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New Proposal for Long-Distance Quantum Networks

In a recent study published in the journal, Physical Review Letters, researchers from the California Institute of Technology and Stanford University collaborated and described a novel approach to constructing a nationwide quantum network utilizing vacuum beam guides, which enable qubits to travel hundreds of miles within tiny, vacuum-sealed tubes.

quantum network
To make a quantum network a reality, researchers in Jiang Group at the University of Chicago Pritzker School of Molecular Engineering have proposed building long quantum channels using vacuum-sealed tubes with an array of spaced-out lenses. Image Credit: Jiang Group

Quantum computing presents significant opportunities to advance data processing, communications, cybersecurity, and other areas. However, to create quantum networks or a quantum internet, several quantum computers must be coupled to enjoy these advantages fully. Creating networks that must transfer quantum information over great distances has proven difficult for scientists.

Researchers at the University of Chicago's Pritzker School of Molecular Engineering (PME) have suggested a novel method for creating lengthy quantum channels using vacuum-sealed tubes with a range of widely spaced lenses.

With a diameter of around 20 cm, these vacuum beam guides would be superior to any current quantum communication method. They would possess capabilities of 10 trillion qubits per second and ranges of millions of kilometers. Through the vacuum tubes, photons of light carrying quantum information would travel and, because of the lenses, stay focused.

We believe this kind of network is feasible and has a lot of potential, it could not only be used for secure communication, but also for building distributed quantum computing networks, distributed quantum sensing technologies, new kinds of telescopes, and synchronized clocks.

Liang Jiang, Study Senior Author and Professor, Pritzker School Molecular Engineering, University of Chicago

Sending Qubits

Quantum computers rely on qubits, which can exhibit quantum phenomena, whereas classical computers encode data in traditional bits, which are represented as a 0 or 1. These phenomena include entanglement, which permits two quantum particles to be correlated with one another even over great distances, and superposition, which is an ambiguous combination of states.

These characteristics enable quantum computers to store and transfer information in novel, secure ways and evaluate new kinds of data. Connecting several quantum computers can make their combined data processing capacity even more potent. Conventional computer networks, however, are not optimal because they cannot preserve the quantum characteristics of qubits.

You cannot send a quantum state over a classical network, you might send a piece of data classically, a quantum computer can process it, but the result is then sent back classically again.

Liang Jiang, Study Senior Author and Professor, Pritzker School of Molecular Engineering, University of Chicago

Researchers have experimented with sending optical photons, which can function as qubits through satellites and fiber-optic links. While photons can pass through current fiber-optic cables for a little distance, they are immediately absorbed and lose their information.

The vacuum of space reduces the number of photons absorbed when they bounce off satellites and land in new locations, but the availability of satellites and the atmosphere's absorption of light limit how far they can travel.

What we wanted to do was to combine the advantages of each of those previous approaches. In a vacuum, you can send a lot of information without attenuation. But being able to do that on the ground would be ideal.

Yuexun Huang, Study First Author and Graduate Student, Pritzker School of Molecular Engineering, University of Chicago

Learning from LIGO

Scientists at the California Institute of Technology's Laser Interferometer Gravitational-Wave Observatory (LIGO) have constructed large ground-based vacuum tubes to hold moving photons of light that can detect gravitational waves. Photons may travel thousands of km inside a practically molecule-free vacuum, as demonstrated by experiments conducted at LIGO.

Jiang, Huang, and associates were inspired by this technique and started to design miniature vacuum tubes to move photons between quantum computers. In their latest theoretical work, they demonstrated that these tubes could transport photons throughout the nation, provided they were correctly ordered and built.

Rather than the ultra-high vacuum (10-11 atmosphere pressure) needed for LIGO, they would need a medium vacuum (10-4 atmosphere pressure), which is far easier to maintain.

Jiang explained, “The main challenge is that as a photon moves through a vacuum, it spreads out a bit, to overcome that, we propose putting lenses every few kilometers that can focus the beam over long distances without diffraction loss.”

Working with Caltech researchers, the team is preparing tabletop tests to validate the concept before utilizing larger vacuum tubes, like those at LIGO, to investigate lens alignment and long-distance photon beam stabilization.

Jiang said, “To implement this technology on a larger scale certain poses some civil engineering challenges that we need to figure out as well. But the ultimate benefit is that we have large quantum networks that can communicate tens of terabytes of data per second.”

The research was funded by the Army Research Laboratory, Air Force Research Laboratory, National Science Foundation, NTT Research, Packard Foundation, the Marshall and Arlene Bennett Family Research Program, and the US Department of Energy.

Journal Reference:

Huang, Y., et al. (2024) Vacuum Beam Guide for Large Scale Quantum Networks. Physical Review Letters.

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