Quantum Memory Entangled Over 50 KM in Optic Fibre Cable

Quantum principles have become the new craze, whether in computing or internet. Scientists are increasingly engaging with quantum computing and also quantum internet for faster transfer of data and super fast computing.

Quantum entanglement is a necessary condition for both quantum computing and quantum internet.   A Chinese team has reported to have achieved a step further in turning quantum entanglement into a reality. In an article published in Nature, a team of researchers reported that they have been successful in achieving quantum memory entangled over a 50 kilometres long coiled fibre cable.

Quantum entanglement is what Einstein called ‘spooky action at a distance’—where two particles are linked and become reliant on each other even if they are thousands of kilometres apart. This theory of quantum entanglement is one of the guiding principles of quantum communication that scientists have been thriving to realise.

Quantum memory is analogous to classical computing memory, with the only difference that it is stored as quantum of information, whereas the classical computing memory is stored in the form of bits. Stored quantum information can also be used at a later time.

“The main significance of this paper lies in extending the entangling distance in [optical] fibre between quantum memories to the city scale," said team leader Jian-Wei Pan, University of Science and Technology of China.

In this experiment, rubidium atoms were used for two storage units of quantum memory. The rubidium atoms were chilled down to a low energy state. Afterwards, when they were coupled with entangled photons, each of the rubidium atoms became part of an entangled system. 

The greater the lengths the photon needs to go through to move between atoms, the greater the risk of the system being disturbed. In this regard, the new research finding is very impressive. 

Earlier, photon particle entanglement was successfully done in empty space and optic fibres at large distances. But adding quantum memory to the entanglement was a tough thing to do. The researchers adopted a different type of strategy where atom-photon entanglement could be done over successive nodes. In this case, the atoms are the nodes and the photons transmit the messages. In other words, photon entanglement has been mixed with atomic matter for producing extra efficiency, reliability and stability. 

Two sets of experiments were used in the research. In the first experiment, a small cloud of atoms were placed in a desired quantum state, which represented a memory state. The reading and writing operations were done using photons. To employ the memory state the researchers allowed photons to interact with atoms in the cloud. After the memory state was set, the cloud emitted a photon which is the signal that the photon is ready.

The photon was polarised hereafter. This allowed the photon to carry information regarding the state of the memory collective. The complicated system was found to be 30% more efficient. 

In the second experiment, two quantum bits of memory from photons were created and sent them through a 50 kilometres long coiled fibre cable.