Scientists move closer to connecting cities with quantum internet

Quantum computing may still be in its early days, but scientists around the world have already started building the quantum internet. Studies conducted independently by researchers at institutes in three different countries have shown that sending quantum bits over a fiber optic cable over long distances is possible. 

Even as the biggest names in the tech industry race to build fault-tolerant quantum computers, the transition from binary to quantum can only be completed with a reliable internet connection to transmit the data. 

Unlike binary bits transported as light signals inside a fiber optic cable that can be read, amplified, and transmitted over long distances, quantum bits (qubits) are fragile, and even attempting to read them changes their state. 

Since light signals cannot be amplified, they cannot travel long distances, making them unsuitable for long-distance transmission. 

Yet, for the quantum internet to scale up rapidly, it must use the existing network of fiber optic cables. 

Three approaches for a quantum internet

Researchers in the Netherlands, China, and the US separately demonstrated how qubits could be stored in “quantum memory” and transmitted over the fiber optic network. 

Ronald Hanson and his team at the Delft University of Technology in the Netherlands encoded qubits in the electrons of nitrogen atoms and nuclear states of carbon atoms of the small diamond crystals that housed them. 

An optical fiber cable traveled 25 miles from the university to another laboratory in Hague to establish a link with similarly embedded nitrogen atoms in diamond crystals. 

At the University of Science and Technology of China (USTC), qubits were encoded into clouds of rubidium atoms. The quantum states were set using a photon, and the research team led by Pan Jian-Wei demonstrated entanglement in three separate labs located at least six miles away. 

Back in the US, Mikhail Lukin, at Harvard University, used diamond-based devices with silicon atoms in them and used quantum states of both the electrons and nucleus, much like Hanson’s lab. 

The devices were used to demonstrate entanglement at two quantum memory nodes separated by an optical fiber link deployed over a looped distance of over 22 miles (35 km), setting a record for storage, processing, and movement of quantum information. 

The road ahead

While the Chinese and Dutch teams’ approach required photons to arrive at a server with precise timing, the one used by the US scientists is relatively easier to execute. 

The two quantum nodes need to be maintained at super-frigid temperatures, but instead of getting the qubits to emit photons for entanglement, the researchers sent one photon that entangled with silicon at the first node, then traveled through the fiber optic cable to graze the silicon atom at the second node and achieved entanglement with the first. 

“Since the light is already entangled with the first node, it can transfer this entanglement to the second node,” explained Can Knaut, a student in Lukin’s lab, in a statement. “We call this photon-mediated entanglement.”

Pan Jian-Wei at USTC  told Nature that at this pace of advancement, his lab would be able to achieve entanglement over 600 miles (1,000 km) by the end of the decade.

Such a system would help transfer sensitive information using cryptographic keys, connect separate quantum computers to build a powerful one or unify an extensive network of optical telescopes located hundreds of miles into one large dish. 

The research findings from Pan and Lukin lab were published in the journal Nature and can be accessed below

Lukin Lab Paper

Pan Lab Paper.