China Breaks Record for Quantum Memory Entanglement Distance

As quantum computing makes technological strides that will allow for its general adoption, many ancillary areas of research need to be explored for it to be usable in the real world. Researchers in China have now managed to entangle two quantum memories (devices that can store information on quantum states for retrieval at a later time) across the greatest distance ever recorded - 12.5 Km. The step brings the concept of a quantum internet closer to fruition: one that allows decentralized communication between quantum computers.

Working with the University of Science and Technology of China and the Jinan Institute of Quantum Technology, the researchers showed that the entangled quantum memories could maintain coherence even when they have an urban environment between them. It is because it was already a known element of entanglement - the process where two quantum units (such as qubits or quantum memories) correlate so that their states - and thus, content - can’t be described separately.

Theoretically, entanglement can be maintained irrespective of distance. The issue is that quantum units’ sensitivity to environmental disruptions such as electromagnetic or thermal interference (also called noise) has the side-effect of collapsing their states, leading to a loss of coherence and entanglement - and thus, information.

The researchers built on their previous 2020 experiments, where they managed to entangle two different qubits across 50 km of fiber optics cabling. However, that feat was achieved within the same lab - the fiber was scaled as much as possible without environmental interference breaking the qubit’s entanglement. It also facilitated control of the qubit’s environment.

This evolution of the initial effort transferred a photon between two different labs, speaking to the improvements in quantum transmission and quantum entanglement stability.

“In 2020, we published a paper in which we demonstrate the entanglement of two quantum memories via a fiber link of 50 km,” Xiao-Hui Bao, one of the researchers who carried out the study, told Phys.org. “In that experiment, both two memories we used were located within one lab and thus not fully independent. The next step in our research was to make the two memories fully independent while placing a long distance between them.”

Currently, physics demands that quantum information always be sent through classical methods such as a fiber optics cable. So the researchers created two quantum ensembles (one in each lab). In the first lab, they entangled one quantum memory, A, which was then hit with a laser, adding energy to it (a process called excitation).

This excess energy is immediately emitted as a photon as the quantum memory naturally returns to its ground state. Furthermore, these photons are inherently entangled according to the quantum memory that emitted them. The researchers then used a fiber optics cable to transmit this emitted photon from an original node, Node 1, through the 12.5 km separating it from the destination node, Node 2. 

This photon arriving in Node 2 meant that the researchers could now use its quantum state information to entangle a new quantum memory, B. Due to it sharing the same state (or at least a correlationally-equivalent state) as the original quantum memory, the two different quantum memories are now said to be entangled - despite the 12.5 km between them.

Transmitting a single photon through 12.5 km of fiber optics without any loss in fidelity is no mean feat - especially considering the emitted photon’s low energy level (near-infrared, at 725 nm), which renders them particularly susceptible to interference from higher-energy-level particles or waves. To circumvent the photon’s low energy, the researchers used “the quantum frequency conversion technique to shift the photon’s wavelength to 1342 nm instead, which improves the overall transmission efficiency significantly.”

The research furthers the arrival of a quantum internet - one where quantum information can more efficiently - and more safely - be sent from one node to another. Furthermore, because the photon is so sensitive to outside interference (remember noise?), anyone attempting to intercept the photon to access its contents would lead it to collapse, thus losing the desired information. It could lead to a new era of quantum-secure communications.

It would also open up the door towards the decentralized operation of quantum computers, which, instead of being located within a single building, could now follow a distributed design, with the same quantum computer transmitting required information from one node to another as needed - a remarkable and necessary step towards a quantum future.