By Leah Poffenberger

The 2019 Frontiers in Optics/Laser Science meeting in Washington, DC, jointly hosted by the Optical Society (OSA) and the APS Division of Laser Science (DLS), featured many visionary speakers with big ideas for developing new technologies. One such idea was the focus of a plenary at the conference: the creation of a quantum internet.

Ronald Hanson, Scientific Director at the QuTech research institute and a professor at the Delft University of Technology, spoke about an ambitious project to create a global quantum internet, starting in the Netherlands. The ultimate vision is a network that enables qubit exchange from anywhere in the world, requiring the ability to transmit states of quantum entanglement across long distances.

In October 1969, the first message was sent between distant interconnected computers: the “L” and “O” of “login” were transmitted from the University of California, Los Angeles, to Stanford University before the system crashed. Over just 50 years, this breakthrough has evolved into today’s internet.

Qubits network image
QuTech

Qubits entangled over long distances will form the quantum network of the future.

Since then, a number of research groups have sought to harness the peculiarities of quantum physics for long-distance communication and computing. In quantum computing, entanglement refers to an interaction between two or more particles that causes them to share a common quantum state and influence each other’s quantum behavior.

In 2017, Chinese scientists showed that photon entanglement could be maintained over 1,200 kilometers between a satellite and ground stations. Other groups have demonstrated that encryption keys generated by quantum processes can be distributed between cities, and there are even commercially available systems to do this. However, these applications rely on classical components.

Now, the goal is to create a truly quantum network with end-to-end quantum technology. The advent of a fully quantum internet would be an important step for quantum computing, allowing quantum processors to transmit information from quantum bits, or qubits, to one another instantaneously.

“We want to make the first entanglement-based internet, and to do that we have to assemble a brilliant, interdisciplinary group of people,” said Hanson in his talk. “It’s a testimony to the type of challenge we have: It’s not just an optics challenge or just a physics challenge. You need people from different disciplines to work together.”

To create such a talented braintrust, Delft University and the Netherlands Organisation for Applied Scientific Research (TNO) joined forces to create QuTech. This advanced research center for quantum computing and the quantum internet provides a place for physicists, computer scientists, and optics engineers to realize the ultimate goal of a network of quantum computers.

QuTech has demonstrated the viability of a fully quantum internet on Delft University’s campus, stretching the entanglement distance from just 3 meters in 2013 to 1.3 kilometers in 2015. And Hanson says they have their sights set on a longer distance for 2020: demonstrating quantum entanglement over 30 kilometers, spanning from the university in Delft to The Hague city center making use of pre-existing telecommunication infrastructure.

To create this interaction from qubits that exist meters to kilometers apart, QuTech is using photons transmitted by fiberoptic cables—the same cables that carry regular internet—to mediate entanglement between particles. These entangled states are what allow for the quantum internet to act as a vehicle for cryptographic key distribution, which allows two quantum computers to trade information in an ultra-secure way.

“The idea here is that you would distribute the sequence key between two parties, and the security of the key actually comes from the physical principles [of the system], and this is something you can’t do in classical machines,” said Hanson.

While there is some focus on creating entanglement over increasing distances, like the 2020 goal, two other areas are also crucial to advancing the quantum internet. Past tests show areas for growth in both entanglement speed—or how quickly the qubits are able to interact—and entanglement fidelity, the stability of the qubit interaction.

“In 2015…we actually got three world records in one experiment: We got the largest distance between two entangled qubits, the entanglement fidelity was the highest anyone had seen,” said Hanson. “But we also got the slowest entangling that anyone had ever done, and that’s not a good record to have.”

Since then, QuTech has been able to increase entanglement speed, leading to higher quantum link efficiency—a term that describes the speed of entanglement happening versus its fidelity—which is a crucial step in creating multi-node networks. QuTech has also made strides towards transmitting entanglement-mediating photons over longer distances by changing their wavelengths.

“We have seen quite some progress in the past years. We can now generate entanglement faster than we lose it, and we have a way of connecting through to the telecom [infrastructure]. And that brings us through the goals for next year: The first goal will be to actually create the first multi-node network. The second goal is to make entanglement between Delft and The Hague—it will be for us the first test case of creating entanglement outside the university,” said Hanson. “Maybe in the long run, we can look back at this time, the experiments that we and other people are doing, and talk about it the same way as we talked about 1969: That these were all crucial steps towards the global quantum internet that we're all enjoying.”

Further Reading

S. Wehner, D. Elkouss, and R. Hanson, "Quantum internet: A vision for the road ahead," Science 362, 303 (2018).

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