud. of nuclei.

“Technically it’s extremely demanding,” said Atatüre, who is also a fellow 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 what we can talk to is an electron: we can communicate with it much like a dog that herds 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 nuclei.

Speaking to the electron, the chaotic set of spins begins to cool down and rally around the shepherd electron; out of this more ordered state, the electron can create spin waves in nuclei.

“If we imagine our spin cloud as a flock of 100,000 sheep moving at random, a sheep suddenly changing direction is hard 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, the injection of a spin wave consisting of a single nuclear spin in the set facilitates the detection of a single nuclear spin among 100,000 nuclear spins.

Using this technique, researchers are able to send information to the quantum bit and listen to what the spins say with minimal disturbance, up to the fundamental limit set by quantum mechanics.

“After exploiting this control and detection capability on this large set of nuclei, our next step will be to demonstrate the storage and recovery of an arbitrary quantum bit from the nuclear spin register,” said the co-prime author Daniel Jackson, a Ph.D. student at the Cavendish lab.

“This step will complete light-connected quantum memory, a major milestone on the road to the quantum Internet,” said co-first author Dorian Gangloff, researcher at St John’s College.

Besides its potential use for a future quantum internet, the technique could also be useful in the development of semiconductor quantum computing.