Researchers have unveiled a new technique that they claim will make it possible to send, store and retrieve highly fragile quantum information, in an achievement that could break new ground in the field of quantum communications, and particularly in the future development of a quantum internet.
The researchers, from the University of Cambridge, designed a method to better control the behavior of a cloud of atomic nuclei in which they had injected a single particle encoded with quantum information, also called a quantum bit. This comes with a noise problem: with every nuclei spinning in a different direction within the cloud, it is near-impossible to identify the particle carrying information.
Using laser beams and a single electron, however, the physicists were able to control the spins of the nuclei, restore some order in the cloud, and as a result, detect the existence of the quantum information much easier.
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The process is similar to finding a needle in a haystack, with a little extra help from experimental physics: the researchers injected a "needle" of quantum information in a "haystack" of nuclei, and then managed to control the behavior of the haystack to make it easier to find the needle. With success: the scientists were able to pin down a single qubit among a cloud of 100,000 spinning nuclei.
As the race to develop a fully fledged quantum computer accelerates around the world, so is interest in quantum communications growing. Closely tied to quantum computing, the field is concerned with developing ways to send and receive quantum information in the form of qubits. The idea is at the heart of the quantum internet, a project pursued by many countries around the world, which seeks to create a network that will let quantum devices exchange quantum information.
Before a large-scale quantum internet sees the light of day, however, a lot of research is left to do to find out how best to encode, compress and transmit information thanks to quantum states. Typically, quantum information can be encoded in electrons that are trapped in artificially made crystals containing thousands of atoms, called quantum dots.
It is one thing to use a quantum dot to store quantum information, but it is another to then find and retrieve the data – and this is where the noisy, messy spin of the atomic nuclei is problematic.
"The solution (…) 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 Mete Atatüre, professor at Cambridge's Cavendish Laboratory, who led the research. "But if we try to communicate with these nuclei like we communicate with bits, they tend to 'flip' randomly, creating a noisy system."
The buzz of the cloud, in other words, makes it extremely challenging to retrieve information from the quantum dot. Cambridge's scientists devised a method to use the light from a laser to control an electron, which in turn can control the chaotic ensemble of nuclei spins. The electron can communicate with the cloud to create a collective spin wave that all nuclei follow – meaning that it is much easier to identify the qubit carrying quantum information when its spin flips.
The researchers compared the process to a dog herding sheep. "If we imagine our cloud of spins as a herd of 100,000 sheep moving randomly, one sheep suddenly changing direction is hard to see," said Atatüre. "But if the entire herd is moving as a well-defined wave, then a single sheep changing direction becomes highly noticeable."
With this new technique, the scientists were able to detect the existence of quantum information as a "flipped quantum bit", with levels of precision that were high enough to see a single qubit flip in the cloud of nuclei. Now that they have harnessed the potential to control the cloud of nuclei, the researchers said that the next step will be to demonstrate the actual storage and retrieval of a qubit from a quantum dot.
"This step will complete a quantum memory connected to light – a major building block on the road to realising the quantum internet," said co-first author Dorian Gangloff, a research fellow at the University of Cambridge.
The findings are likely to be met with enthusiasm from quantum communication physicists across the world. The US recently published a step-by-step strategy to realize a quantum internet, while China is breaking new records on satellite-enabled quantum networks. The EU also formed the Quantum Internet Alliance in 2018 to develop a strategy for a quantum internet.